Beta HCG is secreted by:
Chorionic gonadotropin is a glycoprotein with biological activity similar to that of LH Both LH and HCG act the same LH-hCG receptor. hCG has with a molecular weight of 36,000 to 40,000 Da It has the highest carbohydrate content of any human hormone--30 percent. The 36-hour plasma half-life of intact hCG is much longer than the 2 hours for LH. The hCG molecule is composed of two dissimilar subunits termed a and b subunits. hCG is produced almost exclusively in the placenta, but low levels are synthesized in the fetal kidney. The hCG hormone is structurally related to three other glycoprotein hormones --LH, FSH, and TSH. All four glycoproteins share a common a-subunit.However, each of their b-subunits, although sharing ceain similarities, is characterized by a distinctly different amino acid sequence. A single gene located on chromosome 6 encodes the a-subunit. Seven genes on chromosome 19 encode for the b-hCG-b-LH family of subunits. Six genes code for b-hCG and one for b-LH Before 5 weeks of pegnancy, hCG is expressed both in the syncytiotrophoblast and in cytotrophoblasts. Later, in the first trimester when maternal serum levels peak, hCG is produced almost solely in the syncytiotrophoblast The combined hCG molecule is detectable in plasma of pregnant women 7 to 9 days after the midcycle surge of LH that precedes ovulation. Thus, hCG likely enters maternal blood at the time of blastocyst implantation. Plasma levels riserapidly, doubling every 2 days in the first trimester Peak maternal plasma levels reach approximately 50,000 to 100,000 mIU/mL between the 60th and 80th days after menses. At 10 to 12 weeks' gestation, plasma levels begin to decline, and a nadir is reached by approximately 16 weeks. Plasma levels are maintained at this lower level for the remainder of pregnancy. The principal urinary form is the terminal product of hCG degradation, namely, the b-core fragment. Concentrations of this fragment follow the same general pattern as that in maternal plasma, peaking at approximately 10weeks' gestation. The best-known biological function of hCG is the so-called rescue and maintenance of corpus luteum function--that is, continued progesterone production. Other hCG functions include promotion of relaxin secretion by the corpus luteum
Yolk sac
Liver
Syncytiotrophoblast
Umbilical cord
FMGE 2018