I
INTRODUCTION

Bone Marrow, soft, pulpy tissue that fills the cavities of bones, occurring in two forms, red and yellow. One of the largest tissues in the body, bone marrow accounts for 2 to 5 percent of an adult’s weight. Red marrow, present in all bones at birth, serves as the blood manufacturing center. As an infant matures, most of the red marrow in the shaft of long bones, such as the arm and leg bones, is gradually replaced by yellow marrow. Yellow marrow is composed primarily of specialized fat cells.

II
STRUCTURE

Cross Section of a Bone

Bone marrow, the soft, pulpy tissue that fills bone cavities, contains a network of blood vessels and fibers surrounded by fat and blood-producing cells. In children, the cells that give rise to blood cells can be found throughout the marrow. In adults, these cells are found mostly in the red marrow of the bones of the chest, hips, back, skull, and of the upper arms and legs. The marrow in the long shafts of bones gradually loses its ability to manufacture blood. This marrow, which is dominated by fat cells and takes on a yellowish color, is called yellow marrow. This cross section of a long bone shows yellow bone marrow in the shaft of a long bone.

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Red marrow consists primarily of a loose, soft network of blood vessels and protein fibers interspersed with developing blood cells. The blood vessels are termed the vascular component, and the protein fibers and developing blood cells collectively are referred to as the stroma, or the extravascular component. The protein fibers crisscross the marrow, forming a meshwork that supports the developing blood cells clustered in the spaces between the fibers.

Red marrow contains a rich blood supply. Arteries transport blood containing oxygen and nutrients into the marrow, and veins remove blood containing carbon dioxide and other wastes. The arteries and veins are connected by capillaries, blood vessels that branch throughout the marrow. In various places, the capillaries balloon out, forming numerous thin, blood-filled cavities. These cavities are called sinusoids, and they assist in blood-cell production.

Yellow marrow is so named because it is composed of yellow fat cells interspersed in a rich mesh of connective tissue that also supports many blood vessels. While not usually actively involved in blood formation, in an emergency yellow marrow is replaced by blood-forming red marrow when the body needs more blood.

MARROW FUNCTION

Constituents of Blood

In an average healthy person, approximately 45 percent of the blood volume is cells, produced in the red bone marrow. Within the bone marrow, all blood cells originate from one type of cell called a hematopoietic stem cell. These stem cells divide to become progenitor blood cells, which undergo further specializations to become red blood cells, white blood cells, or platelets. Blood also contains a clear, yellowish fluid called plasma. The test tube on the right has been centrifuged to separate plasma and packed cells by density.

Martin M. Rotker/Science Source/Photo Researchers, Inc.

Red marrow produces all of the body’s blood cells—red blood cells, white blood cells, and platelets. Red blood cells in the circulatory system transport oxygen to body tissues and carbon dioxide away from tissues. White blood cells are critical for fighting bacteria and other foreign invaders of the body (see Immune System). Platelets are essential for the formation of blood clots to heal wounds.

Within red bone marrow, all blood cells originate from a single type of cell, called a hematopoietic stem cell. Stimulated by hormones and growth factors, these stem cells divide to produce immature, or progenitor blood cells. Most of these progenitor cells remain in the stroma and rapidly undergo a series of cell divisions, producing either red blood cells or white blood cells. At any one time, the stroma consists largely of progenitor cells in various stages of development. At the appropriate developmental stage, the fresh, new cells squeeze through the walls of the capillaries. From there, the cells leave the bone and enter the body’s circulatory system. Some progenitor cells migrate to the sinusoids, where they produce platelets, which also travel to the circulatory system via the capillaries.

Although stem cells are relatively rare—about 1 in every 10,000 marrow cells is a stem cell—they typically produce the forerunners of an estimated 2 million red cells per second and 2 billion platelets per day. However, if significant amounts of blood are lost or other conditions reduce the supply of oxygen to tissues, the kidneys secrete the hormone erythropoietin. This hormone stimulates stem cells to produce more red blood cells. To fight off infection, hormones collectively termed colony stimulating growth factors are released by the immune system. These hormones stimulate the stem cells to produce more infection-fighting white blood cells. And in severe cases, the body converts yellow marrow into red marrow to help produce needed blood cells.

BONE MARROW DISEASES

Hairy Cell Leukemia

About 30,000 new cases of cancer of the bone marrow, or leukemia, are diagnosed each year in the United States. There are four types of leukemia, classified by the type of blood cell affected and whether the cells are mature or immature. Pathologists can distinguish various types of leukemia by the appearance of the cancerous cells underneath a microscope. A magnification of a cell affected by hairy cell leukemia, shown here, reveals the characteristic minute, hairlike projections on the cell surface.

Aaron Polliack/Science Photo Library/Photo Researchers, Inc.

Diseases of the bone marrow can be life threatening because they disrupt blood cell production, which is essential for survival. Inadequate production of blood cells results in aplastic anemia. The causes of this relatively rare disease are often unknown, although some cases result from exposure to toxic chemicals, such as lead, benzene, or arsenic. Radiation from nuclear explosions or X rays can also damage the marrow because the radioactive elements involved have a strong affinity for bone marrow.

Leukemias are cancers that affect bone marrow (as well as other tissues). A cell can become cancerous at any state during the series of divisions that produce red or white blood cells. If a progenitor cell becomes cancerous at the beginning of a series of cell divisions, the leukemia is termed acute. Chronic leukemia results when cells in later stages of division become cancerous.

Bone Marrow Transplants

Radiation Treatment

Radiation therapy, sometimes used to shrink collections of cancerous cells, can severely damage a patient’s bone marrow. To compensate for bone marrow lost in radiation treatments, patients are increasingly undergoing a kind of bone marrow transplant called stem cell transplant. In this procedure, prior to radiation treatment a patient’s own stem cells are extracted and the cancerous cells removed. After the radiation therapy is complete, the healthy stem cells are returned to the bloodstream.

Martin Dohrn/Science Source/Photo Researchers, Inc.

Bone marrow transplants treat a variety of blood and bone marrow diseases (see Medical Transplantation). In a conventional transplant, a donor and recipient are matched as closely as possible for blood type. The red marrow from the donor is suctioned from the pelvic bone with a long needle attached to a syringe. The marrow sample is treated to remove the donor’s white blood cells, which otherwise would attack the recipient’s tissues. The treated marrow is then given to the recipient through an intravenous infusion, which introduces immature, but healthy, cells into the bloodstream. These cells migrate to the marrow, where they mature and eventually divide, populating the circulatory system with healthy cells. The transplanted stem cells serve as a continual source of healthy cells. While bone marrow transplants are helpful, they cannot always cure a blood or bone marrow disease because the match between donor and recipient is seldom perfect. The immune system of the recipient may attack some of the donor’s cells, which interferes with the benefits of the transplant.

Increasingly, only the stem cells from a bone marrow are used in a bone marrow transplant. Called a stem cell transplant, this procedure is often used for cancer patients who will be undergoing extensive radiation and chemotherapy treatments that will likely irreparably damage the bone marrow. Prior to undergoing radiation or chemotherapy, some of the patient’s own marrow is removed and screened to eliminate cancer cells in a technique called a stem cell wash. The population of healthy stem cells is kept alive in the laboratory. After the chemotherapy and radiation treatments, the patient’s stem cells are returned to the bloodstream. They travel to the marrow and begin the process of blood cell production. The advantage of this type of stem cell transplant—in which the patient is both donor and recipient—is that the immune system will not be activated to destroy the transplanted cells. The disadvantage is that it takes longer for functioning cells to get into the blood stream, since only stem cells, and not cells in all stages of development, are transferred.

