Structure of the musculature of the chicken small intestine.

The small intestine of the chicken was studied by light and electron microscopy. The musculature, measuring about 180 microns in thickness in the distended intestine, consists of four layers (outer longitudinal, outer circular, inner circular and inner longitudinal) which are directly apposed to one another. There is no layer of connective tissue equivalent to the submucosa of mammalian intestine, and the intestinal glands lie close to the inner longitudinal muscle. Mucosal folds are not formed during isotonic contraction of the intestine. The muscle cells of the chicken small intestine are characterized by large, numerous and sharply outlined dense bodies, by the presence of an extremely thin basal lamina, by prominent dense bands at the cell surface but relatively few intermediate junctions. There are many areas of direct apposition between cell membranes of adjacent cells and little collagen between the muscle cells. The four muscle layers have each distinctive structural features. Gap junctions between muscle cells occur only in the outer circular layer. The outer circular and outer longitudinal layers are closely apposed and numerous junctions of the adherens type link cells of the two layers. Intramuscular blood capillaries are rare and are found virtually only in the outer circular layer; their endothelial cells are joined by tight junctions. In the outer circular layer (but not in the other layers) there are two further cell types, fibroblasts and interstitial cells, which can be clearly distinguished from one another. The latter cells are intimately related to nerve bundles and are connected by gap junctions to some muscle cells.