[Influence of thyroidectomy and PTU treatment on cartilage ultrastructure in the embryo and very young dogfish (scyllium canicula, chondrichthyes) (author's transl)].

In embryos and very young dogfish made hypothyroidian by various treatments (radiothyroidectomy, surgical thyroidectomy and PTU treatment), ultrastructural abnormalities have been observed in all tested cartilages : Meckel's, branchial and pelvic cartilages. Many chondrocytes show a dark nucleus, enlarged REG vesicles and very electron dense hyaloplasm giving the cell a very peculiar reticulated aspect. Other chondrocytes appear unaffected. Cartilage ultrastructure is almost fully restored after a 21 days' recovery time. Around abnormal chondrocytes, extracellular matrix components are scarce. Moreover, matrix mineralization occurs around some abnormal chondrocytes by hydroxyapatite crystalization, never seen in normal embryos. During recovery, these crystals are preserved. When the antithyroid effects of PTU are compensated for by a simultaneous thyroxine treatment, cartilage abnormalities are minimized. If radiothyroidectomy is performed on young embryos, chondrocyte differentiation is delayed. It is concluded from these various experiments that thyroid hormones are required for the normal differentiation of cartilage and for the maintenance of its integrity in this fish.