Fine structure of precapillary arterioles of skeletal muscle in the rat.

The submicroscopic structure of the precapillary arterioles with a diameter of 25-6 mum was followed in m. spinotrapezius of the rat. The walls of the precapillary arterioles of the diameter of 25-20 mum consist of the endothelium, one layer of smooth muscle cells, 3-4 bundles of nerve fibres and a one-layer coat of fibrocytes. Arterioles of the diameter of 20-10 mum have an incomplete fibrocytar coat. Arterioles of the diameter of 10-6 mum also have an incomplete, disseminated muscular layer. The side-branching of true capillaries was not detected. Hence the described arteriolar portion cannot be considered as a metarteriole, but as a regulation device of a segmentary type, regulating the blood flow through the corresponding part of the capillary network as a whole.