Membrane‐coating granules and membrane modifications in keratinizing epithelia

Membrane‐coating granules are found in close association with the Golgi apparatus, and their apparent size, shape and internal substructure varies with the plane of section. After a membrane‐coating granule is discharged into the intercellular space, it dissociates into distinct units, the pentalamellar discs, which are stacked like coins in the intact granule. Subsequently, discs become applied to and appear to fuse with the outer leaflets of the horny cell membranes, resulting in the formation of a nine‐layered membrane‐disc complex. When this complex cleaves at the junction of the thick dense lamella and distal lighter lamella of the disc, a pentalaminar horny‐cell surface membrane results. The more common surface membrane, however, is trilaminar consisting of thickened inner leaflet, lighter middle leaflet, and thickened outer leaflet. The thickened outer leaflet may result from the addition of amorphous intercellular material derived from membrane‐coating granules or from the fusion of two outer leaflets, as in the formation of a tight junction. The thickened inner leaflet results from the deposition of dense material onto the cytoplasmic face of the inner leaflet; there exists a regional variation in its dimensions. The side of the cell facing the surface of the epithelium has a thickened inner leaflet varying in width between 140 and 180 Å, whereas the side facing the underlying cell is only about 100 Å thick. Discs and amorphous material derived from membrane‐coating granules coat and thicken the plasma membrane by modifying the outer leaflet, and thus may play a role in the barrier function of the stratum corneum.