Ultrastructure of growth cartilage in the proximal femur of the frog, Rana temporaria.

The ultrastructural maturation of the proximal femoral growth cartilage in the frog, Rana temporaria, has been described. Reserve, proliferative, hypertrophic and degenerate chondrocyte zones have been identified. There are similarities in the development of amphibian growth cartilage chondrocytes and those found in birds and mammals. The role of matrix production is ascribed to the unfolding of a secretory apparatus involving rough endoplasmic reticulum and Golgi elements, and this is particularly expansive in the hypertrophic chondrocyte. In the young frog, the erosion line of the epiphyseal cartilage is smooth and straight; whilst closed capillary endothelial vessels run parallel to the degenerate chondrocyte zone which itself is uncalcified and devoid of matrix vesicles. In older frogs, mononuclear cells appear to invade this cartilage and in so doing help to create an irregular erosion front. This is accompanied by the development of an open capillary endothelium and the appearance of matrix vesicles in the degenerate chondrocyte zone prior to matrix calcification. In each of the animals examined, a crest of calcification was present in the upper reaches of the epiphyseal cartilage, although its extent increased in older animals. The basic mechanism of cartilage mineralisation is similar to that observed for cartilage generally. Chondrocytes seem actively to produce membrane-bound matrix vesicles from their plasma membranes and processes. Mineralisation appears to be initiated within these structures and, on their rupture, it expands in a radial manner around them, eventually coalescing to form larger areas of calcified matrix. The contribution of growth cartilage to the longitudinal growth of the diaphyseal bone has been discussed.