Alterations in intercellular junctions of the uterine epithelium during the preimplantation phase in the rabbit

In the rabbit, the pseudopregnant uterus has been used as a model for studying alterations characteristic of the preimplantation phase. Alterations in intercellular junctions of the uterine epithelium were investigated during early pseudopregnancy (day 0 to day 6) by means of the freeze-fracture technique. In the uterine epithelium of oestrous females the zonula occludens belongs to the "tight" type of tight junctions. During pseudopregnancy an impressive proliferation of tight junctional belts can be observed. The basal strands proliferate, forming loops perpendicular to the luminal surface, whereas the more or less parallel arrangement of the luminal strands is maintained. At day 4 of pseudopregnancy macular tight junctions begin to develop on the lower portions of the lateral plasmalemma and are extensive by day 6 post hCG. Small gap junctions are infrequent between cells of the uterine epithelium and show no significant changes during the preimplantation phase. The physiological significance of the present morphological observations is discussed in the light of changes occurring during the preimplantation period.