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 these cells de-differentiate during pregnancy-induced arterial dilatation, forming a population of mesenchyme-like myoblasts ready to reconstitute the media after birth. We have studied the re-differentiation of these cells after delivery by means of transmission electron microscopy and immunohistochemistry using antibodies against a panel of cytoskeletal proteins. The data reveal that post partum re-differentiation of the media myoblasts starts immediately after birth where some of the invasive trophoblast cells are still present. The process of re-differentiation is completed at day 8 after parturition. Post partum re-differentiation can be subdivided into two steps: until day 5 after parturition, the central parts of the media are reconstituted out of the reservoir of vimentin-positive myoblasts by stepwise acquisition of desmin, alpha-smooth muscle actin, gamma-smooth muscle actin and smooth muscle myosin. Only thereafter the same re-differentiation takes place in the peripheral parts of the media. On the ultrastructural level immunohistochemical re-expression of cytoskeletal proteins is accompanied by reconstitution of the intracellular contractile apparatus. The data support our earlier notion that the majority of media smooth muscle cells in the guinea pig uterus does not degenerate during trophoblast-invasion but rather de-differentiate temporarily.