Striking changes in the structure and organization of rat fetal membranes precede parturition.

Premature rupture of fetal membranes can harm infant and mother. It is unclear whether structural changes predispose these membranes to breaking. We thus assessed rat visceral yolk sac placenta (VYSP) and amnion by light and by transmission electron microscopy on Days 18-21 of gestation. Light microscope sections were stained for connective tissue (extracellular matrix) components: collagen, glycoprotein, and glycosaminoglycans/proteoglycans. Some tissue was incubated with chondroitinase ABC. We observed that fetal membranes became increasingly fragile, rupturing readily on Day 21. On Days 18-20, the two epithelial layers of the capsular VYSP were separated by a well-developed, well-vascularized connective tissue layer that stained intensely for all matrix components studied; on Day 21, the connective tissue layer was thinner, moderately stained, and less vascularized. On Days 18-20, the two cellular layers of the amnion were separated by a narrow, compact connective tissue layer that stained modestly for all matrix components; on Day 21, this area was widened and stained faintly. Transmission electron microscopy showed that collagen fibrils of the amnion were abundant, closely packed, and well organized on Days 18-20, whereas on Day 21 they were few in number, widely spaced, and disorganized. Similar changes were present after incubation with chondroitinase ABC. In addition, amniotic epithelial cells were moribund and delaminating, basal laminae were deteriorating or absent, and few cells were at the outer surface of the amnion. All changes preceded parturition. We conclude that the structural integrity of rat fetal membranes is impaired before birth through the loss of connective tissue components and cells, changes that presumably underlie membrane rupture. Lastly, the similarity of structural changes in rat and human fetal membranes point to the potential usefulness of the rat model.