The organization of the contractile apparatus of vertebrate smooth muscle.

Abstract

AbstractSmooth muscle of the small intestine of the rat was fixed by vascular perfusion employing aldehydes in a balanced salt solution, followed by immersion fixation in aldehydes and post‐osmication. In such tissue preparations thick filaments approximating 140 Å in diameter are observed in virtually all the smooth muscle cells. The thick filaments are rather uniformly distributed among the more numerous thin filaments. The nearest neighbor distances between the thick filaments range from 400 to 700 Å. The thick to thin filaments ratio is found to apporximate 1:12. Only thin filaments are observed in the most distal segment of terminal processes of muscle cells and the tips of these processes appear to be lined by attachment plaques. A clear segregation of the thick filaments from both dense bodies and attachment plaques is seen. Distally along the tapering extremities of muscle cells progressively more of the plasma membrane is found lined by attachment plaques. These observations are interpreted as strong evidence that the contractile apparatus of the vertebrate smooth muscle cell consists of interdigitating arrays of thick and thin filaments collated into contractile units by the anchoring of the thin filaments in dense bodies and attachment plaques.