Epidermal permeability barrier: transformation of lamellar granule-disks into intercellular sheets by a membrane-fusion process, a freeze-fracture study.
Freeze-fracture replication of lamellar granules and intercellular sheets of the horny layer in mouse, chicken, and snake epidermis reveals a pattern of serial fracture faces which is highly suggestive of polar lipids in a bilayer configuration. The occurrence of alternating wide and narrow fracture faces separated by intervening steps supports the view that epidermal barrier bilayers display lipid asymmetry similar to membranes. Within the lamellar granules, bilayers arrange to form disks which in fact are equivalent to flattened unilamellar liposomes. Stacking of the disks in turn gives rise to the lamellar pattern. After exocytosis into the intercellular space, the disks are arranged parallel to the cell membranes. In tangentially fractured specimens, the cleavage plane jumps back and forth from the plasma membrane to a disk-bilayer, thereby giving rise to the known phenomenon of EF-ridges (on the extracellular fracture face) and PF-grooves (in the plasmatic fracture face) which both represent the level of the plasma membrane sur- or subjacent to the aisles between disks. Concomitantly with the upward movement of the keratinocytes, the ridges and grooves become narrower until they fade away by the second or third cell layer of the stratum corneum. This phenomenon is explained by the fusion of adjacent disks at their highly curved brims due to a mechanism similar to the process of membrane fusion which causes the formation of wide, uninterrupted sheets.