MORPHOLOGIC SITES FOR REGULATING BLOOD-FLOW IN THE EXOCRINE PANCREAS

The exocrine pancreas has a lobular structure and an intricate capilla ry network supplies the lobules. Casts of these capillaries are either straight and of constant width, provided with many shallow crests, or undulating and of varying diameter, provided with bulges and deeper c onstrictions. The mean capillary cast diameter is 6.32 mu m (SD 0.53) and 3.91 mu m (SD 0.84) at constriction sites. The first type correspo nds to non-fenestrated capillaries, makes 24% of capillaries and is mo re frequently provided with pericytes (2.7 +/- 0.9 pericytes per capil lary profile). The second type corresponds to fenestrated capillaries, comprises 76% of the capillaries and is less frequently provided with pericytes (1.5 +/- 0.6 pericytes per capillary profile). The endothel ial cells of capillaries regularly form intermediate junctions and mic rovilli and contain microtubuli and cytoplasmic filaments. Intravital observations show that capillaries are capable of contracting and narr owing the capillary lumen. This contractility is accomplished by endot helial cells both at and apart from their nuclear regions while pericy tes never contracted spontaneously during our in vivo observations. Th e capillary diameters estimated by intravital measurements, 3.53 mu m (SD 1.05), are similar to cast measurements but differ at constricted segments from cast measurements. Flow reduction shows more variability in smaller capillaries and the flow is more reduced in capillaries of 5 pm diameter to about 40% of open capillaries vs. 68% in capillaries with 7.5 mu m diameter Veins are either provided with smooth muscle s phincters or with valves. These results indicate that corrosion castin g accurately shows the geometry of capillaries. However, where the cap illaries are drastically constricted, they might not be filled and the refore may be underestimated during measurements. Since none of the in travital luminal constrictions are small enough to reduce flow (smalle r than I mu m luminal diameter and) because many constrictions are eff ective to reduce flow we conclude that capillaries of the exocrine pan creas are always capable of maintaining continuous blood low yet can i nfluence blood perfusion. The presence of venous valves in association with venous sphincters constitutes a new situation concerning blood d rainage regulation in the exocrine pancreas. (C) 1997 Wiley-Liss, Inc.