Light and electron microscope study on developing intestinal mucosa in rat fetuses with special reference to the obliteration of the intestinal lumen

The development of the small intestine in rat fetuses was studied by light and electron microscopy with special reference to morphological events during the stage of obliteration. Prior to this stage, the epithelium lining the intestinal lumen was simple, tall and columnar and consisted of undifferentiated cells mutually joined by the adluminal junctional complex. At the stage of obliteration, the epithelium appeared stratified and the intestinal lumen was identified as a narrow irregular slit. In addition to the junctional complex surrounding the intestinal lumen, secondary junctional complexes were scattered in the more basal parts of the epithelium. The intra-epithelial cavity was formed within the macula occludens of the secondary junctional complex and became larger by division of the surrounding cells. Since the small intestinal epithelium was essentially simple columnar, mitosis occurred facing the lumen. At the stage of obliteration when the number of epithelial cells was rapidly increasing, mitosis occurred facing not only the main intestinal lumen but also the intra-epithelial cavity. As the intra-epithelial cavities fused with the main intestinal lumen, mesenchyme invaded to form villi and the epithelium returned to a simple columnar form. Consequently, the intestinal lumen was enlarged and villi were formed. At the same time, absorptive cells with a brush border appeared in the villous epithelium and immature goblet cells were occasionally seen.