A newer stem cell transplant technique involves injecting a patient with high doses of the growth factors that stimulate white blood cell production. This causes stem cells to be released into the blood stream. The blood is then drawn and the stem cells are harvested. This technique has the advantage of being less invasive than retrieving stem cells from bone marrow.

Other types of stem cell transplants, which require both a donor and a recipient, are used to correct genetic disorders such as sickle-cell anemia, in which red blood cells are distorted and unable to transport oxygen properly. Sometimes the source of stem cells for these transplants is blood from the umbilical cord or placenta of a newborn infant. New techniques that match the blood cell producing genes of donors and recipients rather than the blood types may offer more success in these transplants.

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Each bone has a special name. We usually say their common names like the kneecap, shin bone, collarbone, etc... Let's learn the scientific names that a nurse, doctor or scientist might use. The chart below provides both the common name and the real name of different bones in our body. Below you will find a diagram of the skeleton. You can use the diagram to locate many of the bones in your body.

Common Name skull jawbone collarbone shoulder blade breast bone funny bone spine hips wrist hip thigh bone kneecap shin bone ankle

Scientific Name cranium mandible clavicle scapula sternum humerus vertebrae pelvis carpals pelvis femur patella tibia tarsals

 

 

Your Skeletal system is all of the bones in the body and the tissues such as tendons, ligaments and cartilage that connect them.
Your teeth are also considered part of your skeletal system but they are not counted as bones. Your teeth are made of enamel and dentin. Enamel is the strongest substance in your body.

How does the Skeletal System help us?

Support
The main job of the skeleton is to provide support for our body. Without your skeleton your body would collapse into a heap. Your skeleton is strong but light. Without bones you'd be just a puddle of skin and guts on the floor.

Protection
Your skeleton also helps protect your internal organs and fragile body tissues. The brain, eyes, heart, lungs and spinal cord are all protected by your skeleton. Your cranium (skull) protects your brain and eyes, the ribs protect your heart and lungs and your vertebrae (spine, backbones) protect your spinal cord.

Movement
Bones provide the structure for muscles to attach so that our bodies are able to move. Tendons are tough inelastic bands that hold attach muscle to bone.

 

Who has more bones a baby or an adult?

Babies have more than adults! At birth, you have about 300 bones. As you grow older, small bones join together to make big ones. Adults end up with about 206 bones.

Are bones alive?

Absolutely. Old bones are dead, dry and brittle. But in the body, bones are very much alive. They have their own nerves and blood vessels, and they do various jobs, such as storing body minerals like calcium. Bones are made of a mix of hard stuff that gives them strength and tons of living cells which help them grow and repair themselves.

What is a bone made of?

A typical bone has an outer layer of hard or compact bone, which is very strong, dense and tough. Inside this is a layer of spongy bone, which is like honeycomb, lighter and slightly flexible. In the middle of some bones is jelly-like bone marrow, where new cells are constantly being produced for the blood. Calcium is an important mineral that bone cells need to stay strong so keep drinking that low-fat milk!

How do bones break and heal?

Bones are tough and usually don't break even when we have some pretty bad falls. I'm sure you have broken a big stick at one time. When you first try to break the stick it bends a bit but with enough force the stick finally snaps. It is the same with your bones. Bones will bend a little, but if you fall the wrong way from some playground equipment or maybe your bike or skateboard you can break a bone. Doctors call a broken bone a fracture. There are many different types of fractures.

Luckily, bones are made of living cells. When a bone is broken your bone will produce lots of new cells to rebuild the bone. These cells cover both ends of the broken part of the bone and close up the break.

How do I keep my bones healthy?

Bones need regular exercise to stay as strong as possible. Walking, jogging, running and other physical activities are important in keeping your bones strong and healthy. Riding your bike, basketball, soccer, gymnastics, baseball, dancing, skateboarding and other activities are all good for your bones. Make sure you wear or use the proper equipment like a helmet, kneepads, shin guards, mats, knee pads, etc... to keep those bones safe.

Strengthen your skeleton by drinking milk and eating other dairy products (like low-fat cheese, frozen yogurt, and ice cream). They all contain calcium, which helps bones harden and become strong.

 

The largest bone in the foot is the calcaneus or the heel bone. The foot bones are held together by strong ligaments and leg muscles that allow us to have a good deal of flexibility while being strong and providing a stable base. When our ligaments weaken we refer to this as fallen arches or flat feet.

Joints are the point of contact between two bones. They are classified into 3 main types according to their degree of movement.

Diarthroses - movable joints

Amphiarthroses - partially movable joints

Synarthroses - immovable joints

Diarthroses - Most of the joints in our body are diarthroses. They tend to have the same structure. These joints allow for considerable movement sometimes in many directions and sometimes in only one or two directions. These movable joints consist of three main parts: articular cartilage, a bursa or a joint capsule and a synovial or joint cavity.

When 2 movable bones meet at a joint their surfaces do not touch one another. The two articular surfaces are covered with a smooth slippery cap of cartilage called the articular cartilage. This cartilage helps to absorb jolts.

Enclosing the 2 articular surfaces of the bone is a tough, fibrous connective capsule called a joint capsule or an articular capsule. Lining the articular capsule is a synovial membrane which secretes synovial fluid. The synovial fluid reduces the friction of the 2 articulating surfaces.

The structure of the joint capsule makes the joint function. Ligaments grow out of the periosteum and connect the bones together more firmly.

The clefts in connective tissue between muscles,tendons, ligaments and bones contain bursa sacs. If the bursa becomes irritated or injured a condition known as bursitis develops. The synovial fluid can be aspirated from the bursal sacs and examined for diagnostic purposes. Typical examinations may include a culture and sensitivity and a micro exam to check for WBC's and RBC's.

As we advance in age the joints undergo degenerative changes. The synovial fluid is not secreted as quickly and the articular cartilage's become ossified. This results in bone outgrowths along the joint edges which tend to stiffen joints causing inflammation, pain and a decrease in mobility.

There are several types of diarthroses joints: ball and socket, hinge, pivot and gliding and condyloid joints. Because they differ in structure they also differ in the amount of movement and range of motion.

Ball and socket joints- These joints allow for the greatest freedom of movement. One bone has a ballshaped head which nestles into a concave socket of the second bone. Examples are hip and shoulder joints.

Hinge joints are like hinges on a door. They allow movement in only two directions namely flexion and extension. Examples of a hinge joints are the knees, elbows and the outer joints of the fingers.

Pivot joints are those with an extension rotating in a second, arch shaped bone. The radius and ulna, wrist and ankle joints are all pivot joints. An example of this joint is between the atlas or the first cervical vertebrae which supports the head and the axis or the second cervical vertebrae which allows the head to rotate.

Gliding joints are those in which nearly flat surfaces glide across each other as in the vertebrae or the spine. These joints enable the torso to bend forward, backward, sideways as well as rotate.

Anatomy of a long bone

The 3-month-old fetus has an early skeleton-like framework composed of cartilage and connective tissue membrane. As the fetus matures, the cartilage and connective tissue change into bone. the formation of bones is called ossification. Ossification occurs in different ways in flat and long bones.

Ossification of flat bones

In the fetus, the flat bones consist of a thin connective tissue membrane. Ossification begins when osteoblasts or bone forming cells, migrate to the region of the flat bones. The osteoblasts secrete calcium and other minerals into the spaces between the thin connective tissue membranes thereby forming bone. This type of ossification involves replacement of thin membrane with bone.

