Skeletal System Help
Structure and Function of Bone
The skeletal system consists of bones, cartilage, and joints. Bones are composed of bone tissue, a connective tissue. The functions of the skeletal system fall into five categories.
- Support. The skeleton forms a rigid framework to which are attached the softer tissues and organs of the body.
- Protection. The skull, vertebral column, rib cage, and pelvic girdle enclose and protect vital organs; sites for blood cell production are protected with in the hollow centers of certain bones.
- Movement. Bones act as levers when attached muscles contract, causing movement about joints.
- Hemopoiesis. Red bone marrow of an adult produces white and red blood cells and platelets.
- Mineral and energy storage. The matrix of bone is composed primarily of calcium and phosphorus; these minerals can be withdrawn in small amounts if needed elsewhere in the body. Lesser amounts of magnesium and sodium are also stored in bone tissue. Lipids stored in adipose cells of yellow bone marrow store energy.
A long bone consists of a diaphysis (or shaft) in the center and an epiphysis on either end (Figure 6-1). Within the diaphysis is a medullary cavity that is lined with a thin layer of connective tissue, the endosteum. The medullary cavity contains fatty yellow bone marrow. The epiphyses consist of spongy bone surrounded by compact bone. Red bone marrow is found within the pores of the spongy bone. Separating the diaphysis and epiphysis is an epiphyseal plate, a region of mitotic activity responsible for linear bone growth (elongation); an epiphyseal line replaces the plate when bone growth is completed. A periosteum of dense regular connective tissue covers the bone and is the site of tendonmuscle attachment and diametric bone growth (widening).
There are several different types of bone cells. Osteogenic cells are progenitor cells that give rise to all bone cells. Osteoblasts are the principal bone-building cells; they synthesize the collagenous fibers and bone matrix, and promote mineralization during ossification. The osteoblasts are then trapped in their own matrix and develop into osteocytes that maintain the bone tissue. Osteoclasts are bone-destroying cells that contain lysosomes and phagocytic vacuoles that demineralize bone tissue.
Ossification (bone formation) begins during the fourth week of prenatal development. Bones develop either through endochondral ossification—going first through a cartilaginous stage—or through intramembranous (dermal) ossification—forming directly as bone. Endochondral ossification of a long bone begins in a primary center in the shaft of the cartilage model with hypertrophy of chondrocytes (cartilage cells) and calcification of the cartilage matrix. The cartilage model is then vascularized, osteogenic cells form a bony collar around the mode, and osteoblasts lay down bony matrix around the calcareous spicules. Ossification from primary centers occurs before birth; from secondary centers in the epiphyses, it occurs during the first 5 years. Most of the bones of the skeleton form through endochondral ossification.
The facial bones, most of the cranial bones, and the clavicle form via intramembranous ossification. During fetal development and infancy, the membranous bones of the top and sides of the cranium are separated by fibrous sutures. There are also six large membranous areas call fontanels that permit the skull to undergo changes in shape during birth. They also permit rapid growth of the brain during infancy. Ossification of the fontanels is complete by 20 to 24 month of age.