(a) The cartilaginous cells of an epiphyseal plate lie in four layers, each of which may be several cells thick. (b) A micrograph of an epiphyseal disk (100x).

from it toward the ends of the cartilaginous structure. Meanwhile, osteoblasts from the periosteum deposit a thin layer of compact bone around the primary ossification center. The epiphyses of the developing bone remain cartilaginous and continue to grow. Later, secondary ossification centers appear in the epiphyses, and spongy bone forms in all directions from them. As spongy bone is deposited in the diaphysis and in the epiphysis, a band of cartilage, called the epiphyseal plate (ep'T-fiz'e-al plat), or metaphysis, remains between the two ossification centers (see figs. 7.2, 7.3b, and 7.8).

Growth at the Epiphyseal Plate

In a long bone, the diaphysis is separated from the epi-physis by an epiphyseal plate. The cartilaginous cells of the epiphyseal plate occur in four layers, each of which may be several cells thick, as shown in figure 7.9. The first layer, closest to the end of the epiphysis, is composed of resting cells that do not actively participate in growth. This layer anchors the epiphyseal plate to the bony tissue of the epiphysis.

The second layer of the epiphyseal plate contains rows of many young cells undergoing mitosis. As new cells appear and as intercellular material forms around them, the cartilaginous plate thickens.

The rows of older cells, which are left behind when new cells appear, form the third layer, enlarging and thickening the epiphyseal plate still more. Consequently, the entire bone lengthens. At the same time, invading osteoblasts, which secrete calcium salts, accumulate in the intercellular matrix adjacent to the oldest cartilaginous cells, and as the matrix calcifies, the cells begin to die.

The fourth layer of the epiphyseal plate is quite thin. It is composed of dead cells and calcified intercellular substance.

In time, large, multinucleated cells called osteoclasts (os'te-o-klasts) break down the calcified matrix. These large cells originate by the fusion of single-nucleated white blood cells called monocytes (see chapter 14, p. 556). Osteoclasts secrete an acid that dissolves the inorganic component of the calcified matrix, and their lysosomal enzymes digest the organic components. Osteoclasts also phagocytize components of the bony matrix. After osteoclasts remove the matrix, bone-building osteoblasts invade the region and deposit bone tissue in place of the calcified cartilage.

A long bone continues to lengthen while the cartilaginous cells of the epiphyseal plates are active. However, once the ossification centers of the diaphysis and

I^H Ossification

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