The collagen fibrils in the collapsed and the chronically stretched intestinal wall.

A partial and progressive obstruction of the ileum (stenosis) was produced by surgery in adult rats and guinea pigs. Oral to the stenosis the accumulation of ingesta imposed a condition of chronic stretch on the intestinal wall: the lumen was much distended and the wall increased in thickness, mainly by hypertrophy of the mucosa and muscle coat. The submucosa too increased in volume and its collagen fibrils showed marked ultrastructural changes: in the rat the collagen fibrils (which were of rather uniform diameter in the control submucosa, mean 87 nm) varied greatly in size and formed two distinct populations: large fibrils (similar to those of controls) and small fibrils (4-5 nm in diameter). The latter were probably newly formed fibrils and their number increased in the more advanced stages of hypertrophy. The small fibrils were usually gathered into groups of 15-60 but were found throughout the entire thickness of the collagen bundles. Aboral to the stenosis the intestine was collapsed. In the submucosa there was a widening of the range of fibril sizes, a small increase in the average size and the occurrence of very large and irregular fibrils. Similar changes occurred in the guinea pig; however, the size of the control collagen fibrils (57 nm) was smaller than in the rat, and in the condition of chronic stretch small and large fibrils could not be clearly separated into distinct populations.