skeletal system
skeletal system
Imagine for a moment that people did not have skeletons.
What comes to mind? Probably that each of us would be a little heap on the
floor, much like a jellyfish out of water. Such an image is accurate and reflects
the most obvious function of the skeleton: to support the body. Although it is
a framework for the body, the skeleton is not at all like the wooden beams that
support a house. Bones are living organs that actively contribute to the
maintenance of the internal environment of the body. The skeletal system
consists of bones and other structures that make up the joints of the skeleton.
The types of tissue present are bone tissue, cartilage, and fibrous connective
tissue, which forms the ligaments that connect bone to bone.
key components of the skeletal system; The skeletal
system has many components besides bones. Ligaments connect bones to
other bones; cartilage is a smooth, shiny substance that coats the bone ends at
joints to keep them from rubbing and scratching against each other; and tendons
connect muscles to bones. Muscles cause joints to move.
FUNCTIONS OF THE SKELETON
1. Provides a framework that supports the body; the muscles
that are attached to bones move the skeleton.
2. Protects some internal organs
from mechanical injury; the rib cage protects the heart and lungs, for example.
3. Contains and protects the red bone marrow, the primary hemopoietic
(blood-forming) tissue.
4. Provides a storage site for excess calcium. Calcium
may be removed from bone to maintain a normal blood calcium level, which is
essential for blood clotting and proper functioning of muscles and nerves.
TYPES OF BONE TISSUE
Recall that
bone cells are called osteocytes, and the matrix of bone is made of calcium
salts and collagen. The calcium salts are calcium carbonate (CaCO3) and calcium
phosphate (Ca3(PO4)2), which give bone the strength required to perform its
supportive and protective functions. Bone matrix is non-living, but it changes
constantly, with calcium that is taken from bone into the blood replaced by calcium
from the diet. In normal circumstances, the amount of calcium that is removed
is replaced by an equal amount of calcium deposited. This is the function of
osteocytes, to regu
late the amount of calcium that is deposited in, or removed
from, the bone matrix. In bone as an organ, two types of bone tissue are
present . Compact bone looks solid but is very precisely structured.
Compact bone is made of osteons or haversian systems, microscopic cylinders of
bone matrix with osteocytes in concentric rings around central haversian
canals. In the haversian canals are blood vessels; the osteocytes are in
contact with these blood vessels and with one another through microscopic
channels (canaliculi) in the matrix. The second type of bone tissue is spongy
bone, which does look rather like a sponge with its visible holes or cavities.
Osteocytes, matrix, and blood vessels are present but are not arranged in
haversian systems. The cavities in spongy bone often contain red bone marrow,
which produces red blood cells, platelets, and the five kinds of white blood
cells.
CLASSIFICATION OF BONES
1. Long bones—the bones of the arms, legs, hands, and feet
(but not the wrists and ankles). The shaft of a long bone is the diaphysis, and
the ends are called epiphyses. The diaphysis is made of compact
bone and is hollow, forming a canal within the shaft. This marrow canal (or
medullary cavity) contains yellow bone marrow, which is mostly adipose tissue.
The epiphyses are made of spongy bone covered with a thin layer of compact
bone. Although red bone marrow is present in the epiphyses of children’s bones,
it is largely replaced by yellow bone marrow in adult bones.
2. Short bones—the
bones of the wrists and ankles.
3. Flat bones—the ribs, shoulder blades, hip
bones, and cranial bones.
4. Irregular bones—the vertebrae and facial bones.
Short, flat, and irregular bones are all made of spongy bone covered with a thin
layer of compact bone. Red bone marrow is found within the spongy bone. The
joint surfaces of bones are covered with articular cartilage, which provides a
smooth surface. Covering the rest of the bone is the periosteum, a fibrous
connective tissue membrane whose collagen fibers merge with those of the tendons
and ligaments thatare attached to the bone. The periosteum anchors these
structures and contains both the blood vessels that enter the bone itself and
osteoblasts that will become active if the bone is damaged.
EMBRYONIC GROWTH OF BONE
During embryonic development, the skeleton is first made of
cartilage and fibrous connective tissue, which are gradually replaced by bone.
