The formation of the chick ileal muscle layers as revealed by α-smooth muscle actin immunohistochemistry

The genesis of intestinal smooth muscle layers was immunohistochemically investigated by use of an antibody to α-smooth muscle actin (α-SMA) in the developing chick ileum. Myoblast cells positive for α-SMA were already found in the presumptive circular muscle layer on E 8.5. On E 11.5 radially oriented muscle fibers were protruded from the outermost layer of the developing circular musculature and then formed a tuft-like aggregates. These radial muscle bundles were bent into an L-shape. The long distal extension of muscle bundles run parallel to the long axis of the ileal loop and developed into the longitudinal muscle layer. The obliquely oriented muscle fibers, locating at the intermuscular space of the muscularis propria, probably are to be considered a remnant of the short extension of radial muscle bundles. The muscularis mucosae was formed by the processes equivalent to the genesis of longitudinal muscle layer. On E 14.5 centripetally oriented muscle fibers emerged from the innermost layer of circular musculature. The long distal extension of centripetal fibers lay along the inner surface of developing circular musculature. On E 19.5 the longitudinal muscle layer of the muscularis mucosae was newly formed by separating from the circular musculature. The villous myoblast cells initially developed from the innermost layer of the muscularis mucosae on E 18.5, and were widely distributed in the lamina propria mucosae on E 20.5. Temporal and chronological pattern in expression of α-SMA was observed during the development of the chick intestinal smooth muscle. By E 14.5 the entire layer of the muscularis propria was intensely immunostained for α-SMA, but from E 15.5 onward the staining intensity gradually began to decrease from the outer half of the circular musculature. Finally, the immunoreactivity was localized in the inner layer of circular muscle and the longitudinal muscle layer. A possible functional role of this inner layer of circular muscle is discussed.