Although bone remodeling begins during the fetal period, it is not very active before birth but accelerates during the first year after birth. The annual rate of bone renewal during the first 2 years after birth is 50%, compared with a rate of 5% in the adult. During the first 2 years after birth, the infant progresses from an essentially helpless state to an erect walking individual.

At birth, the ossification centers present in the skeleton are, with few exceptions, primary centers (see Plate 1-7). The exceptions are the secondary, or epiphyseal, centers in the distal condyle of the femur in the proximal condyle of the tibia and possibly in the head of the humerus; numerous primary centers do not form until a number of years after birth. The mechanical stresses on the skeleton, as the infant begins to acquire increasing voluntary neuromuscular function during its first 2 years, serve to stimulate skeletal growth, ossification, and especially remodeling. During bone remodeling, the attachments of muscles and ligaments are also shifted and modified. Bone remodeling is most active during the growing period but continues throughout life in response to stresses created by an individual’s ever-changing type of physical activity.

DEVELOPMENTAL HISTORY OF BONES

Each of the more than 200 bones of the skeleton has its own developmental history. Some bones have a simple history, whereas others have quite a complicated one. The history of the clavicle and mandible is unique. The clavicle is the first bone in the entire skeleton to ossify (during the 7th week), followed shortly thereafter by the mandible (see Plates 1-5 and 1-6). Both the clavicle and the mandible are originally membrane bones that secondarily develop growth cartilage. The temporal bone is a good example of a bone with a complicated developmental history. It is a composite bone that forms initially as the otic capsule enclosing the organ for audition and equilibrium (inner ear) of the primitive chondrocranium, which then acquires secondary additions. Its squamous part, zygomatic process, and tympanic ring are derived from membrane bones, whereas its styloid process and ear ossicles are derived from the branchial arch skeleton. Although the overall size of the temporal bone is less than half its adult size at birth, the bony labyrinth of the inner ear, the middle ear cavity, the ear ossicles, and the eardrum have attained their adult size at birth. In contrast, the articular tubercle and mastoid process are absent at birth (see Plates 1-6 and 1-7).

HOMEOSTASIS

Homeostasis is the maintenance of constant conditions in the internal environment of the body. There is a constant turnover of bone mineral throughout life in response to mechanical stresses exerted on the skeleton. The bones of athletes become considerably heavier than those of nonathletes. Owing to the atrophy of disuse, the bones of a limb immobilized in a cast become thin and demineralized. In astronauts, a general demineralization of the entire skeleton occurs in response to the weightlessness caused by the lack of gravity in outer space. These alterations in the mineral content of bones allow the skeleton to serve as a dynamic structural support of the body. However, this support function is not really significant until the end of the first year after birth when the child starts to walk. Long before that time, the alterations in the mineral content of the bones are a part of another function of the skeleton related to the homeostasis of the body.

About 56% of the adult human body consists of fluid. There is intracellular fluid within the 75 trillion cells of the body and extracellular fluid outside the cells. The cells are capable of living, growing, and providing their special functions as long as proper concentrations of oxygen, glucose, ions, amino acids, and fatty substances are available in the internal environment. The skeleton plays a vital role in the regulation of calcium metabolism, which is fully described in Section 3, Metabolic Diseases.