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Adaptations of the Rat Vagina in Pregnancy to Accommodate Delivery

Daucher, James A. MD1; Clark, Katherine A. BA2; Stolz, Donna B. PhD2; Meyn, Leslie A. MS1; Moalli, Pamela A. MD, PhD1

doi: 10.1097/01.AOG.0000246798.78839.62
Original Research

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.

The vagina undergoes significant ultrastructural changes during pregnancy and at the time of delivery that facilitate passage of the fetus.

From the 1Magee-Womens Research Institute and the Department of Obstetrics & Gynecology at Magee Womens Hospital; and 2Center for Biological Imaging, Pittsburgh School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania.

See related article on page 136.

This study was supported by the National Institute of Child Health and Human Development—RO1 NIH HD045590.

Corresponding author: Pamela A. Moalli, MD, PhD, Magee-Womens Research Institute, 300 Halket Street, Pittsburgh, PA 15213; e-mail:

© 2007 The American College of Obstetricians and Gynecologists