Answer
The secretion of hCG by the chorion --stimulated by the trophoblastic cells, prolongs the life of the corpus luteum(CL). The corpus lutem secretes estrogen and progesterone , to keep the uterus quiet and stimulate growth and circulation until the fourth month of pregnancy. Then the implantation is established and the placenta takes over the maintenance of the pregnancy. The corpus luteum is absolutely critical in preventing menstruation which would cause the embryo to be lost. If implantation does not takes place, then the life of the CL is shortened, progesterone levels fall, and menstruation takes place.
Work Step by Step
When ovulation takes place, the oocyte is released. The rest of the Graafian follicle develops into a hormone secreting structure called the corpus luteum (CL). About 6-7 days after fertilization implantation of the embryo into the uterine wall takes place. Although the oocyte has been released and fertizlied the CL remains viable . It remains functional for several reasons. First,he lining of the uterus must remain in place after implantation and until the end of pregnancy. This requires the availability of estrogen and progesterone. Successful implantation is usually accomplished by day 12 of pregnancy. It is absolutely essential after this that menstruation does not take place or the embryo would be sloughed off with the uterine lining . Before implantation, the hormone hCG ( a n LH type hormone) is secreted by the trophoblast cells ;this stimulates the Cl to secrete estrogen and progesterone. Progesterone keeps the uterine myometrium calm so there are no contractions to disturb the implanting embryo. After implantation, hCG stilll promotes secretion of progesterone and estrogen from the chorion. This supports the CL and promotes/maintains implantation.
