Ultrastructural basis for maintenance and termination of pregnancy.

Myometrial tissues from guinea pigs, sheep, and human subjects at various stages of gestation and postpartum were quantitatively examined in the electron microscope for the presence of gap junctions between muscle cells. Gap junctions were found in tissues from guinea pigs and sheep which were being delivered or ready to be delivered and in tissues taken immediately post partum. Gap junctions were also present in 19 tissues obtained from 69 women undergoing elective or emergency cesarean section for a variety of reasons. The frequency of their occurrence varies in relation to the presence or absence of labor and other conditions. We propose that gap junctions are required for effective muscle contractions leading to termination of pregnancy in all animals, including human beings. The absence of gap junctions throughout gestation may be necessary for maintenance of pregnancy and the premature appearance of the cell contacts may lead to premature labor.