The placenta is an endocrine organ that produces progesterone and estrogens, hormones essential for the continuance of pregnancy. The placenta also produces protein hormones unique to pregnancy, such as human placental
Remnants of yolk sac
Remnant of extraembryonic coelom
Two stages in the development of the placenta, showing the origin of the membranes around the fetus.
lactogen (hPL) and human chorionic gonadotropin
(hCG). Several peptides and polypeptides, including corti-cotropin-releasing hormone (CRH), GnRH, and insulinlike growth factors, are also synthesized by the placenta and function as paracrine factors.
During the menstrual cycle, the corpus luteum forms shortly after ovulation and produces significant amounts of progesterone and estrogen to prepare the uterus for receiving a fertilized ovum. If the egg is not fertilized, the corpus luteum regresses at the end of the luteal phase, as indicated by declining levels of progesterone and estrogen in the circulation. After losing ovarian steroidal support, the superficial endometrial layer of the uterus is expelled, resulting in menstruation. If the egg is fertilized, the developing embryo signals its presence by producing hCG, which extends the life of the corpus luteum. This signaling process is called the maternal recognition of pregnancy. Syncytiotro-phoblast cells produce hCG 6 to 8 days after ovulation (fertilization), and hCG enters the maternal and fetal circulations. Very similar to LH, hCG has a molecular weight of approximately 38 kDa, binds LH receptors on the corpus luteum, stimulates luteal progesterone production, and prevents menses at the end of the anticipated cycle. It can be detected in the pregnant woman's urine using commercial colorimetric kits.
Human chorionic gonadotropin is a glycoprotein made of two dissimilar subunits, a and p. It belongs to the same hormone family as luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). The a subunit is made of the same 92 amino acids as the other glycoprotein hormones. The P subunit is made of 145 amino acids, with six N- and O-linked oligosaccharide units. It resembles the LH P subunit but has a 24-amino acid extension at the C-terminal end. Because of extensive glycosylation, the half-life of hCG in the circulation is longer than that of LH. Like LH, the major function of hCG in early pregnancy is the stimulation of luteal steroidogenesis. Both bind to the same or similar membrane receptors and increase the formation of pregnenolone from cholesterol by a cAMP-dependent mechanism.
The hCG level in plasma doubles about every 2 to 3 days in early pregnancy and reaches peak levels at about 10 to 15 weeks of gestation. It is reduced by about 75% by 25 weeks and remains at that level until term (Fig. 39.6). Fetal concentrations of hCG follow a similar pattern. The hCG levels are higher in pregnancies with multiple fetuses. During the first trimester, GnRH locally produced by cytotro-phoblasts appears to regulate hCG production by a paracrine mechanism. The suppression of hCG release during the second half of pregnancy is attributed to negative
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