Electron microscope observations on the formation of primitive villi in rat small intestine with special reference to intercellular junctions.

Formation and fusion of intraepithelial cavities have long been considered an essential process in the histogenesis of the intestinal mucosa. By electron microscope observation of thin sections and freeze-fracture replicas of the small intestine of rat fetuses, we first demonstrated the initial steps in the formation of intraepithelial cavities: A focal tight junction (macula occludens) was formed in the abluminal part of the epithelium, after which the membrane of an intracellular cavity was fused with that of the focal tight junction to form an intercellular (intraepithelial) cavity enclosed by a zonula occludens. The present study also revealed that gap junctions appeared and enlarged simultaneously with the formation of primitive villi and differentiation of absorptive cells. These gap junctions gradually came to be confined in the epithelium of intervillous regions where proliferation and differentiation of epithelial cells took place. Absorptive cells in villi rarely had gap junctions. These results suggest that tight and gap junctions play important roles in the histogenesis of the intestinal mucosa, and in the proliferation and differentiation of epithelial cells.