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We have determined the distribution of amelogenin polypeptides in an order of elasmobranchs using indirect immunofluorescence with rabbit polyclonal antibodies prepared to purified murine amelogenins. We find that amelogenins are definitely present within the inner enamel epithelium prior to the production of the extracellular matrix component termed "enameloid" (row II developing tooth organs). During subsequent stages of selachian tooth development (row III tooth organs), immunofluorescence staining data indicated localization of amelogenin antigens within epithelium as well as the enameloid extracellular matrix. The results from these immunohistochemical studies suggest that the 16-20 kdalton amelogenins, which are characteristic of murine inner enamel epithelial cells undergoing terminal biochemical differentiation into secretory ameloblasts, may also be regarded as molecular markers for amelogenesis in developing teeth in the spiny dogfish, Squalus acanthias.

We have determined the general features of epithelial-mesenchymal interactions during fetal and mature adult stages of odontogenesis in the selachian spiny dogfish, Squalus acanthias. The general features of odontogenesis included the formation of an extended dental lamina, bud, cap, bell, and crown stages as identified and described using serial sections from fetal as well as adult specimens and light microscopy. Fetal and adult lengths, the rostrum-caudal fin distance, were correlated with the number of tooth rows and columns present in each specimen. This positional information and terminology was extremely valuable in attempting to acquire reproducible data for analyses. Whereas a number of histologic features of fetal and adult odontogenesis were comparable to those described in Mammalia, we found a number of epithelial-mesenchymal interaction characteristics unique to elasmobranch tooth development, including the persistence of the dental lamina in the adult stages, the coupling of cervical inner enamel epithelia between sequential stages of tooth formation, and the patterns of odontogenesis in the adult dentition.

Ultrastructural features of secretory amelogenesis during selachian tooth development show several similarities to mammalian amelogenesis. However, the following critical differences were noticed: 1) subcellular organelles associated with merocrine-type protein synthesis and secretion were located in both the infranuclear as well as supranuclear regions of the selachian ameloblasts; 2) no evidence for Tomes process formation was found; 3) the basal lamina was not removed during epithelial differentiation into ameloblasts in the selachian model, and the structural features of the basal lamina were significantly altered during amelogenesis in rows III, IV, and VI; and 4) no dentine-enameloid junction was detected. It is suggested that enameloid is an extracellular matrix which is derived from the selachian inner enamel epithelium and appears to be secreted from both the lateral and apical surfaces of ameloblasts.