Ossification of long bones

Ossification of long bones occurs as bone tissue replaces cartilage. The fetal skeleton is comprised largely of cartilage, and the layout of the cartilage in the fetus provides a model for bone formation. As the baby matures, osteoblasts invade the cartilage and gradually replace the cartilage with bone. This process continues in each long bone until all but the articular cartilage and the epiphyseal plate have been replaced by bone. By the time the fetus has fully matured, most cartilage of the body has been replaced by bone. Only isolated pieces of cartilage such as the bridge of the nose and the parts of the ribs, remain.

Intramembranous ossification

Is one of two types of bone formation and is process responsible for the development of flat bones, especially those found in the skull. Unlike endochondral ossification, cartilage is not involved or present in this process

The first step in the process is the formation of bone spicules which eventually fuse with each other and become trabeculae. The periosteum is formed and bone growth continues at the surface of trabeculae. Much like spicules, the increasing growth of trabeculae result in interconnection and this network is called woven bone. Eventually, woven bone is replaced by lamellar bone.

Formation of bone spicules

Embryologic mesenchymal cells (MSC) condense into layers of vascularized primitive connective tissue. Certain mesenchymal cells group together, usually near or around blood vessels, and differentiate into osteogenic cells which deposit bone matrix constitutively. These aggregates of bony matrix are called bone spicules. Separate mesenchymal cells differentiate into osteoblasts, which line up along the surface of the spicule and secrete more osteoid, which increases the size of the spicule.

Formation of woven bone

As the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of trabeculae. When osteoblasts become trapped in the matrix they secrete, they differentiate into osteocytes. Osteoblasts continue to line up on the surface which increases the size. As growth continues, trabeculae become interconnected and woven bone is formed. The term primary spongiosa is also used to refer to the initial trabecular network.

Primary centre of ossification

The periosteum is formed around the trabeculae by differentiating mesenchymal cells. The primary centre of ossification is the area where bone growth occurs between the periosteum and the bone. Osteogenic cells that originate from the periosteum increase appositional growth and a bone collar is formed. The bone collar is eventually mineralized and lamellar bone is formed.

Formation of osteon

Osteons are units or principal structures of compact bone. Durinng the formation of bone spicules, cytoplasmic processes from osteoblasts interconnect. This becomes the canaliculi of osteons. Since bone spicules tend to form around blood vessels, the perivascular space is greatly reduced as the bone continues to grow. When replacement to compact bone occurs, this blood vessel becomes the central canal of the osteon.

Endochondral ossification is one of two types of bone formation and is the process responsible for much of the bone growth in vertebrate skeletons, especially in long bones. As the name might suggest (endo - within, chondro - root for cartilage), endochondral ossification occurs by replacement of hyaline cartilage.

Primary centre of ossification

The first site of ossification occurs in the primary centre of ossification, which is in the middle of diaphysis (shaft). The following steps then occur:

Formation of periosteum: Once vascularized, the perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells which later become osteoblasts.

Formation of bone collar: The osteoblast secretes osteoid against the shaft of the cartilage model. This serves as support for the new bone.

Calcification of matrix: Chondrocytes in the primary centre of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Nutrients can no longer diffuse if the matrix becomes sufficiently calcified and the chondrocytes subsequently die. This creates cavities within the bone.

Invasion of periosteal bud: A periosteal bud, which consist of blood vessels, lymph vessels and nerves, invades the cavity left by the chondrocytes. The vascularization utlimately carries hemopoietic cells, osteoblasts and osteoclasts inside the cavity. The hemopoietic cells will later form the bone marrow.

Formation of trabeculae: Osteoblasts use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts break down spongy bone to form the medullary (bone marrow) cavity.

Secondary centre of ossification

Cartilage is retained in the epiphyseal plate, located between the diaphysis (shaft) and the epiphysis (end) of the bone. These areas of cartilage are known as secondary centres of ossification. Cartilage cells undergo the same transformation as above. As growth progresses, the proliferation of cartilage cells in the epiphyseal plate slows and eventually stops. The continuous replacement of cartilage by bone results in the obliteration of the epiphyseal plate, termed the closure of the epiphysis. Only articular cartilage remains.

Appositional bone growth

The growth in diameter of bones around the diaphysis occurs by deposition of bone beneath the periosteum. Osteoclasts in the interior cavity continue to degrade bone until its ultimate thickness is achieved, at which point the rate of formation on the outside and degradation from the inside is constant.

Histology

Part of a longitudinal section of the developing femur of a rabbit. a. Flattened cartilage cells. b. Enlarged cartilage cells. c, d. Newly formed bone. e. Osteoblasts. f. Giant cells or osteoclasts. g, h. Shrunken cartilage cells. (From "Atlas of Histology," Klein and Noble Smith.)During endochondrol ossification, four distinct zones can be seen at the light-microscope level.

Zone of resting cartilage. This zone contains normal, resting hyaline cartilage.

Zone of proliferation. In this zone, chondrocytes undergo rapid mitosis, forming dinstinctive looking stacks.

Zone of maturation / hypertrophy. It is during this zone that the chondrocytes undergo hypertrophy (become enlarged). Chondrocytes contain large amounts of glycogen and begin to secrete alkaline phosphatase.

Zone of calcification. In this zone, chondrocytes are either dying or dead, leaving cavities that will later become invaded by bone-forming cells.

Maturation from infancy to adulthood is characterized by two types of bone growth. Bones grow longitudinally and determine the height of an individual. Bones also grow thicker and become wider so as to support the weight of the adult body.

Growing taller

Longitudinal bone growth occurs at the epiphyseal plate or growth plate. Longitudinal bone growth ceases when the growth plate becomes ossified or hardened. This plate or disc is sensitive to the effects of certain hormones, especially growth hormone and sex hormones. GH stimulates growth at the plate, making the child taller. The sex hormones estrogen and testosterone, however, cause the plate to seal or fuse, thereby inhibiting further longitudinal growth.

The growth plates or epiphyseal plates are generally more sensitive to the effects of estrogen than to those of testosterone. During puberty in the female, the rising levels of estrogen seal the epiphyseal plate earlier than testosterone does in males. The effects of the male hormone, testosterone, are felt at a later stage. Thus, females stop growing earlier than males do.

Because the epiphyseal disc or plate plays such a crucial role in longitudinal bone growth, injury to the plate can severely retard bone growth. A child who injures the plate in a tibia, for instance, may end up with that leg considerably shorter than the non-injured leg.

The surface of bone appears irregular and bumpy. This appearance is due to numerous ridges, projections, depressions, and grooves called bone markings. The projecting bone marking serves as points of attachment for muscles, tendon, and ligaments. The grooves and depressions form the routes traveled by blood vessels and nerves as they pass over and through the bones and joints. The projections and depressions also help to form joints. The head of the upper arm bone, for instance, fits into a depression in a shoulder bone, forming the shoulder joint. Please refer to handout.

 

The arrangement of compact and spongy tissue in long bone accounts for its strength. Long bones contain sites of growth and reshaping and structures associated with joints. The parts of a long bone include the following:

Diaphysis- The diaphysis is the long shaft of the bone. It is composed primarily of compact bone and therefore provides considerable strength.

Epiphysis- The enlarged ends of the long bone are the epiphyses. The epiphyses of a bone articulates, or meets, with a second bone at a joint. Each epiphysis consists of a thin layer of compact bone overlying spongy bone. The epiphyses are covered by cartilage.

Epiphyseal disc or plate- A growing bone contains a band of cartilage located at the ends of long bones, between the epiphyisis and the diaphysis. This band of cartilage is the epiphyseal plate. It is here that longitudinal bone growth occurs.

