Morphometric analysis of gap junctions in nonpregnant and term pregnant human myometrium.
Some structures in the human myometrium, namely; caveolae, extracellular space and gap junctions were quantified in tissues from nonpregnant women, women who were not in labor, and women in spontaneous labor or oxytocin-induced labor, in order to reveal morphologic differences that would contribute to explaining the events leading to labor. Transmission electron micrographs taken from tissues from twenty-four pregnant and ten nonpregnant women were studied. Differences were observed in cell growth which was accompanied by enlargement of the extracellular space, and in gap junctions. Gap junctions were demonstrated in nonpregnant women, and even in one postmenopausal woman. Their frequency and size increased gradually between groups of pregnant women in the order of non-labor, oxytocin-induced labor and spontaneous labor. The increase in size of the junctions was less than the increase in number. Gap junctions are most frequently found in human myometrial cells in spontaneous labor, which strongly suggests a role for these structures in the termination of pregnancy. The effect of oxytocin on the appearance of gap junctions was negligible; hence, the contribution of oxytocin to establish myometrial contractile synchronicity is thought to be minor. Caveolae are probably not relevant for the initiation of labor as their distribution did not differ between the groups.