This tissue is the specialized, highly modified endometrium of pregnancy. The transformation of secretory endometrium to decidua is dependent upon the action of estrogen and progesterone and other stimuli provided by the implanting blastocyst (or maternal platelets) during trophoblast invasion of the endometrium and its blood vessels. The special relationship that exists between the endometrium/decidua and the invading trophoblast seemingly defies the laws of transplantation immunology. The success of this unique autograft is not only a scientific curiosity but may involve processes that harbor insights into more successful transplantation surgery and perhaps the control of neoplasia as well.
[...] The physiological role of prolactin produced in decidua is not known. Because all (or most) of prolactin produced in decidua enters amnionic fluid, it has been speculated that there may be a role for this hormone in solute and water transport across the chorioamnion, and thus in the maintenance of amnionic fluid volume homeostasis. It also has been shown, however, that prolactin receptors are present in a number of bone marrow- derived immune cells, and that prolactin acts on bone cells to modify selected immune functions (Pellegrini and colleagues, 1992). [...]
[...] The presence of necrotic decidua obtained through curettage after spontaneous abortion in the first trimester should not, therefore, necessarily be interpreted as either a cause or an effect of the abortion. PROLACTIN IN THE DECIDUA Convincing evidence was presented by Riddick and co-workers (1979) and Golander and associates (1978) that the decidua is a source of prolactin, which is present in enormous amounts in the amnionic fluid during human pregnancy. Decidual prolactin is not to be confused with placental lactogen which is produced only by the syncytiotrophoblast. [...]
[...] Early in pregnancy, the spongy layer of the decidua consists of large distended glands, often exhibiting marked hyperplasia but separated by minimal stroma. At first, the glands are lined by typical cylindrical uterine epithelium with abundant secretory activity. Presumably, the glandular secretion contributes to the nourishment of the blastocyst during its histotrophic phase, before the establishment of a placental circulation. As pregnancy progresses, the epithelium gradually becomes cuboidal or even flattened, later degenerating and sloughing to a greater extent into the lumens of the glands. [...]
[...] The blood supply to the decidua capsularis is lost as the embryo-fetus grows and expands into the uterine cavity. The blood supply to the decidua parietalis through the spiral arteries persists, as in the endometrium during the luteal phase of the cycle. The spiral arteries in the decidua parietalis retain a smooth muscle wall and endothelium and thereby remain responsive to vasoactive agents that act upon the smooth muscle or endothelial cells of these vessels. The (spiral) arterial system supplying the decidua basalis directly beneath the implanting blastocyst, and ultimately the intervillous space surrounding the syncytiotrophoblast, however, are altered remarkably. [...]
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