Medullary cavity- The medullary cavity is the hollow center of the diaphysis. In infancy, the cavity is filled with red bone marrow for blood cell production. In the adult, the medullar cavity is filled with yellow bone marrow and functions as a storage site for fat; at this stage it is not associated with blood cell production. The inside of the medullary cavity is lined with connective tissue called the endosteum.

Periosteum- The periosteum is a tough fibrous connective tissue membrane that covers the outside of the diaphysis. It is anchored firmly to the outside of the bone on all surfaces except the articular cartiglage. The periosteum protects the bone, serves as a point of attachment for muscle, and contains blood vessels that nourish the underlying bone. Because the periosteum carries the blood supply to the underlying bone, any injury to this structure has serious consequences to the health of the bone. Like any other organ the loss of blood supply can cause its death.

Articular cartilage- The articular cartilage is found on the outer surface of the epiphysis. It forms a smooth, shiny surface that decreases friction within a joint. Because a joint is also called an articulation, this cartilage is called articular cartilage.

 

Bone cells are called osteocytes, and the matrix of the bone is made of calcium salts and collagen. The calcium salts give bones the strength for its supportive and protective functions. The function of osteocytes is to regulate the amount of calcium that is deposited in or removed from the bone matrix.

Bone is an organ, it has its own blood supply and is made up of two types of tissue; compact and spongy bone.

There are two types of bone tissue: compact and spongy. The names imply that the two types of differ in density, or how tightly the tissue is packed together. There are three types of cells that contribute to bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.

Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae) of matrix. Between the rings of matrix, the bone cells (osteocytes) are located in spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone, the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone.

 

 

Spongy (cancellous) bone is lighter and less dense than compact bone. Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow. The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.

 

Bones

A. Functions of Bones

1. Support. Provide a hard framework.

2. Protection of many vital organs.

3. Movement. Act as levers with skeletal muscles moving them. Joints control possible movements.

4. Mineral storage. Especially calcium and phosphate, critical minerals for cellular function. Continuous deposition and withdrawal. Exquisite control of Ca++ (calcium ions) levels necessary for function of nerves, muscles, blood coagulation and other functions. Most of Ca++ in body in bones. Osteoclasts & osteoblasts controlled by hormones which regulate blood levels of Ca++.

5. Blood cell formation. Certain bones have active marrow.

B. Structure

1. Compact-Dense outer layer, looks smooth and solid. Contains cylinder of concentric layers with central canals.
a. Haversian system = circles of bone (lamella) with central canal (Haversian canal)
b. Central canal contain blood vessels & nerves. Connected at right angles to network.
c. Perforating small canals - blood vessels & nerves go through lamellar bone to supply osteocytes. Connect to periosteum.
d. Osteocytes live in bone, maintain it. Live in holes called lacunae. Connect to each other and central canal via canaliculi, little canals. Pass nutrients, waste products

2. Spongy- honeycombed, open spaces. Same structure as compact but less regular.

Withstand maximum stress with least weight. In bone interiors & weird weight bearing bones like head of femur. Not organized in lamella. Trabeculae are arranged along lines of stress. Osteocytes interconnected by canaliculi. Nutrients reach osteocytes by diffusing through the canaliculi from capillaries. Osteoporosis - More bone resorption than deposition, very weak bone.

Skeletal System

A. Axial skeleton

Principal supportive structure of the body includes skull, vertebrae, sternum & ribs. Central column of the skeleton from which arms and legs & bones that help them hang.

B. Appendicular skeleton

Provides fairly freely movable frame for upper & lower limbs. Includes pectoral (shoulder) & pelvic (hip) girdles, arms, forearms, wrists, hands, thighs, legs & feet.

Joints

Bones -> framework; muscles -> power; joints provide mechanism that allows body to move

A joint is where 2 adjacent bones or cartilages or combination thereof meet.

Most joints movable, some not.

General Structure

1. Articular cartilage

2. Joint (synovial) cavity

3. Articular capsule - external layer = fibrous capsule, inner layer is a synovial membrane

4. Synovial fluid-occupies all free spaces within the joint capsule, fluid derived by filtration from blood flowing thorough the capillaries in the synovial membrane

5. Reinforcing ligaments

Skeletal System

INTRODUCTION

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.

Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means “to set in motion.” Today more than 100 hormones have been identified.

Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.

Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones is steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action.

II
HOW HORMONES WORK

Plasma Membrane

The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including some hormones, to freely cross the membrane and activate changes within the cell. Proteins embedded in the plasma membrane carry out a variety of functions. When hormones bind with receptor proteins embedded in the cell membrane, both molecules undergo structural changes that activate chemical changes within the cell.

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Most hormones are released directly into the bloodstream, where they circulate throughout the body in very low concentrations. Some hormones travel intact in the bloodstream. Others require a carrier substance, such as a protein molecule, to keep them dissolved in the blood. These carriers also serve as a hormone reservoir, keeping hormone concentrations constant and protecting the bound hormone from chemical breakdown over time.

Hormones travel in the bloodstream until they reach their target tissue, where they activate a series of chemical changes. To achieve its intended result, a hormone must be recognized by a specialized protein in the cells of the target tissue called a receptor. Typically, hormones that are water-soluble use a receptor located on the cell membrane surface of the target tissues. A series of special molecules within the cell, known as second messengers, transport the hormone’s information into the cell. Fat-soluble hormones, such as steroid hormones, pass through the cell membrane and bind to receptors found in the cytoplasm. When a receptor and a hormone bind together, both the receptor and hormone molecules undergo structural changes that activate mechanisms within the cell. These mechanisms produce the special effects induced by the hormone.

Receptors on the cell membrane surface are in constant turnover. New receptors are produced by the cell and inserted into the cell wall, and receptors that have reacted with hormones are broken down or recycled. The cell can respond, if necessary, to irregular hormone concentrations in the blood by decreasing or increasing the number of receptors on its surface. If the concentration of a hormone in the blood increases, the number of receptors in the cell wall may go down to maintain the same level of hormonal interaction in the cell. This is known as downregulation. If concentrations of hormones in the blood decrease, upregulation increases the number of receptors in the cell wall.

Some hormones are delivered directly to the target tissues instead of circulating throughout the entire bloodstream. For example, hormones from the hypothalamus, a portion of the brain that controls the endocrine system, are delivered directly to the adjacent pituitary gland, where their concentrations are several hundred times higher than in the circulatory system.

III
HORMONAL EFFECTS

Hormonal effects are complex, but their functions can be divided into three broad categories. Some hormones change the permeability of the cell membrane. Other hormones can alter enzyme activity, and some hormones stimulate the release of other hormones.

Recent studies have shown that the more lasting effects of hormones ultimately result in the activation of specific genes. For example, when a steroid hormone enters a cell, it binds to a receptor in the cell’s cytoplasm. The receptor becomes activated and enters the cell’s nucleus, where it binds to specific sites in the deoxyribonucleic acid (DNA), the long molecules that contain individual genes. This activates some genes and inactivates others, altering the cell’s activity. Hormones have also been shown to regulate ribonucleic acids (RNA) in protein synthesis.

A single hormone may affect one tissue in a different way than it affects another tissue, because tissue cells are programmed to respond differently to the same hormone. A single hormone may also have different effects on the same tissue at different times in life. To add to this complexity, some hormone-induced effects require the action of more than one hormone. This complex control system provides safety controls so that if one hormone is deficient, others will compensate.

IV
TYPES OF HORMONES

Hormones exist in mammals, including humans, as well as in invertebrates and plants. The hormones of humans, mammals, and other vertebrates are nearly identical in chemical structure and function in the body. They are generally characterized by their effect on specific tissues.

