Adaptations of the rat vagina in pregnancy to accommodate delivery.

OBJECTIVE: To characterize ultrastructural changes in the rat vagina in pregnancy, delivery, and postpartum, focusing on collagen architecture and smooth muscle cell morphology.
METHODS: The vagina of four virgin, four midpregnant, four late pregnant, four immediate, and four late post-vaginal-delivery rats were examined by transmission electron microscopy. Images were classified into one of four categories based on collagen fibril area fraction, with group 1 containing the highest number of collagen fibers per unit area and group 4 containing the lowest. Smooth muscle cells were characterized into three cell types ("synthetic," "intermediate," and "contractile") based on the volume fraction of cytoplasm occupied by organelles compared with myofibrils.
RESULTS: Quantitative analysis demonstrated that 76% of collagen fibers in virgin rats were categorized as group 1 or 2 compared with 49% in midpregnant, 40% in late pregnant, and 23% in immediate postpartum animals (P=0.006). Late postpartum tissue seemed similar to virgin tissue (77%). Midpregnant (37%), late-pregnant (34%) and immediate postpartum animals (43%) contained a higher proportion of synthetic smooth muscle cells compared with virgins (20%) and late postpartum animals (21%) (P=.02). Contractile smooth muscle cells predominated in virgin (64%) and late postpartum animals (70%) compared with midpregnant (42%), late pregnant (50%) and immediate postpartum (50%, P=.05).
CONCLUSION: In pregnancy, collagen fiber area decreased while smooth muscle cells transformed from a contractile to a synthetic phenotype. The late postpartum period returned to prepregnant levels for both collagen and smooth muscle cell morphologies. It is likely that these changes represent adaptations to minimize trauma to the vagina during passage of the fetus.