Summary

Summary

The skeleton protects and supports other systems­ of the body. Bones are classified according to their shapes such as flat, irregular, long, short, and sesamoid­ (round) . Long bones consist of an epiph-ysis at each end and are covered with articular ­cartilage that connects with other bones. The shaft of the bone is called the diaphysis. The three types of bone cells are osteoblasts, osteocytes, and osteo-clasts (osteophages). Bone tissue is mostly made up of collagen and inorganic salts such as calcium phosphate. The primary types of bone are compact and spongy. Organic components of bone include osteoid and bone cells, while inorganic components are mineral salts.

Bone repair involves hematoma formation, for-mation of a fibrocartilaginous callus, formation of a bony callus, and bone remodeling. Control of bone remodeling is through genetic factors, a negative feed-back hormonal loop, and responses to gravitational and mechanical forces. PTH is the primary hormonal controller of bone remodeling.

The skeleton is divided into two major por-tions: the axial skeleton and the appendicular skel-eton. The major parts of the axial skeleton include the skull (22 bones), vertebral column (24 movable vertebrae, the sacrum, and the coccyx), and thoracic cage (12 pairs of ribs, the sternum, and the thoracic vertebrae). All bones of the skull, except for the tem-poromandibular joints, are joined by immovable sutures. The cranium contains 8 bones and the face contains 14 bones. In infants, the cranial bones are connected by fibrous membranes through soft spots called fontanels, which allow the cranium to slightly change shape. The primary curvatures of the verte-bral column are the thoracic and sacral curvatures, whereas the secondary curvatures are the cervical and lumbar. The vertebral column is C-shaped at birth, with only thoracic and sacral curvatures being present, and reverts back toward this shape in the elderly. The major parts of the appendicular skele-ton include the pectoral girdle (one clavicle and one scapula on each side), upper limbs (30 bones each), pelvic girdle (2 hip bones and the sacrum), and lower limbs (37 bones each).

Though the skeleton changes continually through life, the most significant changes occur during childhood. The earliest significant changes involve the cranium and facial bones. Long bones continue to grow until late adolescence. In females, the pelvis changes during puberty to prepare for childbirth. Once an adult reaches full height, the skeleton remains mostly the same until late middle age. Aging leads to reductions in height because of changes in the intervertebral discs and osteoporosis. Inadequate bone ossification is called osteopenia, which affects all adults eventually.