Ultrastructure research on the vascularization of the enamel organ in the developing molar teeth of the rat.

Vascularization of the enamel organ was studied in the developing molar teeth of the rat with light and electron microscopy. Capillaries were first observed in the enamel organ at the embryonic age of 21 days. Differentiation of mesenchymal cells of the dental papilla into odontoblasts had taken place by that time, but deposition of inorganic substances had not started. The occurrence of capillaries before the nutritional supply from the dental papilla is interrupted by deposition of dentin may possibly be due to high nutritional requirement of ameloblasts following differentiation from the inner enamel epithelium.In addition to the basal lamina of the capillary endothelia, another basal lamina was recognized around the cells in contact with the endothelia. The findings indicate that, although indistinguishable from cells of the stellate reticulum because of the tendency to flatten and widen the intercellular spaces, the latter cells are the constituents of the outer enamel epithelium. The assumption was further supported by the presence of half-desmosomes characteristic cf the basal lamina of the outer enamel epithelium in the basal lamina of these cells. It is concluded, therefore, that the capillaries in the enamel organ result from indentation or invagination of the outer enamel epithelium in association with their growth and are not the result of direct invasion into the stellate reticulum by penetration through the layer of the outer enamel epithelium.The capillaries on the surface of the outer enamel epithelium at the embryonic age of 19 days maintained the continuity of endothelia, but they became fenestrated with the growth into the enamel organ and their endothelia contained many pinocytotic and coated vesicles. This may be regarded as a change to enable rapid and sufficient supply of metabolic substances needed for amelogenesis.