A freeze-fracture evidence for lateral expansion of the plasma membrane of rat retinal pigment epithelial cells.

Distribution of the membrane particles of the retinal pigment epithelial cells of Wistar rats was studied by using freeze-fracture replicas. The overall view of the basal surfaces shows a mosaic pattern owing to membrane infoldings and interdigitated cell margins, where the P-face particles are as dense as 4.4 x 10(3)/micron2 and many particle-free patches can be seen on the basal membranes infolded into the cytoplasm. The particles of the lateral membrane are packed most closely as 6.7 x 10(3)/micron2 through the plasma membrane of the cell, where elaborate junctional complexes are seen surrounding the cell. In particular, on the side-junctional zone of the lateral membrane, they are counted as 7.7 x 10(3)/micron2, while the number of particles on the basolateral membrane is 5.5 x 10(3)/micron2. On the apical membrane the density of particles is 4.9 x 10(3)/micron2 and it is as many as on the basal membrane. Such a decreasing gradient of the distribution of the particles from the lateral to the basal or apical membrane is discussed with respect to their function.