Ultrastructure of the enamel layer in developing teeth of the shark Carcharhinus menisorrah

Abstract

The first two rows of teeth at the posterior end of the dental lamina in a 60 cm specimen of Carcharhinus menisorrah were uncalcified, but calcification had begun in the tooth-cap layer at the tips of third-row teeth. Enamel crystals developed within hollow enameline fibrils (tubules) which polymerized beneath the basement menbrane underlying ameloblasts. Vesicles containing fine granules were present in the apical cytoplasm of ameloblasts in tooth buds prior to calcification. Fine granular material accumulated extracellularly between ameloblasts and basement membrane, and also in the enameline matrix on the pulpal side of the basement membrane. The morphology suggests that ameloblasts secrete a granular precursor for the mineralizing enameline fibrils. Enamel crystals with their fibrous coatings were tightly packed in mineralizing zones. Crystals became indefinitely long and equilaterally hexagonal in cross section. They were aligned in parallel within bundles of fibrils interwoven in the mineralizing zones. Odontoblast processes and myelinated nerve fibres penetrated into the cap layer between mineralizing zones. Giant fibres with a banding periodicity of 14.5 nm occurred in the partitioning matrix between zones of mineralization. Their origin and nature are uncertain. Conventional collagen fibrils developed in the connective tissue within the base of the tooth, and in dentine after it began to differentiate. Crystals of mineralized dentine were needle-shaped as in mammals. The cap layer of the shark's tooth is considered to be composed of tubular enamel in which the mineralized zones are probably homologous with mammalian enamel, but which is penetrated by odontoblast processes of mesodermal origin.