Endometrium/decidua is the anatomical site of blastocyst apposition, implantation, and placental development. The endometrium is the mucosal lining of the uterine cavity and the decidua is the highly modified and specialized endometrium of pregnancy. From an evolutionary perspective, the human endometrium is highly developed in order to accommodate a hemochorioendothelial type of placentation. Endometrial development of a magnitude similar to that observed in women, that is with special spiral (or coiling) arteries, is restricted to only the catarrhine primates—such as humans, great apes, and Old World monkeys. Trophoblasts of the blastocyst invade these endometrial arteries during implantation and placentation to establish uteroplacental vessels.
[...] of immunological acceptance of the conceptus, maternal recognition of pregnancy, placental development, pregnancy maintenance, and fetal nutrition are quickly established. THE CARDINAL FUNCTION OF THE UTERUS Biologically, the single salutary function of the uterus is the accommodation of a conceptus (pregnancy). There is no known endocrine or other physiological function of the endometrium or myometrium, independent of pregnancy, that affects the metabolic homeostasis or the physical well- being of women. There is no evidence that the uncomplicated removal of the myometrium, endometrium, and cervix (hysterectomy) adversely affects the life span or general good health of women. [...]
[...] Estradiol-17ß, the biologically potent, naturally occurring estrogen, which is secreted by the granulosa cells of the dominant ovarian follicle, acts to promote responses of the endometrium in a manner that is classical for steroid hormone action. By use of estrogens of high specific radioactivity, Jensen and Jacobson (1962) demonstrated that nonmetabolized (radiolabeled) estradiol-17ß is sequestered in estrogen-responsive tissues, notably the uterus. These research findings marked the beginning of the contemporary era of the study of the mechanisms of steroid action through specific steroid receptor proteins. [...]
[...] The same is true of the decidua during pregnancy, at least at the juncture of the lower pole of the amnionic sac with the decidua parietalis. There is, however, functional closure of the cervical canal during pregnancy that is effected by a mucus "plug" that has antimicrobial properties. Therefore, the decidua must function in an exceptionally successful manner to limit bacterial colonization at the lower pole of the chorion laeve-decidual interface. Many investigators have suggested that infections that originate by ascending spread of microorganisms from the cervix of vaginal fluid may constitute one cause of the onset of preterm labor (Chap p. 281). [...]
[...] The single physiological and metabolic function of the endometrium/decidua is to serve as the maternal tissue interface of pregnancy. The endometrium is the optimal site for blastocyst implantation and embryo-fetal/placental development; but it cannot be claimed that this function of the endometrium/decidua is unique because of the known success, albeit limited, of ectopic pregnancies. "Decidualization" of tissues in which ectopic pregnancies implant is prominent. There also may be a role for the endometrium in sperm capacitation; but again, it cannot be argued that this function is unique to this tissue, as evidenced by the success of sperm capacitation and fertilization of human ova in vitro. [...]
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