Morphometric investigation of the microvascular system of pancreatic exocrine and endocrine tissue in the rat.

Morphometric data were obtained from the microvasculature in exocrine and endocrine tissue of the rat pancreas. Two groups of anesthetized rats (100 and 300 g body wt, respectively), received intravenous injections of fluorochrome-coupled globulins in order to label the entire vascular system. After circulation of fluorescein-isothiocyanate (FITC)-globulin through the vascular system for 30 min and lissamine rhodamine B 200 (RB 200)-globulin for 5 min, the pancreas was frozen rapidly. In frozen sections of the organ, both dyes showed the same intravascular distribution. The mean number of sectioned vascular elements was 970 +/- 40/mm2 in the exocrine parenchyma and 1430 +/- 60/mm2 in islet tissue. The cross-sectional area fraction of the microvascular system, which is equivalent to the intravascular volume fraction, amounted to 0.027 +/- 0.001 mm2/mm2 in the exocrine system and to 0.061 +/- 0.002 mm2/mm2 in the endocrine pancreas. The corresponding values for microvascular surface area (mm2/mm3) were 18.0 +/- 0.8 (exocrine tissue) and 34.2 +/- 2.0 (endocrine tissue). Mean diameters of vessels up to 10 microns in exocrine tissue amounted to 5.5 +/- 0.2 and 6.1 +/- 0.1 microns in exocrine and endocrine tissues, respectively; when all vessels up to 30 microns in diameter were considered, these values amounted to 5.7 +/- 0.2 and 6.7 +/- 0.2 microns, respectively. Especially in light of the fact that flow rate was observed to be much higher in islets than in acini, the present results support the existence of an insular microcirculatory system which is specialized in terms of its exchange capacity.