Formation and release of vesicles from the basal surfaces of rat eye non-pigmented ciliary epithelial cells: a novel secretory mechanism?

When rat ciliary body is processed by high pressure freezing and freeze substitution, numerous membrane-bound vesicle profiles are seen in the vitreous associated with the pars plana and in the valleys between the ciliary processes. They consist of a homogeneously distributed fine granular matrix and varying numbers of ribosome-like structures. The mechanism by which these vesicles are secreted appears to follow an apocrine-type pattern, albeit at the basal cell surface. Matrix material accumulates between the basal plasma membrane of non-pigmented ciliary epithelial cells and a cortical layer of cytoskeletal components; the blebs thus formed protrude through a discontinuity in the basal lamina and, by a progressive narrowing of the neck region, are eventually pinched off, giving rise to free vesicles. Under conventional aqueous chemical fixation conditions, most of these vesicles are washed away or their contents solubilized and extracted, which accounts for their not having been identified hitherto as genuine morphological structures. They are nonetheless apparent, albeit in reduced numbers and mostly empty. Such vesicles are also observed in tissue processed according to several other chemical fixation techniques, namely, conventional fixation in the presence of the cationic dye ruthenium hexamine trichloride, simultaneous glutaraldehyde/osmium tetroxide fixation, and microwave fixation. In the latter instance, comparable vesicle preservation to that obtained by high pressure freezing/freeze substitution may be achieved if fixation is followed by cryoprotection, plunge freezing, and freeze substitution instead of conventional post-fixation and dehydration procedures.