The placenta is a complex endocrine organ that synthesizes and metabolizes a large variety of steroid and peptide hormones as well as numerous other substances in large quantities for release into maternal and fetal circulations. Most of the substances appear to be synthesized by trophoblast elements such as the binucleate cells of sheep. The precise roles of placental and fetal hormones, the mechanisms by which they are involved with placental functions, fetal growth and development, and function and metabolism of maternal systems are discussed in more detail by Rurak.
The most heavily studied protein hormones produced by the placenta are the placental lactogens and chorionic gonadotrophins. Placental lactogen is a polypeptide structurally similar to growth hormone and prolactin, and has been identified in several species, including humans, rodents, and ruminants. Placental secretion increases with increased placental size (e.g., twin vs. single pregnancy), and increases as pregnancy advances, but the pattern of secretion differs among species. Human placental lactogen (hPL) is synthesized by syncytiotro-phoblast and released primarily into maternal circulation. In contrast, ovine placental lactogen (oPL) is produced by binucleate cells. Plasma levels are higher in the fetus than in the ewe until about 50 d postmating, but increases dramatically in maternal plasma as pregnancy advances. Placental lactogen cross-reacts with both growth hormone and prolactin receptors in ruminant tissues, and may act as a glucose-sparing agent that decreases peripheral sensitivity to insulin. Maternal utilization of protein is also reduced, thereby increasing amino acid availability to the conceptus.
Chorionic gonadotrophins have been heavily studied in the horse and human. Human chorionic gonadotrophin (hCG) is a double-chain glycoprotein, with the a chain identical to that in leuteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH) and a distinct p chain. It first appears at the time of implantation and is synthesized by the blastocyst. Equine chorionic gonadotrophin (eCG or PMSG) is similar in structure to hCG and is synthesized by unique populations of binucleate trophoblast cells. Both hCG and eCG function to maintain corpora lutea function in early pregnancy until placental progesterone production is sufficient to maintain pregnancy.
Chorionic gonadotrophin is not produced by ruminants. In those species, interferon tau appears to have similar functions in pregnancy recognition and corpra lutea maintenance. Interferon tau is a 172 amino acid type I interferon that is not inducible by viruses, but is synthesized constitutively by the blastocyst trophectoderm just prior to implantation. A similar mechanism may operate in the pig.
Prolactin is produced by placental tissue of several species. Suggested roles include serving as a fetal growth hormone, stabilization of uterine prostaglandin production, and perhaps in the regulation of amnionic fluid volume. Other peptide hormones have been identified or claimed to be products of placental synthesis. Most are analogues of corresponding pituitary or hypothalamic hormones and include ACTH, FSH, TSH, and a-MSH.
Placental relaxin has been identified as a product of the placenta of humans, horses, cats, pigs, rabbits, and monkeys. It is present in low quantities in endometrial and placental tissues of the sheep, but is absent in the bovine placenta.
Steroid hormones, including estrone, estradiol, estriol, and progesterone, are produced by placental tissues in many species, although sites of production and patterns of secretion vary greatly among species. Progesterone is synthesized from maternal cholesterol, which is taken up by the trophoblast as low-density lipoprotein (LDL). Cytochrome P450 side chain cleavage enzyme is the key enzyme in a series of reactions converting the 27-carbon cholesterol to the 21-carbon progesterone. Part of the progesterone passes to the maternal circulation, where it supplements progesterone produced by the corpus luteum. Progesterone and pregnenolone pass to the fetus, which uses these steroids to produce dehydroepiandrosterone and androstanedione, which are subsequently converted to estradiol and estriol by the placenta. In the human, ewe, cow and mare, maintenance of pregnancy is dependent on corpus luteum progesterone secretion until about 30 60, 50 60, 210 230, and 150 299 days of gestation, respectively. The goat and sow are dependent on corpus luteum production throughout pregnancy.
The key enzymes required for the conversion of the 21-carbon progestogenic steroids to the 18-carbon estrogens are 17a-hydroxylase/17,20-lyase and aromatase. There are substantial species differences in the extent to which these enzymes are present in placental tissues. In the sheep and cow, the placenta has low levels of 17a-hydroxylase/17,20-lyase during early pregnancy, thus limiting placental androstenedione production, but there is increased expression of the enzyme during late gestation. Placental androstenedione production may be supplemented by the fetal adrenal gland. Androstenedione may then be aromatized in the placenta to estrogens. In contrast, the rat placenta lacks aromatase. Thus, the placenta produces androgens during late gestation and releases them to the maternal circulation.
Placental tissues are sources of a large number of cytokines, chemokines, prostaglandins, and related factors. Regulation of arachidonic acid metabolism and prostanoid production in placental tissues is essential for the maintenance of pregnancy and initiation and progression of parturition. Cytokines (e.g., TNF-a, IL-p) and chemokines (e.g., MIP-1a) act in a coordinated fashion at multiple points of the prostanoid biosynthetic pathway to regulate prostaglandin production.
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