Bone matrix is produced by cells called osteoblasts(a blastcell is a “growing”
or “producing” cell, and osteo means “bone”). In the embryonic model of the
skeleton, osteoblasts differentiate from the fibroblasts that are present. The
production of bone matrix, called ossification, begins in a center of
ossification in each bone. The cranial and facial bones are first made of fibrous
connective tissue. In the third month of fetal development, fibroblasts
(spindle-shaped connective tissue cells) become more specialized and
differentiate into osteoblasts, which produce bone matrix. From each center of
ossification, bone growth radiates outward as calcium salts are deposited in the
collagen of the model of the bone. This process is not complete at birth; a
baby has areas of fibrous connective tissue remaining between the bones of the
skull. These are called fontanels , which permit compression of the
baby’s head during birth without breaking the still thin cranial bones. The
fontanels also permit the growth of the brain after birth. You may have heard
fontanels referred to as “soft spots,” and indeed they are. A baby’s skull is
quite fragile and must be protected from trauma. By the age of 2 years, all the
fontanels have become ossified, and the skull becomes a more effective
protective covering for the brain. The rest of the embryonic skeleton is first
made of cartilage, and ossification begins in the third month of gestation in
the long bones. Osteoblasts produce bone matrix in the center of the diaphyses
of the long bones and in the center of short, flat, and irregular bones. Bone
matrix gradually replaces the original cartilage The long bones
also develop centers of ossification in their epiphyses. At birth, ossification
is not yet complete and continues throughout childhood. In long bones, growth
occurs in the epiphyseal discs at the
junction of the diaphysis with each epiphysis. An epiphyseal
disc is still cartilage, and the bone grows in length as more cartilage is
produced on the epiphysis side. On the diaphysis side,
osteoblasts produce bone matrix to replace the cartilage. Between the ages of
16 and 25 years (influenced by estrogen or testosterone), all of the cartilage
of the epiphyseal discs is replaced by bone. This is called closure of the
epiphyseal discs (or we say the discs are closed), and the bone lengthening
process stops. Also in bones are specialized cells called osteoclasts(a
clastcell is a “destroying” cell), which are able to dissolve and reabsorb the
minerals of bone matrix, a process called resorption. Osteoclasts are very
active in embryonic long bones, and they reabsorb bone matrix in the center of
the diaphysis to form the marrow canal. Blood vessels grow into the marrow
canals of embryonic long bones, and red bone marrow is established. After
birth, the red bone marrow is replaced by yellow bone marrow. Red bone marrow
remains in the spongy bone of short, flat, and irregular bones. For other functions
of osteoclasts and osteoblasts, : Fractures and Their Repair.
FACTORS THAT AFFECT BONE GROWTH AND MAINTENANCE
1. Heredity—each person has a genetic potential for height,
that is, a maximum height, with genes inherited from both parents. Many genes
are involved, and their interactions are not well understood. Some of these
genes are probably those for the enzymes involved in cartilage and bone
production, for this is how bones grow.
2. Nutrition—nutrients are the raw
materials of which bones are made. Calcium, phosphorus, and protein become part
of the bone matrix itself. Vitamin D is needed for the efficient absorption of
calcium and phosphorus by the small intestine. Vitamins A and C do not become
part of bone but are necessary for the process of bone matrix formation
(ossification). Without these and other nutrients, bones cannot grow properly.
Children who are malnourished grow very slowly and may not reach their genetic
potential for height. 3. Hormones—endocrine glands produce hormones that
stimulate specific effects in certain cells.
References
•Bloom, M.D., and D.W. Fawcett (1975) A Textbook of Histology, W.B. Saunders
•Campbell, N.A., J.B. Reece, L.G. Mitchell, and M.R. Taylor (2003) Biology: Concepts and Connections, Benjamin/Cummings
•Hoar, W.S. (1983) General and Comparative Physiology, Prentice-Hall
•Snell, R.S.(2003) Clinical Neuroanatomy for Medical Students. Lippincott Williams &Wilkins
•Rosenzweig, M.R., S.M. Breedlove, and A.L. Leiman (2002) Biological Psychology: An Introduction to Behavioural, Cognitive and Clinical Neuroscience. Sinauer Associates
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