Quantitative analysis of rough endoplasmic reticulum approaches to the cell membrane in the secretory ameloblast of the rat incisor.

The distribution of approaches of rough endoplasmic reticulum to the cell membrane within the supranuclear region of the secretory ameloblast of the rat incisor was quantitated using a Zeiss MOP-3. In ameloblasts cut in cross section, most of these approaches appear as circular profiles representing cross sections of elongated cisternae, which are aligned parallel to the long axis of the cell. Because of their position, orientation, and distribution of ribosomes, these approaches were consistent with the appearance of subsurface cisternae. Using cross-sectioned ameloblasts, the lengths of apposed plasma membranes either between or within rows of cells were measured from electron micrographs. Along these lengths, matched approaches of rough endoplasmic reticulum from opposite sides of the apposed plasma membranes were counted. Approaches from either side that were unmatched were also counted. Thirteen percent of the approaches were matched between rows of ameloblasts, and 13.5% of the approaches were matched within rows, demonstrating no significant difference between the two sites. Furthermore, mathematical analysis showed that the theoretical probability of two approaches coinciding is 17%. The experimental values are not statistically different from the theoretical probability, and it is concluded that the matching of rough endoplasmic reticulum approaches to the plasma membrane, or subsurface cisternae, occurs at random.