Size and distribution of endothelial plasmalemmal vesicles in consecutive segments of the microvasculature in cat skeletal muscle

Abstract

Vessels belonging to five individual microvascular units (arteriole-capillary-venule sequence) in the tenuissimus muscle of cat were identified in the electron microscope by utilizing a serial sectioning technique. Morphometric analyses were perormed on the plasmalemmal vesicle population in four defined vessel segments: terminal arterioles, arteriolar fourth of capillaries, venular fourth of capillaries, and postcapillary venules. The sizes and numbers of vesicles, classified as luminal, abluminal, and free vesicles, were assessed. The abluminal vesicles were regularly more numerous than the luminal ones in the same segment. There were markedly different patterns of vesiculation along the five microvascular units. This finding was interpreted tentatively as indicating a reactivity of the endothelial transport function to factors of an unknown nature in the local microenvironment.