Ultrastructural studies of the rabbit placenta in the last third of gestation.

Placental attachment and the ultrastructure of the decidua and placental labyrinth have been studied in rabbits during the final third of gestation. The placenta became progressively easier to separate from the uterine wall as gestation proceeded. This ease of separation was associated with degenerative changes in the decidual tissue, but disruption of the placental labyrinth was not observed until the last 24 hr of pregnancy. Two types of decidual cells were observed; smaller uninucleate glycogen-containing cells and larger multinucleate cells with lipid inclusions. The ageing placentae exhibited increasing decidual degeneration associated with deposition of extracellular fibrous materials. Glycogen became less widely distributed over the period of study and changed from the beta- to the alpha-configuration. In contrast to the observed disruption of the decidual tissue, the placental labyrinth maintained its integrity until the final stages of pregnancy. A dramatic increase in subcellular activity was observed in the syncytiotrophoblast after 28 days of gestation.