Morphology of joints and patterns of cartilage calcification in the endoskeleton of the batoid Raja cf. polystigma

The skeleton of the batoid fish consists of a mixture of calcified and uncalcified cartilage with a typical layout of mineral deposition toward the outer border, leaving an uncalcified central core in most of the skeleton segments. An exception is observed in the radials, where mineral deposition is central. Joints and endoskeleton segments were studied in two adult samples of Raja cf. polystigma. Histomorphology, mineral deposition pattern, and zonal chondrocyte duplication activity were compared among several endoskeleton segments, but with particular attention to the fin rays; in the first, the uncalcified cartilage is central with an outer layer ranging from mineralized tesserae to a continuous calcified coating, whereas in the second, the uncalcified cartilage surrounds one or more central calcified columns. The diarthroses have a joint cavity closed by a fibrous capsule and the sliding surfaces rest on the base of mineralized tesserae, whereas the interradial amphiarthroses show a layer of densely packed chondrocytes between the flat, calcified discs forming the base of neighboring radials. In the endoskeleton segments, three types of tesserae are distinguished, characterizing the phases of skeletal growth and mineralization which present differences in each endoskeleton segment. The chondrocyte density between central core, subtesseral layer, and radial external cartilage did not show significant differences, while there was a significant difference in chondrocyte density between the latter zones and the type c tesserae of the pelvic girdle. The histomorphology and morphometry observed in Raja cf. polystigma suggest a model of cartilage growth associated with structural stiffening without remodeling. A key point of this model is suggested to be the incomplete mineralization of the tesseral layer and the continuous growth of cartilage, both enabling fluid diffusion through the matrix fibril network of scattered, uncalcified cartilage zones inside and between the tesserae.