MICROMANIPULATION OF SMALL BLOOD VESSELS IN THE MOUSE

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Abstract
Observations were made on the arterioles, capillaries, and venules of the mesentery, intestinal wall, and ear of the living white mouse. The mesenteric vessels were, in addition, subjected to micromanipulation under high magnifications (450-1350 X). Arterioles showed rhythmic, spontaneous alterations in caliber, and responded to mechanical prodding by a rapid constriction extending some distance below and above the point of stimulation. "Spontaneous" activity in venules consisted of a gradual dilatation and return to original diam., dependent primarily on changes in capillary blood flow, and secondarily on substances liberated when surrounding tissue was irritated with a microneedle. Capillaries were of 2 types: muscular capillaries or arteriovenous bridges, and non-muscular or true capillaries. The former differed from the latter by the presence of pericapillary muscle cells and by the presence of a continuous flow of blood. Spontaneous, rapid alterations in capillary diam. such as occur in arterioles, were not seen in either the muscular or non-muscular capillaries. Gradual changes in diam. both in the direction of increase and decrease were observed chiefly in the latter. Mechanical stimulation of pericapillary cells produced no changes in the diam. of the true capillaries. In the case of arteriovenous bridges certain perivascular cells did cause a partial constriction of the lumen. Direct stimulation of capillary endothelial cells resulted in a localized contraction of these cells. This did not involve the entire circumference of the vessel and did not impede circulating blood. A "reflex" arteriolar dilatation occurred when sufficient tissue surrounding the corresponding cap-pillary bed was mechanically irritated. The possible relationship of these expts. to certain clinical paling phenomenon observed in the skin is discussed. Emptying and paling of a capillary can occur without changes in its diam. much of the confusion regarding pericapillary muscle cells. and regarding the ability of capillaries to contract may be due 1) to the existence of both muscular and non-muscular capillaries with varying proportions of each type in different tissues; and 2) to unjustified conclusions about capillary contractility drawn from indirect, stimulation expts.