Alterations in uterine epithelial tight junction structure during the oestrous cycle and implantation in the pig

The structure of the pig uterine epithelium tight junction has been studied using freeze-fracture methods during oestrus and implantation. Compared with non-pregnant non-cyclic pigs, in both pregnancy and during the cycle the number of intersections per unit area of the tight junction ridges increases to a maximum 16 days after ovulation. There was no significant alteration in the depth or number of ridges in the tight junction band. The only significant difference between tight junctions in cyclic and pregnant pigs was a brief increase in the depth during the cycle (12-15 days after ovulation). The number of intersections is thought to be a far more important measure than depth in determining the permeability of the tight junction. It therefore seems unlikely that changes in tight junction structure play any direct role in the rescue of the corpus luteum or in establishing pregnancy but merely reflect the alterations in progesterone concentrations. However, the decrease in uterine permeability indicated by the increase in intersection frequency has been reported for other species and may reflect an important change in the uterine milieu in preparation for a possible pregnancy, apparently induced by alterations in progesterone concentration.