A
Human Hormones

Menstrual Cycle

A typical menstrual cycle lasts 28 days. It begins with three to five days of menstruation, the shedding of the uterine lining, during which hormone levels are low. At the end of menstruation, a pituitary hormone stimulates new follicles to develop in the ovary. These secrete estrogen as they mature, causing cells in the lining of the uterus to proliferate. Mid-cycle, one mature follicle releases an egg. The empty follicle forms the corpus luteum, an endocrine body that secretes progesterone. Under the added influence of progesterone, the uterine lining thickens further and swells in preparation for the implantation of a fertilized egg. If fertilization does not take place, the corpus luteum dies and hormone levels fall. Without hormonal support, the uterine lining disintegrates and discharges, beginning a new menstrual period and cycle.

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Human hormones significantly affect the activity of every cell in the body. They influence mental acuity, physical agility, and body build and stature. Growth hormone is a hormone produced by the pituitary gland. It regulates growth by stimulating the formation of bone and the uptake of amino acids, molecules vital to building muscle and other tissue.

Sex hormones regulate the development of sexual organs, sexual behavior, reproduction, and pregnancy. For example, gonadotropins, also secreted by the pituitary gland, are sex hormones that stimulate egg and sperm production. The gonadotropin that stimulates production of sperm in men and formation of ovary follicles in women is called a follicle-stimulating hormone. When a follicle-stimulating hormone binds to an ovary cell, it stimulates the enzymes needed for the synthesis of estradiol, a female sex hormone. Another gonadotropin called luteinizing hormone regulates the production of eggs in women and the production of the male sex hormone testosterone. Produced in the male gonads, or testes, testosterone regulates changes to the male body during puberty, influences sexual behavior, and plays a role in growth. The female sex hormones, called estrogens, regulate female sexual development and behavior as well as some aspects of pregnancy. Progesterone, a female hormone secreted in the ovaries, regulates menstruation and stimulates lactation in humans and other mammals.

Other hormones regulate metabolism. For example, thyroxine, a hormone secreted by the thyroid gland, regulates rates of body metabolism. Glucagon and insulin, secreted in the pancreas, control levels of glucose in the blood and the availability of energy for the muscles. A number of hormones, including insulin, glucagon, cortisol, growth hormone, epinephrine, and norepinephrine, maintain glucose levels in the blood. While insulin lowers the blood glucose, all the other hormones raise it. In addition, several other hormones participate indirectly in the regulation. A protein called somatostatin blocks the release of insulin, glucagon, and growth hormone, while another hormone, gastric inhibitory polypeptide, enhances insulin release in response to glucose absorption. This complex system permits blood glucose concentration to remain within a very narrow range, despite external conditions that may vary to extremes.

Hormones also regulate blood pressure and other involuntary body functions. Epinephrine, also called adrenaline, is a hormone secreted in the adrenal gland. During periods of stress, epinephrine prepares the body for physical exertion by increasing the heart rate, raising the blood pressure, and releasing sugar stored in the liver for quick energy.

Insulin Secretion

This light micrograph of a section of the human pancreas shows one of the islets of Langerhans, center, a group of modified glandular cells. These cells secrete insulin, a hormone that helps the body metabolize sugars, fats, and starches. The blue and white lines in the islets of Langerhans are blood vessels that carry the insulin to the rest of the body. Insulin deficiency causes diabetes mellitus, a disease that affects at least 10 million people in the United States.

Astrid and Hanns-Frieder Michler/Photo Researchers, Inc.

Hormones are sometimes used to treat medical problems, particularly diseases of the endocrine system. In people with diabetes mellitus type 1, for example, the pancreas secretes little or no insulin. Regular injections of insulin help maintain normal blood glucose levels. Sometimes, an illness or injury not directly related to the endocrine system can be helped by a dose of a particular hormone. Steroid hormones are often used as anti-inflammatory agents to treat the symptoms of various diseases, including cancer, asthma, or rheumatoid arthritis. Oral contraceptives, or birth control pills, use small, regular doses of female sex hormones to prevent pregnancy.

Initially, hormones used in medicine were collected from extracts of glands taken from humans or animals. For example, pituitary growth hormone was collected from the pituitary glands of dead human bodies, or cadavers, and insulin was extracted from cattle and hogs. As technology advanced, insulin molecules collected from animals were altered to produce the human form of insulin.

With improvements in biochemical technology, many hormones are now made in laboratories from basic chemical compounds. This eliminates the risk of transferring contaminating agents sometimes found in the human and animal sources. Advances in genetic engineering even enable scientists to introduce a gene of a specific protein hormone into a living cell, such as a bacterium, which causes the cell to secrete excess amounts of a desired hormone. This technique, known as recombinant DNA technology, has vastly improved the availability of hormones.

Recombinant DNA has been especially useful in producing growth hormone, once only available in limited supply from the pituitary glands of human cadavers. Treatments using the hormone were far from ideal because the cadaver hormone was often in short supply. Moreover, some of the pituitary glands used to make growth hormone were contaminated with particles called prions, which could cause diseases such as Creutzfeldt-Jakob disease, a fatal brain disorder. The advent of recombinant technology made growth hormone widely available for safe and effective therapy.

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Gonads—in the male, the testes (singular, testis), and in the female, the ovaries—are the organs that produce gametes and sex hormones. The male gamete is the spermatozoan, produced by cell division in the seminiferous tubules of the adult testes. Typically, several hundred million sperm reach maturity in the epididymis and are stored in the vas deferens each day. Whatever is not released in ejaculation is reabsorbed, part of a continuous cycle. In the female, the ovaries produce eggs, or ova. At birth, about 2 million oocytes, or immature eggs, are present in the ovaries. Once the female reaches puberty, one egg matures approximately every 28 days inside a saclike Graafian follicle. Ovulation occurs when the mature egg bursts from the follicle and the ovary, beginning its journey down the fallopian tube toward the uterus.

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Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

 

Called the master gland, the pituitary secretes hormones that control the activity of other endocrine glands and regulate various biological processes. Its secretions include growth hormone (which stimulates cellular activity in bone, cartilage, and other structural tissue); thyroid stimulating hormone (which causes the thyroid to release metabolism-regulating hormones); antidiuretic hormone (which causes the kidney to excrete less water in the urine); and prolactin (which stimulates milk production and breast development in females). The pituitary gland is influenced both neurally and hormonally by the hypothalamus.

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Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

 

I
INTRODUCTION

Human Anatomy Illustrations

Learn about the ten systems in the human body.

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Endocrine System, group of specialized organs and body tissues that produce, store, and secrete chemical substances known as hormones. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. Because of the hormones they produce, endocrine organs have a great deal of influence over the body. Among their many jobs are regulating the body's growth and development, controlling the function of various tissues, supporting pregnancy and other reproductive functions, and regulating metabolism.

Endocrine organs are sometimes called ductless glands because they have no ducts connecting them to specific body parts. The hormones they secrete are released directly into the bloodstream. In contrast, the exocrine glands, such as the sweat glands or the salivary glands, release their secretions directly to target areas—for example, the skin or the inside of the mouth. Some of the body's glands are described as endo-exocrine glands because they secrete hormones as well as other types of substances. Even some nonglandular tissues produce hormone-like substances—nerve cells produce chemical messengers called neurotransmitters, for example.

