Luminal constrictions on corrosion casts of capillaries and postcapillary venules in rat exocrine pancreas correspond to pericyte processes.

The rat exocrine pancreas was studied as a model to demonstrate morphological features of different types of capillaries, using scanning electron microscopy of vascular corrosion casts and transmission electron microscopy of tissue sections. Two types of capillaries were discerned. The first type represents less undulated, slender, straight capillaries with numerous, shallow, circular or semilunar furrows on its cast's surface. In tissue sections, this type probably corresponds to non-fenestrated capillaries. The numerous grooves on its cast correspond to pericyte processes beneath the endothelial lining. The second type comprises capillaries of an undulated course and variable diameter with less numerous furrows. In addition, these casts showed circumscribed, smooth surfaced bulging areas defined by the grooves described. In tissue sections, this type probably corresponds to fenestrated capillaries, the bulging areas on its cast correspond to fenestrated regions of the endothelium. Fenestrated areas of capillary endothelium are less reinforced; pericyte processes are not present beneath these regions in tissue sections. The hypothesis that pericyte processes are responsible for surface indentations on capillary casts was supported by observations on postcapillary venules. Casts of these vascular segments showed also numerous circularly running furrows. Accordingly, the wall of postcapillary venules is provided with pericytes while smooth muscle cells are missing.