The dimensional characteristics of smooth muscle in rat blood vessels. A computer-assisted analysis.

This investigation was undertaken to provide precise information about the dimensional characteristics of vascular smooth muscle cells as related to their paracellular matrix. The representative types of vessels were fixed at the mean blood pressures of adult male Wistar rats. The shapes, positions of the nucleus, linear dimensions, volumes, and orientation within the vessel wall were determined by a computer-assisted reconstruction of the cells from serial sections. Wall-to-lumen and cellular-to-paracellular ratios also were assessed. The smooth muscle cells were elongate, but whereas some are spindle shaped, most are not, and may be shaped like flattened triangles, paddles, boomerangs, or hourglasses, and in addition, any one of these shapes may be forked. The nucleus tended to be in the largest part of the cell, wherever that region occurred. Thus, the majority of the nuclei (61%) were not centrally located, but overlapped the middle and end thirds of the elongate cells. Of the three arteries investigated, the muscular type tail artery had cells with volumes two to three times larger (P less than 0.01) than cells in a musculoelastic (femoral) or elastic (mesenteric) artery, and six times larger (P less than 0.01) than those of the portal vein. Therefore, the smooth muscle cells of the vein were significantly smaller than those in any artery (P less than 0.01). The smooth muscle cells were aligned at a steeper angle in the vessel wall (15 degrees +/- 2 degrees) of the muscular artery than in those with more elastic tissue (9 degrees +/- 2 degrees), with a higher percentage of circumferential cells in the latter. The wall-to-lumen ratios decreased as the relative amount of paracellular matrix, particularly elastic tissue, increased in the three arteries. Therefore, irregularly shaped cells, with the nucleus in the thickest region, and having characteristic cell volumes depending on the type of vessel, form the vascular smooth muscle tissue. These factors are relevant if stereology, or measuring from two dimensions, is used to estimate size characteristics in cardiovascular disease such as hypertension. In addition, the optimum angle at which vascular strips are cut would vary, for example, when used in testing pharmacological agents.