Fine structure of differentiating ameloblasts in the kitten.

The fine structure of differentiating ameloblasts was studied in the lower second molar of 1-week-old kittens after perfusion fixation with and without subsequent decalcification. The differentiation zone was divided into three phases. In Differentiation 1, ameloblasts are about 27 mum long and face an uninterrupted basal lamina. The predentin adjacent to the basal lamina contains a few collagen fibrils oriented mainly at right angles to the ameloblast surface. The specialized predentin forms a well-defined layer, up to 1.5 mum thick, referred to as the junctional layer. In Differentiation 2, ameloblast processes extend through the basal lamina and the thickness of the junctional layer. The processes consist of cytoplasmic sheets forming a honeycomb-like network. Dentin starts to calcify after process-formation is underway. Two distinct types of odontoblast processes, having different shapes and contents, come in contact with the ameloblasts and push into the ameloblastic layer. In Differentiation 3, stippled material appears in the extracellular spaces between ameloblasts. Later, stippled material-like substances appear in the predentin close to the ameloblast apex and close to odontoblast processes within the dentin. Ameloblasts now are up to 40 mum high. Enamel secretion starts in small circumscribed areas which gradually enlarge, leading to the disappearance of the ameloblast processes. These findings are compared with results obtained in other species, including man, and their possible functional significance is discussed.