Reichert's membrane - A continuing enigma for developmental biologists.

Reichert's membrane (RM) is a basement membrane of gigantic proportions that surrounds the mammalian embryo following implantation. It is part of the parietal yolk sac, which originates from the wall of the preimplantation blastocyst. RM persists from implantation to birth in rodents and analogous structures occur in other mammals, including primates. RM fulfils a vital role in early postimplantation rodent development, by mechanically buffering the elongating, pre-gastrulation embryo against vigorous uterine contractions, which occur at that stage of pregnancy. It persists and enlarges throughout the remainder of gestation, to match the increasing volume of the embryo/fetus and its extraembryonic membranes, although its function at these later stages is unknown. By contrast, in whole embryo culture, RM fails to expand sufficiently to enable normal embryonic development, so that routine preparation for culture includes opening and removal of RM. Modifying the culture conditions can improve embryonic outcomes when RM is intact, but does not normalise development. Possible reasons for the dichotomy between the in vivo and in vitro significance of RM include: the absence of uterine contractions in vitro, which may serve to induce RM expansion in vivo; the absence of a decidual influence, which plays a vital role in maintaining embryos beyond implantation; failure of RM to grow in vitro, owing to insufficient proliferation or survival of the parietal endoderm, which synthesises RM; insufficient cellular recruitment into the parietal endoderm from the visceral endoderm; failure of RM to lose its physical integrity in vitro, thereby limiting its expansion. Distinguishing between these possible mechanisms will require new experimental research on RM.