The desmosome: Fine structural studies with freeze-fracture replication and tannic acid staining of sectioned epidermis

Desmosomes of larval and post-metamorphic newt epidermis have been studied by freeze-fracture replication both with and without prior glutaraldehyde fixation. Characteristic particles of a diameter (70-130 A) similar to that of typical membrane associated particles are found clustered on the exposed internal faces of adherent desmosomal membranes. They remain attached to the B-face in unfixed material, but occupy the desmosomal A-face after fixation. Membrane associated particles of nondesmosomal surfaces are found predominantly on the A-face in both fixed and unfixed epidermis. Suitably oriented replicas of unfixed desmosomes reveal profiles of apparent fine filaments extending from the region of tonofilament loops through the desmosomal plaque to traverse the cytoplasmic leaflet of the plasmalemma. They can be traced onto the B-face. Their position correlates to fine linear profiles noted in tannic acid/glutaraldehyde-fixed and sectioned desmosomes. The possibility that these represent a mechanism for anchorage of tonofilaments to the plaque and to the membrane is discussed. These and other fine structural features are compared and contrasted to the properties of hemidesmosomes described in the preceding report.