Pregnancy-induced de-differentiation of media smooth muscle cells in uteroplacental arteries of the guinea pig is reversible after delivery

The majority media of smooth muscle cells of uteroplacental arteries of the guinea pig is not destroyed during trophoblast invasion. Rather, most of t hese cells de-differentiate during pregnancy-induced arterial dilatation, f orming a population of mesenchyme-like myoblasts ready to reconstitute the media after birth. We have studied the re-differentiation of these cells af ter delivery by means of transmission electron microscopy and immunohistoch emistry using antibodies against a panel of cytoskeletal proteins. The data reveal that post partum re-differentiation of the media myoblasts starts i mmediately after birth where some of the invasive trophoblast cells are sti ll present. The process of re-differentiation is completed at day 8 after p arturition. Post partum re-differentiation can be subdivided into two steps : until day 5 after parturition, the central parts of the media are reconst ituted out of the reservoir of vimentin-positive myoblasts by stepwise acqu isition of desmin, a-smooth muscle actin, gamma-smooth muscle actin and smo oth muscle myosin. Only thereafter the same re-differentiation takes place in the peripheral parts of the media. On the ultrastructural level immunohi stochemical re-expression of cytoskeletal proteins is accompanied by recons titution of the intracellular contractile apparatus. The data support our e arlier notion that the majority of media smooth muscle cells in the guinea pig uterus does not degenerate during trophoblast-invasion but rather de-di fferentiate temporarily. (C) 2000 Harcourt Publishers Ltd.