CYTOCHEMICAL CHARACTERIZATION OF BASEMENT-MEMBRANES IN THE ENAMEL ORGAN OF THE RAT INCISOR

Ameloblasts are unique epithelial cells, in that once they have deposi ted the entire thickness of enamel and the process of maturation begin s, they reform a basal lamina-like structure at their apical surface. In order to characterize further this basal lamina, its composition wa s analysed using (1) lectin-gold cytochemistry for glycoconjugates, (2 ) high-iron diamine (HID) staining for sulfated glycoconjugates and (3 ) immunogold labeling for collagen type IV and laminin. The labeling p atterns were compared to that of other more ''typical'' basement membr anes found in the enamel organ. Sections of rat incisor enamel organs embedded in Lowicryl K4M were stained with Helix pomatia agglutinin (H PA), Ricinus communis I agglutinin (RCA), wheat germ agglutinin (WGA) and Ulex europaeus I agglutinin (UEA). Samples from the late maturatio n stage were also reacted en bloc with lectins and embedded in Epon fo r transmission electron microscopic examination or prepared for scanni ng electron microscopy. Such samples were also stained with HID and co nventionally processed for Epon embedding. Tissue sections were then r eacted with thiocarbohydrazide-silver proteinate (TCH-SP). Analysis of the lectin labeling suggested that the region of extracellular matrix immediately adjacent to ameloblasts, where the basal lamina is situat ed, was intensely reactive with HPA and RCA, moderately reactive with WGA, and weakly reactive with UEA. In general, other basement membrane s were mildly reactive with all lectins used. No HID-TCH-SP staining w as observed directly over the basal lamina while numerous stain deposi ts were present over other basement membranes of the enamel organ. Imm unolocalization of collagen type IV and laminin yielded a weak and var iable labeling over the basal lamina. These results are consistent wit h the concept of basement membrane heterogeneity and, although the pre cise nature and composition of the basal lamina associated with matura tion stage ameloblasts remain to be determined, they suggest that it m ay possibly function as a specialized basement membrane with particula r compositional characteristics.