Arrangement of smooth muscle cells and intramuscular septa in the taenia coli.

Bands of electron-dense material beneath the cell membrane of smooth muscle cells of the guinea-pig taenia coli provide attachment to thin myofilaments and to intermediate (10nm) filaments; about 50% of the cell membrane is occupied by dense bands in muscle cells transversely sectioned at the level of their nucleus, and between 50 and 100% in small cell profiles nearer the cell's ends. In addition to the known cell-to-cell junctions (intermediate contacts), more complex apparatuses anchor muscle cells together, either end-to-end or end-to-side of side-to-side. They consist of elaborate folds, invaginations and protrusions accompanied by large amounts of basal lamina material. In the end-to-end anchoring apparatuses numerous finger-like and laminar processes from the two cells interdigitate. Other muscle cells have a star-shaped profile in the last few microns of their length, or show longitudinal invaginations occupied by a thickened basal lamina and occasionally by collagen fibrils. The septa of connective tissue extend only for a few hundred microns along the length of the taenia. In taeniae fixed in condition of mild stretch the muscle cells form an angle of about 5 degrees with the septa. In muscles fixed during isotonic contraction the angle increases to about 29--22 degrees, and in longitudinal sections the muscle cells appear arranged in a herring-bone pattern. The collagen concentration in the taenia coli is 4--6 times greater than in skeletal and cardiac muscles. These various structures are discussed in terms of their possible role in the mechanism of force transmission.