Ultrastructural observations on chondroid bone in the teleost fish Hemichromis bimaculatus

This paper presents transmission electron microscopical observations on the chondroid bone (CB) supporting the neurocraniad articulation facet of the upper pharyngeal jaws of juvenile specimens of Hemichromis bimaculatus (an acellular-boned teleost fish). Chondroid bone, a skeletal tissue morphologically intermediate between cartilage and bone, is composed of a dense mineralized collagenous matrix, resembling that of woven-fibred bone, and large chondrocyte-like cells. The latter vary considerably in their morphological features (functional cells, cells containing a large vacuole and degenerating cells). The CB is mineralized except for its upper layer. Mineralization is initiated in matrix vesicles. Clusters of apatite crystals coalesce at the mineralization front. Distally, the tissue grows by incorporation of cells which exhibit the features of osteoblasts, and which derive from less differentiated fibroblast-like cells located in the outermost layer of the tissue. Proximally, the CB is subjected to erosion by multinucleated clastic cells. The deposition of bone against the wall of lacunae which have been opened by clastic resorption may suggest a possible active involvement of the CB cells. Further studies should point out whether this bone substantially contributes to the acellular dermal dentigerous bone located below.