The earliest reference to the endocrine system comes from ancient Greece, in about 400 bc. However, it was not until the 16th century that accurate anatomical descriptions of many of the endocrine organs were published. Research during the 20th century and in the early 21st century vastly improved our understanding of hormones and how they function in the body. Today, endocrinology, the study of the endocrine glands, is an important branch of modern medicine. Endocrinologists are medical doctors who specialize in researching and treating disorders and diseases of the endocrine system.

II
COMPONENTS OF THE ENDOCRINE SYSTEM

Pituitary Gland

Called the master gland, the pituitary secretes hormones that control the activity of other endocrine glands and regulate various biological processes. Its secretions include growth hormone (which stimulates cellular activity in bone, cartilage, and other structural tissue); thyroid stimulating hormone (which causes the thyroid to release metabolism-regulating hormones); antidiuretic hormone (which causes the kidney to excrete less water in the urine); and prolactin (which stimulates milk production and breast development in females). The pituitary gland is influenced both neurally and hormonally by the hypothalamus.

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The primary glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal body, and reproductive glands—the ovary and testis. The pancreas, an organ often associated with the digestive system, is also considered part of the endocrine system. In addition, some nonendocrine organs are known to actively secrete hormones. These include the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta. Almost all body cells can either produce or convert hormones, and some secrete hormones. For example, glucagon, a hormone that raises glucose levels in the blood when the body needs extra energy, is made in the pancreas but also in the wall of the gastrointestinal tract. However, it is the endocrine glands that are specialized for hormone production. They efficiently manufacture chemically complex hormones from simple chemical substances—for example, amino acids and carbohydrates—and they regulate their secretion more efficiently than any other tissues.

The hypothalamus, found deep within the brain, directly controls the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. Acting as liaison between the brain and the pituitary gland, the hypothalamus is the primary link between the endocrine and nervous systems.

Ovary Releasing an Ovum

The ovary is the female organ that produces the reproductive cells called eggs, or ova. This false-color electron micrograph shows the release of a mature ovum at ovulation. The ovum (red) is surrounded by cells and liquid from the ruptured ovarian follicle.

Professors P. M. Motta and J. Van Blerkom/Science Source/Photo Researchers, Inc.

Located in a bony cavity just below the base of the brain is one of the endocrine system's most important members: the pituitary gland. Often described as the body’s master gland, the pituitary secretes several hormones that regulate the function of the other endocrine glands. Structurally, the pituitary gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breast-feeding. Hormones secreted by the anterior lobe include adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.

The posterior lobe of the pituitary gland contains the nerve endings (axons) from the hypothalamus, which stimulate or suppress hormone production. This lobe secretes antidiuretic hormones (ADH), which control water balance in the body, and oxytocin, which controls muscle contractions in the uterus.

Pancreas

The pancreas has both a digestive and a hormonal function. Composed mainly of exocrine tissue, it secretes enzymes into the small intestine, where they help break down fats, carbohydrates, and proteins. Pockets of endocrine cells called the islets of Langerhans produce glucagon and insulin, hormones that regulate blood-sugar level.

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The thyroid gland, located in the neck, secretes hormones in response to stimulation by TSH from the pituitary gland. The thyroid secretes hormones—for example, thyroxine and three-iodothyronine—that regulate growth and metabolism, and play a role in brain development during childhood.

The parathyroid glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood.

Structure of Human Gonads

Gonads—in the male, the testes (singular, testis), and in the female, the ovaries—are the organs that produce gametes and sex hormones. The male gamete is the spermatozoan, produced by cell division in the seminiferous tubules of the adult testes. Typically, several hundred million sperm reach maturity in the epididymis and are stored in the vas deferens each day. Whatever is not released in ejaculation is reabsorbed, part of a continuous cycle. In the female, the ovaries produce eggs, or ova. At birth, about 2 million oocytes, or immature eggs, are present in the ovaries. Once the female reaches puberty, one egg matures approximately every 28 days inside a saclike Graafian follicle. Ovulation occurs when the mature egg bursts from the follicle and the ovary, beginning its journey down the fallopian tube toward the uterus.

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Located on top of the kidneys, the adrenal glands have two distinct parts. The outer part, called the adrenal cortex, produces a variety of hormones called corticosteroids, which include cortisol. These hormones regulate salt and water balance in the body, prepare the body for stress, regulate metabolism, interact with the immune system, and influence sexual function. The inner part, the adrenal medulla, produces catecholamines, such as epinephrine, also called adrenaline, which increase the blood pressure and heart rate during times of stress.

The reproductive components of the endocrine system, called the gonads, secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads, the ovaries, produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy.

Located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.

The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine gland, secreting digestive enzymes into the gastrointestinal tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagon, and somastatin. These hormones all participate in regulating energy and metabolism in the body.

The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel.

III
HOW THE ENDOCRINE SYSTEM WORKS

Hormones from the endocrine organs are secreted directly into the bloodstream, where special proteins usually bind to them, helping to keep the hormones intact as they travel throughout the body. The proteins also act as a reservoir, allowing only a small fraction of the hormone circulating in the blood to affect the target tissue. Specialized proteins in the target tissue, called receptors, bind with the hormones in the bloodstream, inducing chemical changes in response to the body’s needs. Typically, only minute concentrations of a hormone are needed to achieve the desired effect.

Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperature is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body to increase the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if an individual becomes ill, the adrenal glands increase the secretions of certain hormones that help the body deal with the stress of illness. The adrenal glands work in concert with the pituitary gland and the brain to increase the body’s tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness is gone.

Long-term changes in hormone levels can influence the endocrine glands themselves. For example, if hormone secretion is chronically low, the increased stimulation by the feedback mechanism leads to growth of the gland. This can occur in the thyroid if a person's diet has insufficient iodine, which is essential for thyroid hormone production. Constant stimulation from the pituitary gland to produce the needed hormone causes the thyroid to grow, eventually producing a medical condition known as goiter.

IV
DISEASES OF THE ENDOCRINE SYSTEM

Goiter

Goiter, a disease characterized by the enlargement of the thyroid gland, can result from insufficient levels of iodine in the diet. The thyroid requires iodine to synthesize the hormone thyroxine and an iodine deficiency causes the thyroid to swell. In some cases goiter may also produce either lowered or elevated levels of basal metabolism. Treatments include the ingestion of small doses of iodine, or, in extreme cases, the removal of the thyroid gland.

AFIP/Science Source/Photo Researchers, Inc.

Endocrine disorders are classified in two ways: disturbances in the production of hormones, and the inability of tissues to respond to hormones. The first type, called production disorders, are divided into hypofunction (insufficient activity) and hyperfunction (excess activity). Hypofunction disorders can have a variety of causes, including malformations in the gland itself. Sometimes one of the enzymes essential for hormone production is missing, or the hormone produced is abnormal. More commonly, hypofunction is caused by disease or injury. Tuberculosis can appear in the adrenal glands, autoimmune diseases can affect the thyroid, and treatments for cancer—such as radiation therapy and chemotherapy—can damage any of the endocrine organs. Hypofunction can also result when target tissue is unable to respond to hormones. In many cases, the cause of a hypofunction disorder is unknown.

Hyperfunction can be caused by glandular tumors that secrete hormone without responding to feedback controls. In addition, some autoimmune conditions create antibodies that have the side effect of stimulating hormone production. Infection of an endocrine gland can have the same result.

