RESUMO
Direct visualization of the skin barrier formation process by cryotransmission electron microscopy of vitreous epidermal sections has been performed. The results obtained differ in many aspects from those of classical chemical fixation electron microscopy. Here we show that (i) a new "organelle or branched tubular structure" containing nonlamellar or partly lamellar material, and closely corresponding in numbers, dimensions, and localization to lamellar bodies of classical chemical fixation electron micrographs, and (ii) a new "ribosome complex-like structure", not preserved in classical electron micrographs, are omnipresent at apparent active sites of skin barrier formation. Evidence that skin barrier formation may not take place via extensive membrane fusion involving discrete lamellar bodies, but rather by a morphologically continuous membrane folding process are: (i) the often clearly nonlamellar content of structures corresponding to lamellar bodies with concomitant visualization of multilamellar membrane structures of the intercellular space; (ii) the "multifolded" appearance of the lipid matrix of the intercellular space; and (iii) the identification of extended "intracellular" multilamellar continuous structures with an optical density profile closely corresponding to that of the lipid matrix of the intercellular space. Based on the cryo-electron microscopic data presented in this study we propose that a membrane transition from cubic-like to multilamellar may take place already inside the "tubuloreticular cisternal membrane system" of upper granular cells recently reported by Elias et al.