Scanning electron microscopic studies of smooth muscle cells and their collagen fibrillar sheaths in empty, distended and contracted urinary bladders of the guinea pig.

This study was performed to clarify the structure of muscle cells and the arrangement of surrounding collagen fibrils in the guinea-pig urinary bladder by scanning electron microscopy with two different chemical-digestion methods. Morphological changes in the muscle cells and the collagen fibrils under stretching and contraction were also examined. The smooth muscle cells in the empty bladder were shaped like an unbranched stick 130 microns long. They elongated to 360 microns in the distended bladder, and 48 microns in the contracted bladder. These cells have short lateral processes touching the neighboring muscle cells in an end-to-side fashion. Longitudinal striations on the cell surface due to rows of caveolae and dense bands were recognizable irrespective of the extended or contracted conditions. Transversely arranged wrinkles, which were considered to be produced by surface enfolding during contraction, were marked on the surface of contracted muscle cells. Each muscle cell was accommodated by a thin lace-like sheath made of both separate and small-bundled collagen fibrils. That part of the fibrils in one sheath often extended to participate in another sheath suggested that the collagen sheaths not only modulate cell shape but also provide a structure serving the co-ordinated motion of neighboring muscle cells.