Alessandro Castriota-Scanderbeg, M.D.
Each limb consists of four segments, including a root or zonoskeleton; a proximal segment or stylopodi-um, consisting of a single bone (humerus, femur); a medial segment or zeugopodium, consisting of two bones (radius and ulna; tibia and fibula); and a distal part or autopodium, corresponding to hand and foot. The complex processes involved in the formation of these segments and pertinent genetic hints have been reviewed in Chapter 6.
Development of the bones in the limbs takes place for the most part by virtue of endochondral bone formation, that is, transformation of the primitive mesenchyma into an intermediate cartilage model, which subsequently ossifies. First, a network of immature, woven-fibered trabeculae (the primary spongiosa) is produced. Later, the primary spongiosa is replaced by secondary bone,which is either trabec-ular or cortical, depending on the location (Frost 1983). Ossification starts at approximately the midpoint of the cartilaginous model (primary ossification center) and proceeds toward both ends of the bone until a plate of cellular activity is created at the interface between the diaphysis and the epiphysis. This growth plate, or physis, allows for longitudinal growth of the bone until its final length is achieved. The primary ossification centers for the femur make their appearance around the 7th week of gestation, while those for the humerus, radius, ulna, tibia, and fibula appear around the 8th week of gestation. As endochondral ossification proceeds, the mesenchy-mal cells surrounding the cartilaginous model undergo transformation into osteoblasts (intramem-branous bone formation) and lay down bone in the subperiosteal zone, which is destined to form the cortex of the developing bone. By ossification of the secondary centers within the epiphyses at the ends of the bone, the epiphyseal cartilage is converted to bone, except for a thin peripheral layer, which persists as articular cartilage of the intervening joint. The tubular bones with secondary ossification centers at both ends are termed, by convention, 'long' bones, while those with a center at only one end are termed 'short' bones. The secondary ossification cen ters for the head of the humerus, distal femur, and proximal tibia make their appearance around the 36th week of gestation, while those for the femoral head and capitulum of humerus do not appear until the 2nd to the 6th month after birth. The distal epiphysis of radius usually appears around 12 months, and the greater trochanter of femur and proximal epiphysis of fibula at about 3 years of age. Ossification of the long bones extends a long way into childhood and early adolescence, with the patella developing at about 4 years, the capitulum of the radius at
5 years, the medial epicondyle of the humerus at
6 years, the distal epiphysis of the ulna at 7 years, the olecranon of the ulna at 10 years, and the lateral epi-condyle of the humerus and tubercle of the tibia at 11 years (Garn et al. 1967). With further growth, the physeal plate becomes progressively narrowed and finally disappears, allowing fusion between the epi-physis and the diaphysis.
At any stage of development, even when growth is complete, the normal bone is an active, dynamic tissue in which the process of bone formation is balanced by that of bone resorption. This balance is accomplished by the integrated activity of specialized bone cells,i.e., osteoblasts and osteoclasts,which are enrolled into the process of growth, fracture healing, modeling, and remodeling of the living human skeleton (Resnick et al. 1995). Modeling is the dynamic process by which major adjustments in the size and shape of the bones are produced. The process of modeling, which depends heavily upon the mechanical forces applied to the skeleton, is most prominent in the immature skeleton until adolescence, and results in a net increase in the amount of bone tissue, especially in the sub-periosteal location. Remarkable examples of bone modeling in the tubular bones include: (a) drifting of the midshaft, accomplished by endosteal bone resorption and periosteal bone formation; and (b) flaring of the metaphyses, accomplished by resorption along the periosteal surface and apposition in the endosteal surface. In this way the wide metaphysis is substituted by a narrow diaphysis as the bone grows in length. Remodeling is the dynamic process that modifies bone quality, causing the structurally inferior woven-fibered bone of the infant to give way to the more compact lamellar bone of the adult (Resnick et al. 1995). In addition, remodeling replaces aged or injured bone tissue with new bone, a process requiring a tight balance between resorption and formation of the cortical and trabecular bone.
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