[2] In some areas of the sheep placenta, called placentomes, there is aggressive interdigitation between trophoblast villi on the fetal side (cotyledon) and the

uterus on the maternal side (caruncle), and at points the epithelia form a common syncytium allowing for more efficiency of gas and nutrient exchange. Pigs have a similar but more diffuse placental structure than sheep with less aggressive interdigitation.[2, 17] The human/primate uterus is a single muscular organ different structurally from the two-horned uterus of rodents (for mice see Margaret J Cook’s LY294002 datasheet book at www.jax.org), pigs,[18] rabbits,[16] or sheep.[19] While the electro-mechanics of the human/primate uterus may be fundamentally different from that seen in other species,[20, 21] the uteri of rodents,[22] rabbits[23] sheep,[24] and pigs[18] respond to oxytocin, suggesting a common expression

of the receptor, and most have been used to study the mechanisms underlying uterine contractility in vitro. In addition to hormones such as estrogen (discussed elsewhere), progesterone is a key hormone of pregnancy that appears to be differentially regulated in humans and animals.[25] The particulars of the responsiveness to this hormone and its interaction with estrogen in successful pregnancy remain learn more a topic of intense investigation. In humans, the corpus luteum is the major site of progesterone expression with help from chorionic

gonadotropin released by the early conceptus.[26] Blockade of progesterone during this time causes pregnancy loss.[26] Major production of progesterone switches to the placenta by 5–6 weeks’ gestation. Maternal serum levels of progesterone raise post-conceptionally and continue to elevate beyond parturition.[25, 27] However, progesterone has been given with variable success to treat women with recurrent miscarriage[28] and antiprogesterone given late in pregnancy can cause cervical ripening and delivery in some women[29] suggesting a complex biology. Human fetal membranes can produce[30] TCL and metabolize progesterone,[31] and locally produced progesterone metabolites may be important in uterine quiescence and activation.[32] The human uterus can produce an inhibitory progesterone receptor which increases before parturition.[33] Finally, progesterone receptor regulation at multiple levels in the cytoplasm and the nucleus may regulate functional progesterone activity leading to parturition.[34] Progesterone’s regulation during pregnancy in related non-human primates is similar to human pregnancy in several respects including dependence on early production of progesterone by the corpus luteum[35] that early pregnancy can be interrupted by antiprogestins[36] and that there is not systemic withdrawal before parturition.