Les cartilages calcifiés de la petite roussette (Scyliorhinus canicula L., Chondrichthyens): histologie et ultrastructure

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Abstract
The ultrastructure of the mandibular tesserae and of the calcifications of the double cone in the vertebrae were studied in the lesser spotted dogfish, Scyliorhinus canicula. New evidence is proved that shows with certainty that chondrocytes degenerate in the tesserae. Ultrastructural study of the collagen substrate in the tesserae also suggested new concepts on collagen organization in the tessarae: very thick collagen fibers have a radial arrangement in the superficial portion of each tessera and evidence also points to the probable noncollagenic nature of the substrate of the Liesegang rings. Observation of early tesseral stages strongly suggests that these formations have a dual origin, which might explain the different configurations of the tesserae; globular on the inner surface and parallel to the collagen fibers on the external surface. The presence or absence of bone in the tesserae will have to be examined anew since this study seem to indicate that the tesserae are entirely cartilaginous in this dogfish. Examination of serial section of the axial skeleton has shown that the complex structures of the skeleton have very different aspects at different sectional levels in the vertebra. However, at all sectional levels, the double calcified cone seems to be composed of a calcified fibrous cartilage in which chondrocytes degenerate. Our observations on the tesserae and on the double cone clearly demonstrate the degenerative process of the chondrocytes in these structures. Our results also show that, in contrast to other species, this dogfish has no vascularization in any of its cartilages, calcified or not.