Shortly after birth, a secondary ossification center develops in the proximal epiphysis. The cartilage cells undergo hypertrophy and degenerate. As in the diaphysis, calcification of the matrix occurs, and blood vessels and osteogenic cells from the perichondrium invade the region, creating a new marrow cavity (see illustration 7 of Fig. 8.17). Later, a similar epiphyseal ossification center forms at the distal end of the bone (see illustration 8 of Fig. 8.17). This center is also regarded as a secondary ossification center, although it develops later. With the development of the secondary ossification centers, the only cartilage that remains from the original model is the articular cartilage at the ends of the bone and a transverse disc of cartilage, known as the epiphyseal growth plate, which separates the epiphyseal and diaphyseal cavities (Plate 13, page 248).
Cartilage of the epiphyseal growth plate is responsible for maintaining the growth process.
For a bone to retain proper proportions and its unique shape, both external and internal remodeling must occur as the bone grows in length. The proliferative zone of the epiphyseal plate gives rise to the cartilage on which bone is later laid down. In reviewing the growth process, it is important to realize the following:  The thickness of the epiphyseal plate remains relatively constant during growth.  The amount of new cartilage produced (zone of prolifera tion) equals the amount resorbed (zone of resorption).  The resorbed cartilage is, of course, replaced by spongy bone.
Actual lengthening of the bone occurs when new cartilage matrix is produced at the epiphyseal plate. Production of new cartilage matrix pushes the epiphysis away from the diaphysis, elongating the bone. The events that follow this incremental growth—namely, hypertrophy, calcification, resorption, and ossification—simply involve the mechanism by which the newly formed cartilage is replaced by bone tissue during development.
Bone increases in width or diameter when appositional growth of new bone occurs between the cortical lamellae and the periosteum. The marrow cavity then enlarges by resorption of bone on the endosteal surface of the cortex of the bone. As bones elongate, remodeling is required. It consists of preferential epiphysis enlarges by growth of epiphyseal
FIGURE 8.20 • Diagram of external remodeling of a long bone. This diagram shows two periods during the growth of the bone. The younger bone profile (before remodeling) is shown on the right; the older (after remodeling), on the left. Superimposed on the left side of the figure is the shape of the bone (left half only) as it appeared at an earlier time. The bone is now longer, but it has retained its general shape. To grow in length and retain the general shape of the particular bone, bone resorption occurs on some surfaces, and bone deposition occurs on other surfaces, as indicated in the diagram. (Based on Ham AW. J Bone Joint Surg Am 1952;34:701.) resorption of bone in some areas and deposition of bone in other areas, as described previously and as outlined in Figure 8.20.
When an individual achieves maximal growth, proliferation of new cartilage within the epiphyseal plate terminates.