Development of dentition and dermal skeleton in embryonic Scyliorhinus canicula.

Serial sections ranging from very young embryos to hatched juveniles and whole embryos of Scyliorhinus show that dentition and dermal skeleton belong to two independent secondary developmental fields that differ both developmentally and structurally. The development of the dentition starts very early, with a thickening of the ectoderm in the region of the mouth (stage 04), the invagination of the dental lamina (stage 18), and the formation of the germs of the first generation (stage 20). Tooth replacement movements start only near the end of embryogenesis (stage 35). Scale germs, on the other hand, first begin to form at stage 24. Scales erupt shortly before the animal hatches (stage 43). Only one scale generation is formed during embryogenesis. The forces which erupt the scales may come from fluid pressures in vacuoles of the fibrous layer of the dermis. Those which erupt the teeth probably also result from similar fluid pressures. The crown and upper part of the base of scales and teeth are formed by cells of the inner dental epithelium which are differentiated from the ectoderm. They are also formed by odontoblasts which are derived from the vascular layer of the dermis. However, the basal plates of scales and teeth containing the anchoring fibers are formed by osteoblasts, which are derived from the fibrous layer of the dermis.