Membrane specializations of the cells of the Harderian gland of the rabbit with particular reference to the mechanism of exocytosis.

Membrane specializations of the cells of the red and white lobe of the Harderian gland, especially membrane differentiations involved in exocytosis, as well as granule interactions were investigated by means of the freeze-fracture and thin-section techniques. Gap junctions are numerous between both types of glandular cells. Tight junctions consist of about five strands, loosely arranged and often fragmented. Large secretion droplets of both cell types containing different kinds of lipids are formed by fusion. The adjacent granule bilayers are devoid of intramembrane particles (IMP) at the fusion site. During exocytosis the large secretion droplets of the red-lobe cells often cause the plasma membrane to bulge out extensively into the lumen. The overlying plasmalemma as well as the granule membrane show IMP-free patches. Secretion granules of the white-lobe cells, smaller in volume, deform the plasma membrane only slightly when lying in close apposition. The distribution of IMP on the plasmalemma of the white-lobe cells remains random at the side of impending fusion. Thin sections sometimes reveal, in this stage of exocytosis, a single unit membrane but never a pentalaminar structure. Both cellular components of the Harderian gland, when pretreated identically, show different membrane reactions with respect to IMP-clearing during exocytosis. This leads to the conclusion that IMP-clearing may not be responsible for membrane fusion but for other complex events in the regulation of membrane-to-membrane interactions.