A freeze-fracture study of the papillary layer of the rat incisor enamel organ.

After tooth enamel has been secreted it undergoes maturation or hardening. This process is mediated by ruffled and smooth-ended ameloblasts and associated papillary layer cells. The cells of the papillary layer are characterized by large numbers of mitochondria, coated vesicles, microvilli, and gap junctions. These features have led numerous investigators to speculate that the papillary layer is an ion-transporting epithelium. We have conducted freeze-fracture studies of the rat papillary layer in order to better characterize the surface features of these cells. The cell membranes of the papillary cells contained large numbers of intramembrane particles of various sizes ranging from 4 to 9 nm in diameter. Gap junctions were present at the cell surface and in the cytoplasm in the form of annular gap junctions. The intramembrane particles or connexons of both types of gap junctions were about 8-9 nm wide and were either packed randomly or present in the so-called 'crystallized' state. At the interface between smooth-ended ameloblasts and papillary layer cells, a well-developed zonula occludens was present along the basal surfaces of the ameloblasts and several large gap junctions were formed between the two cell types. The capillary network associated with the papillary layer was characterized by a thin endothelium containing large numbers of fenestrations.