Junctional complexes in fetal rat small intestine during morphogenesis

Abstract

During the 7 days prior to birth (Days 15–22), the small-intestinal epithelium of the fetal rat changes from primitive stratified to simple columnar epithelium which lines villi at 19 days. As seen in thin sections, this remodeling involves rapid formation of new junctional complexes and secondary lumens between epithelial cells deep in the stratified epithelium. We have examined the formation and reorganization of junctional complexes in proximal small intestine of 15- to 19-day fetal rats using freeze-fracture techniques. On Days 15 and 16 the epithelial cells surrounding the primary lumen are joined by conventional apical junctional complexes. Additionally, macular junctional complexes are located on deeper epithelial cells. These display no polarity and consist of tight-junction strands intermixed with gap junction-like arrays and desmosomes. On Days 17 and 18 nonluminal, macular junctional complexes enlarge and secondary lumens develop within their centers. As the secondary lumens expand, microvilli appear and the junctional complex polarizes about the secondary lumen; tight-junction strands become parallel to the luminal surface, desmosomes migrate basolaterally, and gap junction-like arrays disappear. By Day 19, secondary lumens have fused with the primary lumen; concomitant loss of apical cells results in formation of villi lined by simple columnar epithelium with polarized apical tight junctions. The observed pattern of junctional complex formation may play a role in maintaining barrier function and establishing epithelial cell polarity as the epithelium is remodeled.