HCG is secreted by:
Ans. b (Syncytiotrophoblasts). (Ref. Ganong Physiology 21st ed. -453)HUMAN CHORIONIC GONADOTROPIN (hCG)# It is a glycoprotein (molecular wt 36,000 to 40,000 d) with highest carbohydrate (30%) content of any human hormone. It is structurally related to three other glycoprotein hormones--LH, FSH, and TSH.# The 36-hour plasma half-life of intact hCG is much longer than the 2 hours for LH.# Biosynthesis:- Source: Syncytiotrophoblasts of placenta- A single gene located on chromosome 6 encodes the -subunit of all four glycoprotein hormones--hCG, LH, FSH, and TSH.- The secretion of b-hCG roughly corresponds to placental mass, whereas the rate of secretion of the complete hCG molecule is maximal at 8 to 10 weeks' gestation. Plasma cone of beta-hCG, although a minor portion of intact hCG at 10 weeks, can represent 30 to 50 percent during the last trimester.# Concentrations of hCG in Serum and Urine:- The intact hCG molecule is detectable in plasma of pregnant women about 7 to 9 days after the midcycle surge of LH that precedes ovulation. Thus, it is likely that hCG enters maternal blood at the time of blastocyst implantation. Blood levels increase rapidly, doubling every 2 days, with maximal levels being attained at about 8 to 10 weeks' gestation.- Because hCG circulates as multiple highly related isoforms with variable cross-reactivity between commercial assays, there is considerable variation in calculated serum hCG levels among the more than 100 assays. Peak levels reach about 100,000 mlU/mL between the 60th and 80th days after the last menses. Beginning at about 10 to 12 weeks' gestation, maternal plasma levels of hCG begin to decline, and a nadir is reached by about 20 weeks. Plasma levels are maintained at this lower level for the remainder of pregnancy.- The pattern of appearance of hCG in fetal blood is similar to that in the mother, however, fetal plasma levels are only about 3 percent of those in maternal plasma.# Elevated or Depressed hCG Levels in Maternal Plasma or Urine:- Significantly higher plasma levels of hCG are sometimes found in:- Women pregnant with multiple fetuses or with a single erythroblastotic fetus resulting from maternal D-antigen isoimmunization.- Hydatidiform mole or choriocarcinoma.- Women carrying a fetus with Down's syndrome, and this finding can be used in biochemical screening tests# Relatively lower levels of hCG in plasma are found with:- Ectopic pregnancies and impending spontaneous abortion.# Biological Functions of hCG- The best-known biological function of hCG is the so-called rescue and maintenance of function of the corpus luteum--that is, continued progesterone production.- Used to detect pregnancy because it appears in the urine 8 days after successful fertilization (blood and urine tests available).- Elevated hCG in women with hydatidiform moles or choriocarcinoma.- It stimulates fetal testicular testosterone secretion that is maximum at approximately the same time that maximal levels of hCG are attained. hCG enters fetal plasma from the syncytiotrophoblast, acts as an LH surrogate, and stimulates replication of testicular Leydig cells and testosterone synthesis to promote male sexual differentiation.- There is hCG stimulation of the maternal thyroid gland.- Other functions include:* Promotion of relaxin secretion by the corpus luteum.* LH-hCG receptors are found in myometrium and in uterine vascular tissue, and it has been hypothesized that hCG may act to promote uterine vascular vasodilatation and myometrial smooth muscle relaxation.Note: Peak levels reached on 72nd day or 12th week of pregnancy.
Cytotrophoblasts
Syncytiotrophoblasts
Both
Habfaur cells
Endocrine Control of the Menstrual Cycle