Photomicrograph of a mixed bone spicule formed during endochondral bone formation. In this Mallory-Azan-stained section, bone has been deposited on calcified cartilage spicules. In the center of the photomicrograph, the spicules have already grown to create an anastomosing trabecula. The initial trabecula still contains remnants of calcified cartilage, as shown by the light-blue staining of the calcified matrix compared with the dark-blue staining of the bone. In the upper part of the spicule, note a lone osteoclast (arrow) aligned near the surface of the spicule, where remodeling is about to be initiated. x275.
• Zone of proliferation, which is adjacent to the zone of reserve cartilage in the direction of the diaphysis. In this zone, the cartilage cells undergo division and organize into distinct columns. These cells are larger than those in the reserve zone and actively produce matrix.
• Zone of hypertrophy, which contains greatly enlarged cartilage cells. The cytoplasm of these cells is clear, which is a reflection of the glycogen that they normally accumulate (and that is lost during fixation). The matrix is compressed into linear bands between the columns of hypertrophied cartilage cells.
• Zone of calcified cartilage, in which the enlarged cells begin to degenerate and the matrix becomes calcified.
• Zone of resorption, which is the zone nearest the diaphysis. The calcified cartilage here is in direct contact with the connective tissue of the marrow cavity.
In the zone of resorption, small blood vessels and accompanying connective tissue invade the region occupied by the dying chondrocytes. They form a series of spearheads, leaving the calcified cartilage as longitudinal spicules, at least as seen in longitudinal sections of bone. Actually, in a cross section of the bone, the cartilage appears as a honeycomb because the invading vessels and connective tissue have migrated into the sites previously occupied by the cartilage cells.
Bone deposition occurs on the cartilage spicules in the same manner as described for the formation of the initial ossification center
As bone is laid down on the calcified spicules, the cartilage is resorbed, ultimately leaving a primary spongy bone. This spongy bone undergoes reorganization through osteoclastic activity and addition of new bone tissue, thus accommodating the continued growth and physical stresses placed on the bone.
Shortly after birth, a secondary ossification center develops in the upper 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 illustrations 6 and 7 of Fig. 8.14). Later, a similar epiphyseal ossification center forms at the lower end of the bone (see illustration 8 of Fig. 8.14). 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 plate, which separates the epiphyseal and diaphyseal cavities.
Cartilage of the epiphyseal 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.
• The thickness of the epiphyseal plate remains relatively constant during growth.
• The amount of new cartilage produced (zone of proliferation) equals the amount resorbed (zone of resorption).
• The resorbed cartilage is, of course, replaced by spongy bone.
In reviewing the growth process, it is important to realize that
• 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
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