Accurately diagnosing an endocrine disorder can be extremely challenging, even for an astute physician. Many diseases of the endocrine system develop over time, and clear, identifying symptoms may not appear for many months or even years. An endocrinologist evaluating a patient for a possible endocrine disorder relies on the patient's history of signs and symptoms, a physical examination, and the family history—that is, whether any endocrine disorders have been diagnosed in other relatives. A variety of laboratory tests—for example, a radioimmunoassay—are used to measure hormone levels. Tests that directly stimulate or suppress hormone production are also sometimes used, and genetic testing for deoxyribonucleic acid (DNA) mutations affecting endocrine function can be helpful in making a diagnosis. Tests based on diagnostic radiology show anatomical pictures of the gland in question. A functional image of the gland can be obtained with radioactive labeling techniques used in nuclear medicine.

One of the most common diseases of the endocrine systems is diabetes mellitus, which occurs in two forms. The first, called diabetes mellitus Type 1, is caused by inadequate secretion of insulin by the pancreas. Diabetes mellitus Type 2 is caused by the body's inability to respond to insulin. Both types have similar symptoms, including excessive thirst, hunger, and urination as well as weight loss. Laboratory tests that detect glucose in the urine and elevated levels of glucose in the blood usually confirm the diagnosis. Treatment of diabetes mellitus Type 1 requires regular injections of insulin; some patients with Type 2 can be treated with diet, exercise, or oral medication. Diabetes can cause a variety of complications, including kidney problems, pain due to nerve damage, blindness, and coronary heart disease. Recent studies have shown that controlling blood sugar levels reduces the risk of developing diabetes complications considerably.

Diabetes insipidus is caused by a deficiency of vasopressin, one of the antidiuretic hormones (ADH) secreted by the posterior lobe of the pituitary gland. Patients often experience increased thirst and urination. Treatment is with drugs, such as synthetic vasopressin, that help the body maintain water and electrolyte balance.

Gigantism

Gigantism results from the overproduction of growth hormone during childhood or adolescence. The arms and legs grow especially long, and height can surpass 2.4 m (8 ft). The disorder is caused by a pituitary tumor that, if untreated, usually kills the patient by early adulthood. If the tumor develops after growth of the long bones is complete, the result is a condition called acromegaly, characterized by a long face, jutting jaw, and large feet and hands.

Martin Rotker/Phototake NYC

Hypothyroidism is caused by an underactive thyroid gland, which results in a deficiency of thyroid hormone. Hypothyroidism disorders cause myxedema and cretinism, more properly known as congenital hypothyroidism. Myxedema develops in older adults, usually after age 40, and causes lethargy, fatigue, and mental sluggishness. Congenital hypothyroidism, which is present at birth, can cause more serious complications including mental retardation if left untreated. Screening programs exist in most countries to test newborns for this disorder. By providing the body with replacement thyroid hormones, almost all of the complications are completely avoidable.

Addison's disease is caused by decreased function of the adrenal cortex. Weakness, fatigue, abdominal pains, nausea, dehydration, fever, and hyperpigmentation (tanning without sun exposure) are among the many possible symptoms. Treatment involves providing the body with replacement corticosteroid hormones as well as dietary salt.

Cushing's syndrome is caused by excessive secretion of glucocorticoids, the subgroup of corticosteroid hormones that includes hydrocortisone, by the adrenal glands. Symptoms may develop over many years prior to diagnosis and may include obesity, physical weakness, easily bruised skin, acne, hypertension, and psychological changes. Treatment may include surgery, radiation therapy, chemotherapy, or blockage of hormone production with drugs.

Thyrotoxicosis is due to excess production of thyroid hormones. The most common cause for it is Graves' disease, an autoimmune disorder in which specific antibodies are produced, stimulating the thyroid gland. Thyrotoxicosis is eight to ten times more common in women than in men. Symptoms include nervousness, sensitivity to heat, heart palpitations, and weight loss. Many patients experience protruding eyes and tremors. Drugs that inhibit thyroid activity, surgery to remove the thyroid gland, and radioactive iodine that destroys the gland are common treatments.

Acromegaly and gigantism both are caused by a pituitary tumor that stimulates production of excessive growth hormone, causing abnormal growth in particular parts of the body. Acromegaly is rare and usually develops over many years in adult subjects. Gigantism occurs when the excess of growth hormone begins in childhood.

Contributed By:
Gad B. Kletter

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

 

The nervous system sends electrical messages to control and coordinate the body. The endocrine system has a similar job, but uses chemicals to “communicate”. These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by a group of specialized cells called an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel to elsewhere in the body to target organs, upon which they act. Note that this is in contrast to our digestive glands, which have ducts for releasing the digestive enzymes.

Pheromones are also communication chemicals, but are used to send signals to other members of the same species. Queen bees, ants, and naked mole rats exert control of their respective colonies via pheromones. One common use for pheromones is as attractants in mating. Pheromones are widely studied in insects and are the basis for some kinds of Japanese beetle and gypsy moth traps. While pheromones have not been so widely studied in humans, some interesting studies have been done in recent years on pheromonal control of menstrual cycles in women. It has been found that pheromones in male sweat and/or sweat from another “dominant” female will both influence/regulate the cycles of women when smeared on their upper lip, just below the nose. Also, there is evidence that continued reception of a given man’s pheromone(s) by a woman in the weeks just after ovulation/fertilization can significantly increase the chances of successful implantation of the new baby in her uterus. Pheromones are also used for things like territorial markers (urine) and alarm signals.

Each hormone’s shape is specific and can be recognized by the corresponding target cells. The binding sites on the target cells are called hormone receptors. Many hormones come in antagonistic pairs that have opposite effects on the target organs. For example, insulin and glucagon have opposite effects on the liver’s control of blood sugar level. Insulin lowers the blood sugar level by instructing the liver to take glucose out of circulation and store it, while glucagon instructs the liver to release some of its stored supply to raise the blood sugar level. Much hormonal regulation depends on feedback loops to maintain balance and homeostasis.

There are three general classes (groups) of hormones. These are classified by chemical structure, not function.

• steroid hormones including prostaglandins which function especially in a variety of female functions (aspirin inhibits synthesis of prostaglandins, some of which cause “cramps”) and the sex hormones all of which are lipids made from cholesterol,
• amino acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine, and
• peptide hormones (like insulin) which is the most numerous/diverse group of hormones.

The major human endocrine glands include:
(clipart edited from Corel Presentations 8)

