Quantification of intestinal microvascular growth during maturation: techniques and observations.

This study was undertaken to determine what changes occur in the intestinal microvasculature during the rapid growth associated with juvenile maturation. A technique was developed that permitted the comparison of the same microvessels in exactly the same intestinal region at two time periods of an animal's life. A region of the terminal ileum of 5-week-old rats was exposed and marked, and photographs and video recordings were made of the microvessels. Four weeks later, the marked intestinal region was located, and photography and videography of the microvessels were repeated. Comparison of indexes for body, intestinal, and microvascular growth for the treated rats and age- and colony-matched controls revealed no significant differences. The number and branching pattern of arterioles observed in the marked region remained remarkably constant during the 4 weeks between observation periods, even though body and bowel mass of the treated animals increased approximately 2.5 times. The lengths of the arterioles were increased (18%) by almost the same proportion as the axis of bowel (22%) in which they were oriented. The average distance between capillaries in the radial intestinal muscle layer was also increased by about the same percentage (24%) as that of tissue elongation (22%). The overall data are consistent with the hypothesis that during the growth spurt of juvenile life, the arterioles present at the weanling stage are elongated and new branches do not develop. The net effect of tissue growth with a minimal change in numbers of arterioles is a decreased ratio of number of arterioles to tissue mass as a normal consequence of maturation.