SCANNING AND TRANSMISSION ELECTRON-MICROSCOPY AND HIGH-RESOLUTION INTRAVITAL VIDEO-MICROSCOPY OF CAPILLARIES IN THE MOUSE EXOCRINE PANCREAS, WITH SPECIAL EMPHASIS ON ENDOTHELIAL-CELLS

Capillaries in the mouse exocrine pancreas were studied by scanning el ectron microscopy of microvascular corrosion casts, transmission elect ron microscopy of tissue sections, and high resolution intravital vide o-microscopy. Two types of capillaries were discerned by corrosion cas ting. The first type was rather straight, had a constant diameter of 5 -6 mum, and its surface showed multiple, circumferential furrows. The frequency of such constrictions was less in the second type, which was more undulated and had a diameter of 7-9 mum. In the second type, the se constrictions defined bulged areas of the capillary cast. Correspon ding tissue sections also showed two types of capillaries, fenestrated and non-fenestrated capillaries. Microtubules were abundant in all ca pillary endothelial cells, whereas bundles of microfilaments were scar ce. Microtubules were arranged along the long axis of endothelial cell s as well as parallel to endothelial cell border regions. Endothelial cells were joined by intermediate junctions along cell borders running both circumferentially and longitudinally. Flow reversal in capillari es and spontaneous endothelial contractions were documented in vivo. E ndothelial cells bulged into the lumen, either at their nuclear region or distant from it. Spontaneous contraction of pericytes was not obse rved. These results suggest that contraction of capillaries is carried out by endothelial cells, representing an autonomous flow regulatory device. Capillary contraction in exocrine pancreas may be influenced b y blood-borne agents, probably by those released in Langerhans islets. (C) 1993 Wiley-Liss, Inc.