Ultrastructure of growth cartilage in the proximal femur of the frog, Rana temporaria
Dickson, G.R.
Journal of Anatomy 135(Pt 3): 549-564
1982
ISSN/ISBN: 0021-8782 PMID: 7153173 Document Number: 187683
Relative to mammals and birds, little is known about the epiphyseal apparatus in amphibia. The epiphyseal ends of the limb bones in the frog, R. temporaria, apparently are enlarged and calcify independently of the centers in the shafts. The calcified epiphyses apparently united with the shaft centers relatively late in life. Several anuran species were studied and their long bones were found to consist of totally uncalcified cartilage. Proximal femora from frogs (weight range 7.6-31.9 g) were processed for conventional transmission electron microscopy, to elucidate the ultrastructural characteristics of frog growth cartilage and to establish the existence or otherwise of calcified cartilage in anuran long bones. There are similarities in the development of amphibian growth cartilage chondrocytes and those found in birds and mammals. Reserve, proliferative, hypertrophic and degenerate chondrocyte zones were identified. The role of matrix production is ascribed to the unfolding of 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, while closed capillary endothelial vessels run parallel to the degenerate chondrocyte zone which itself is uncalcified. 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. Chondrocytes seemed actively to produce membrane-bound matrix vesicles from their plasma membranes and processes. Mineralization appeared to be initiated within these structures and, on their rupture, it expanded 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 is also discussed.