The conspicuous similarities of the rat to the human placenta make rat models very attractive in the study of placental diseases. Understanding species differences in depth is required to project from animal models to humans.

Placentas of pregnant rats were examined on gestational days 13 (onset of endovascular trophoblastic invasion), 16 and 21 (term), with special emphasis on the endovascular trophoblasts (EVT). Sections were immunostained for cytokeratin (CK), a-smooth muscle actin (aSMA), a heavy chain of smooth muscle myosin (SMM), non-muscle myosin (NMM), Rho proteins and endothelin receptors A and B (ETA and ETB). Transmission electron microscopic (TEM) study was also performed.

Expression of aSMA in varying degrees was noted in most EVT in the central artery on day 21, co-localizing with CK. Small bundles of thin fibers consistent with actin fibers, were observed in these cells by TEM. Localization of aSMA at the periphery of the cells, in accordance with the cellular localization of the thin fibers by TEM was evident by confocal microscopy of double-immunofluorescent stained frozen sections. Expression of SMM, NMM and Rho proteins as well as ETA and ETB was also seen in EVT. Expression of smooth muscle contractile proteins was observed as early as day 13 of gestation.

The expression of smooth muscle contractile proteins and endothelin receptors in EVT, suggests that some control of central artery contraction is retained in the rat placenta despite gestational arterial modification, and may play a role in situations of dysregulation of the vasoactive systems.

Authors:

Ilana Ariel, Galina Skarzinski, Tally Kossovski, David Knigin, Michael Bursztyn

The Hadassah-Hebrew University Medical Center, Mount Scopus, Jerusalem