Structural Modulations of Plasmalemmal Vesicles

Abstract

Structural modulations affecting a small fraction of the population of plasmalemmal vesicles of vascular endothelia are described. They include forms which are apparently produced by the fusion of the vesicular membrane with the plasmalemma and by the successive elimination of the layers of the two fused membranes. Such modulations are assumed to represent stages in the discharge process of vesicular contents. Other forms, characterized by their flask shape and elongated neck, are assumed to represent stages in the formation and loading of membrane invaginations, followed by their being pinched off to form isolated vesicles. Stages in a membrane-fusion process leading to the formation of apertured fenestrae and channels are also described in fenestrated endothelia. The visualization of these structural details is greatly facilitated by staining tissue specimens with uranyl acetate before dehydration.According to a previously advanced hypothesis (1-4), transport of fluid in quanta across the endothelium of blood capillaries implies the following sequence of events: (a) appearance of a pocket by invagination of the plasmalemma on either cell front; (b) formation of a closed vesicle by a pinching off (membrane fission) of the pocket; (c) movement of the vesicle across the endothelial cytoplasm; (d) fusion of the vesicle membrane with the plasmalemma on the opposite cell front; and finally (e) discharge of the vesicular content into the extracellular medium.Most electron micrographs so far published seem to represent either (1) invaginations of the cell membrane or (2) closed vesicles assumed to be in transit across the endothelial cell. Recently, intermediate stages have tentatively been identified in the form of vesicles provided with a narrow neck (3) or an aperture (diaphragm) (5, 6). The first are considered intermediary forms between (a) and (b), whereas the second are assumed to represent vesicles approaching the discharge stage (3, 5, 6).A priori, clear visualization of the stratified structure of plasmalemmal and vesicular membranes should allow the recognition of more graded intermediary forms, especially during those phases of the process which involve membrane fusion prior to vesicle opening and discharge. For instance, appearances representing contact and fusion of the two membranes, as well as subsequent progressive elimination of their layers, should be encountered with a frequency proportional to the time taken by these events in a complete traverse of the endothelium.The present paper reports the occurrence of modulations in vesicle structure which probably represent such intermediate stages. MATERIALS AND METHODSOur observations were carried out on (a) blood capillaries of the tongue, myocardium, diaphragm, intestine, and pancreas; (b) lymphatic capillaries of the tongue and intestine; and (c) endocardium of adult rats and guinea pigs.

PV1 Is a Key Structural Component for the Formation of the Stomatal and Fenestral Diaphragms

“…Judged by its magnitude and importance for the underlying tissues, a critical function of the endothelium is to mediate the fluid and solute exchanges between the blood plasma and the interstitial fluid, in short microvascular permeability. Although the molecular mechanisms underlying this function are still largely unknown, several endothelial specialized subcellular structures have been involved in these processes: caveolae, transendothelial channels (TEC), fenestrae, sinusoidal gaps, vesiculo-vacuolar organelles (VVOs), and intercellular junctions (for reviews see Palade, 1991;Michel and Curry, 1999;Dvorak and Feng, 2001;Bendayan, 2002;Stan, 2002).…”

“…Based on electron microscopy (EM) studies, the SDs of caveolae, TEC and VVOs and the FDs have the same morphology: thin (5-7 nm) protein barrier provided with a central density or "knob" (Palade and Bruns, 1968;Palade, 1969a, 1969b). The FD has an octagonal symmetry (Maul, 1971;Bearer and Orci, 1985), being constituted of radial fibrils starting at the rim and interweaving in a central mesh (Bearer and Orci, 1985).…”

Temperature dependence of protein transport across lymphatic endothelium in vitro.

“…Horseradish peroxidase (HRP),`for instance, when introduced into the general circulation, appears rapidly in cytoplasmic vesicles of lymphatic endothelium (45). Parallel evidence, both structural (7,27) and functional (28), supports the view that vesicles subserve a transport function across blood vascular endothelium.…”

Arrest of membrane fusion events in mast cells by quick-freezing.

“…In thin sections of stimulated mast cells, granule membranes often contact the plasma membrane at sites where the intervening cytoplasm has been expressed (7,21), forming what Palade and Bruns (25) termed a "pentalaminar figure ." The occurrence of these structures in many types of secretory cells during exocytosis has suggested that they are a preliminary stage in membrane fusion (2,24,29,30) .…”