Morphology of smooth muscle and its diversity as studied with scanning electron microscopy

Smooth muscles exhibit remarkable morphological diversity among different organs in their geometrical arrangement, shape, size, amount, and packing density of muscle fibers, as extensively reviewed by Burnstock (1) and Gabella (2,3). These parameters have direct relevance to the overall behavior of individual organs and organ-specific physiological and mechanical properties of the muscles (4-8). The morphology of smooth muscles has been investigated on sections in a variety of organs, particularly and most intensively in the gastrointestinal tract by Gabella (9,10). However, the parameters have not yet been fully scrutinized at the ultrastructural level in any given organ, primarily because of the intrinsic technical limitations of transmission electron microscopy (TEM). Reconstruction of ultra-thin serial sections has been the only means for this sort of investigation, and has been occasionally adopted to assess the size and arrangement of smooth muscle fibers (11-15). Nevertheless, it involves considerable work and requires a skilled hand.