Histogenesis of the mucosa of the descending colon in mouse fetuses.

The present study clarify intraepithelial events occurring during the formation of intestinal crypts in the descending colon. In early stage, the mucosa is lined with a simple columnar epithelium consisting of undifferentiated cells which are mutually joined by the junctional complex surrounding the intestinal lumen. The epithelium becomes thickened and stratified, and secondary junctional complexes are formed in the abluminal part of the epithelium. An intracellular cavity opens to the focal tight junction of the secondary junctional complex to form an intraepithelial (intercellular) cavity surrounded by the tight junction. The intraepithelial cavity grows by the fusion of intracellular cavities and vesicles as well as by the division of surrounding cells. Intraepithelial cavities fuse with each other and with the main intestinal lumen. This, together with mesenchymal invasion, results in the formation of intestinal crypts lined with a simple columnar epithelium, where various cell types begin to differentiate. These processes seem essentially similar to those occurring in the developing small intestine except that villi, not crypts, are formed in the small intestine. It is suggested that the formation site and elongation of the tight junction provide for the location and fusion of intraepithelial cavities and eventually result in the formation of villi or crypts, alternatively.