The postnatal development of the alimentary canal in the opossum. III. Small intestine and colon.

The duodenum of the newborn opossum exhibits a patent lumen containing scattered elongate villi, whereas the distal segments of the small intestine are smaller in diameter and are filled with short immature villi. The muscularis externa through the small intestine consists of a single layer of myoblasts. Interposed between the intestinal lining epithelium and the muscularis externa is an extensive capillary bed that occupies a considerable proportion of the intestinal wall. Additional villi appear to form during the postnatal period as a result of evaginations of the epithelium, together with underlying connective tissue and vasculature, into the intestinal lumen. Intestinal glands are not observed until 8.5cm, and are shallow in depth even in the adult. The epithelium of the entire small intestine is modified for absorption until just prior to weaning. The principal intestinal lining cells show an extensive apical endocytic complex, large supranuclear vacuoles and numerous cytoplasmic inclusions. Intestinal epithelial cells of the colon also appear to be modified for absorption during the first two weeks after birth. Although goblet cells and Paneth cells are present during the suckling period, they do not comprise a significant population in the intestinal epithelium until after weaning. In contrast to the small intestine, goblet cells are numerous in the colon by the ninth postnatal day. The significance of macromolecular absorption and the possibility of passive immunity being transmitted in the opossum during suckling are discussed and related to similar events that occur in the slckling young of several eutherian species. The possible functional significance of two large membranes that develop in the lamina propria of the intestines after weaning also is discussed.