Ultrastructure of basement membranes in developing shark tooth.

Based on studies of the tooth of largely mammalian species, the dental basement membranes are shown to be specialized for various roles significant in the development and maintenance of the tooth. Comparative studies with the nonmammalian tooth will facilitate further clarification of the mechanisms of mammalian tooth formation. In this study, basement membranes of the shark tooth in successive developmental stages was ultrastructurally examined for elucidation of their roles in odontogenesis. Teeth of a shark, Cephaloscyllium umbratile, were processed for thin section electron microscopy. Throughout the developmental stages the lamina densa of the basement membrane was made up of a fine network of "cords," irregular anastomosing strands known to be the major component of mammalian basement membranes. In the presecretory stage of the shark tooth, dental papilla cells were immobilized for their differentiation into odontoblasts by means of the binding of their processes to numerous narrow extensions of the lamina densa of the inner dental epithelium. In the secretory stage, a number of cords of the widened lamina densa were extended towards and bound to tubular vesicles of the forming enameloid. During the mineralization stage, fragments of the degrading enameloid matrix appeared to be moving through the lamina densa to the epithelial cells for processing. In the maturation stage, half of the lamina densa facing the enameloid was mineralized forming an advancing edge of mineralization of the enameloid. It provided strong binding and smooth transition of organic to mineral phase which may allow transportation of substances across the phases for enameloid maturation in a way similar to that reported in the mammalian tooth. These observations indicate that basement membranes of the developing shark tooth, as those in the mammalian tooth, play various roles, including anchoring, firm binding, and possible mediation of the transport of substances that are known to be vital for the development of the tooth.