1. the hypothalamus and pituitary gland
   The pituitary gland is called the “master gland” but it is under the control of the hypothalamus. Together, they control many other endocrine functions. They secrete a number of hormones, especially several which are important to the female menstural cycle, pregnancy, birth, and lactation (milk production). These include follicle-stimulating hormone (FSH), which stimulates development and maturation of a follicle in one of a woman’s ovaries, and leutinizing hormone (LH), which causes the bursting of that follicle (= ovulation) and the formation of a corpus luteum from the remains of the follicle.
   There are a number of other hypothalamus and pituitary hormones which affect various target organs.
   One non-sex hormone secreted by the posterior pituitary is antidiuretic hormone or ADH. This hormone helps prevent excess water excretion by the kidneys. Ethanol inhibits the release of ADH and can, thus, cause excessive water loss. That’s also part of the reason why a group of college students who go out for pizza and a pitcher of beer need to make frequent trips to the restrooms. Diuretics are chemicals which interfere with the production of or action of ADH so the kidneys secrete more water. Thus diuretics are often prescribed for people with high blood pressure, in an attempt to decrease blood volume.
   Another group of non-sex hormones that many people have heard of is the endorphins, which belong to the category of chemicals known as opiates and serve to deaden our pain receptors. Endorphins, which are chemically related to morphine, are produced in response to pain. The natural response to rub an injured area, such as a pinched finger, helps to release endorphins in that area. People who exercise a lot and push their bodies “until it hurts” thereby stimulate the production of endorphins. It is thought that some people who constantly over-exercise and push themselves too much may actually be addicted to their own endorphins which that severe exercise regime releases.
2. the thyroid gland
   Thyroid hormones regulate metabolism, therefore body temperature and weight. The thyroid hormones contain iodine, which the thyroid needs in order to manufacture these hormones. If a person lacks iodine in his/her diet, the thyroid cannot make the hormones, causing a deficiency. In response to the body’s feedback loops calling for more thyroid hormones, the thyroid gland then enlarges to attempt to compensate (The body’s plan here is if it’s bigger it can make more, but that doesn’t help if there isn’t enough iodine.). This disorder is called goiter. Dietary sources of iodine include any “ocean foods” because ocean-dwelling organisms tend to accumulate iodine from the seawater, and would include foods like ocean fish (tuna) and seaweeds like kelp. Because of this, people who live near the ocean do not have as much of a problem with goiter as people who live inland and don’t have access to these foods. To help alleviate this problem in our country, our government began a program encouraging salt refiners to add iodine to salt, and encouraging people to choose to consume this iodized salt.
3. the pancreas
   This organ has two functions. It serves as a ducted gland, secreting digestive enzymes into the small intestine. The pancreas also serves as a ductless gland in that the islets of Langerhans secrete insulin and glucagon to regulate the blood sugar level. The -islet cells secrete glucagon, which tells the liver to take carbohydrate out of storage to raise a low blood sugar level. The -islet cells secrete insulin to tell the liver to take excess glucose out of circulation to lower a blood sugar level that’s too high. If a person’s body does not make enough insulin (and/or there is a reduced response of the target cells in the liver), the blood sugar rises, perhaps out of control, and we say that the person has diabetes mellitus.
4. the adrenal glands
   These sit on top of the kidneys. They consist of two parts, the outer cortex and the inner medulla. The medulla secretes epinephrine (= adrenaline) and other similar hormones in response to stressors such as fright, anger, caffeine, or low blood sugar. The cortex secretes corticosteroids such as cortisone. Corticosteroids are well-known as being anti-inflammatory, thus are prescribed for a number of conditions. However, these are powerful regulators that should be used with caution. Medicinal doses are typically higher than what your body would produce naturally, thus the person’s normal feedback loops suppress natural secretion, and it is necessary to gradually taper off the dosage to trigger the adrenal glands to begin producing on their own again. Because the corticosteroids suppress the immune system, their use can lead to increased susceptibility to infections, yet physicians typically prescribe them for people whose immune systems are hard at work trying to fight off some pathogen. For example, back when I was in grad school, I was diagnosed with mono, and the campus doctor prescribed penicillin and cortisone. Since mono is a virus and penicillin only is effective against some bacteria, about all it did was kill off the friendly bacteria in my body, therefore causing me to develop a bad case of thrush. At the same time, the cortisone was supressing my immune system so my body could not as efficiently fight off the mono and the thrush. People with high blood pressure should be leery of taking prescription corticosteroids: they are known to raise blood pressure, thus can cause things like strokes. My mother-in-law had high blood pressure and was being treated with diuretics. Her physician also had her on large doses of cortisone for her arthritis. While he was on vacation, she started having significant back pain and was referred to an orthopedic surgeon. This man decided the back pain was just due to arthritis, and without carefully checking on what dosage she was already taking, prescribed more cortisone. Simultaneously, because of difficulty walking due to her arthritis, she decided to decrease the amount of diuretics she was taking so she didn’t have to make as many “long” trips to the other end of the house. The combination of lowered dose of diuretics and high dose of cortisone raised her blood pressure to the point where a blood vessel in her brain burst, causing a stroke. When the EMTs took her blood pressure, as I recall the systolic was way over 200 mm Hg.
5. the gonads or sex organs
   In addition to producing gametes, the female ovaries and male testes (singular = testis) also secrete hormones. Therefore, these hormones are called sex hormones. The secretion of sex hormones by the gonads is controlled by pituitary gland hormones such as FSH and LH. While both sexes make some of each of the hormones, typically male testes secrete primarily androgens including testosterone. Female ovaries make estrogen and progesterone in varying amounts depending on where in her cycle a woman is. In a pregnant woman, the baby’s placenta also secretes hormones to maintain the pregnancy.
6. the pineal gland
   This gland is located near the center of the brain in humans, and is stimulated by nerves from the eyes. In some other animals, the pineal gland is closer to the skin and directly stimulated by light (some lizards even have a third eye). The pineal gland secreted melatonin at night when it’s dark, thus secretes more in winter when the nights are longer. Melatonin promotes sleep (makes you feel sleepy). It also affects reproductive functions by depressing the activity of the gonads. Additionally, it affects thyroid and adrenal cortex functions. In some animals, melatonin affects skin pigmentation. Because melatonin production is affected by the amount of light to which a person is exposed, this is tied to circadian rhythm (having an activity cycle of about 24 hours), annual cycles, and biological clock functions. SAD or seasonal affective disorder (syndrome) is a disorder in which too much melatonin is produced, especially during the long nights of winter, causing profound depression, oversleeping, weight gain, tiredness, and sadness. Treatment consists of exposure to bright lights for several hours each day to inhibit melatonin production. It has also been found that melatonin levels drop 75% suddenly just before puberty, suggesting the involvement of melatonin in the regulation of the onset of puberty. Studies have been done on blind girls (with a form of blindness in which no impulses can travel down the optic nerve and reach the brain and pineal gland), which showed that these girls tended to have higher levels of melatonin for a longer time, resulting in a delay in the onset of puberty. While some older people, who don’t make very much melatonin, thus don’t sleep well, might benefit from a melatonin supplement, I’m skeptical of the recent melatonin craze in this country. When so many people apparently are suffering from SAD, I question the wisdom of purposly ingesting more melatonin, especially since the pineal gland is one of the least-studied, least-understood of the endocrine glands.

Local regulators are hormones with target cells nearby or adjacent to the endocrine gland in question. For example, neurotransmitters are secreted in the synapses of our nervous system and their target cells are in the same synapses.

 

http://biology.clc.uc.edu/courses/bio105/endocrin.htm

The pituitary gland (often called the master gland) is located in a small bony cavity at the base of the brain. A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes. The anterior pituitary is glandular.

The endocrine system in females and males. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The hypothalamus contains neurons that control releases from the anterior pituitary. Seven hypothalamic hormones are released into a portal system connecting the hypothalamus and pituitary, and cause targets in the pituitary to release eight hormones.

The location and roles of the hypothalamus and pituitary glands. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Growth hormone (GH) is a peptide anterior pituitary hormone essential for growth. GH-releasing hormone stimulates release of GH. GH-inhibiting hormone suppresses the release of GH. The hypothalamus maintains homeostatic levels of GH. Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. During adolescence, sex hormones cause replacement of cartilage by bone, halting further bone growth even though GH is still present. Too little or two much GH can cause dwarfism or gigantism, respectively.

Hypothalamus receptors monitor blood levels of thyroid hormones. Low blood levels of Thyroid-stimulating hormone (TSH) cause the release of TSH-releasing hormone from the hypothalamus, which in turn causes the release of TSH from the anterior pituitary. TSH travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures.

Gonadotropins and prolactin are also secreted by the anterior pituitary. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production. .

The Posterior Pituitary

The posterior pituitary stores and releases hormones into the blood. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary where they are dumped into the blood. ADH controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth.

Information provided by: http://gened.emc.maricopa.edu

 

http://www.cartage.org.lb/en/themes/sciences/LifeScience/GeneralBiology/Physiology/EndocrineSystem/NervousEndocrine/NervousEndocrine.htm