Vascular smooth muscle structure and juvenile growth in rat intestinal venules.

The morphological structure of individual vascular smooth muscle cells from intestinal venules was evaluated with a combination of quantitative scanning (SEM) and transmission (TEM) electron microscopy techniques. In addition, growth of individual venular smooth muscle cells and of the overall vessel wall was compared from measurements of these variables during the rapid juvenile growth spurt from ages 4 to 6 and 10 to 12 weeks in Wistar-Kyoto rats. SEM revealed that smooth muscle cells of intestinal venules in weanling rats are very long (379 +/- 91 [SD] microns) and wide (6.0 +/- 1.3 microns) and very little further cell enlargement occurs during rapid juvenile growth. TEM studies indicated that passive inner vessel diameter and total muscle layer cross-sectional area of both the largest and intermediate diameter venules of young rats, as well as the percentage of the total wall area as muscle tissue in each venule type, did not significantly increase during body growth. These observations indicate that both the intestinal venules and their smooth muscle cells reach mature dimensions at a very early stage of life. Comparison of intestinal vascular smooth muscle cell dimensions indicates that venular smooth muscle cells are much larger in both cell length and volume than comparable arteriolar smooth muscle cells.