The McGill Physiology Virtual Lab

Biochemical/molecular techniques

Background concepts:
Reproduction hormones

During pregnancy, normal trophoblast cells of the placenta secrete a glycoprotein hormone: human chorionic gonadotropin (hCG). Because, under normal circumstances, hCG is produced only by placental tissue and is conveniently found in the blood or urine of a pregnant woman, hCG detection is a sensitive chemical test of early pregnancy.

Early development, implantation and placentation
After reaching the uterus, the fertilized egg or zygote - getting nutrients from the intrauterine fluid - undergoes further cell divisions and reaches the stage known as blastocyst whereby the cells begin to differentiate. The blastocyst consists of an outer layer of cells, the trophoblast, an inner cell mass and a central fluid-filled cavity. The inner cell mass will give rise to the embryo and some of the membrane associated with it. The trophoblast will surround the embryo throughout development and be involved in its nutrition as well as in the secretion of several important hormones.
 Blastocyst  implanted in the endometrium

The zygote-to-blastocyst development period corresponds with days 14 to 21 of the typical menstrual cycle. At this time, the uterine lining is being prepared by progesterone secreted by the corpus luteum to receive the blastocyst. By the 21st day of the cycle or 7 days after ovulation, the blastocyst embeds itself into the endometrium. The trophoblast cells are very sticky especially in the region around the inner cell mass. It is this portion of the blastocyst which adheres to the endometrium and initiates implantation. Rapid trophoblast proliferation occurs and cells penetrate between endometrial cells.
The nutrient-rich endometrial cells provide what is necessary for early growth of the embryo, but this function is replaced after the first few weeks by the placenta: a combination of interdigitating fetal and maternal tissues. The fetal portion of the placenta is supplied by the outermost layers of trophoblast cells: the chorion. Fingerlike projections  of the trophoblast cells (chorionic villi) extend from the chorion into the endometrium. Human chorionic gonadotropin (hCG) starts to be secreted by the trophoblast cells around the time they start their endometrial invasion. The hCG enters the maternal circulation and thus can be detected.
Fetal and maternal tissues

Function of the human chorionic gonadotropin: without pregnancy, the corpus luteum degenerates within two weeks after its formation. The function of HCG is to maintain the corpus luteum. This hormone also strongly stimulates steroid secretion by the corpus luteum in the beginning stages of gestation.

Hormonal changes during pregnancy
The secretion of hCG reaches a peak 60 to 80 days after the last menstrual period, decreasing rapidly afterwards. By the end of the third month it has reached a low level which will remain constant for the duration of the pregnancy. With this decrease of hCG secretion, the placenta begins to secrete large quantities of estrogen and progesterone and the dependence on the corpus luteum for the maintenance of pregnancy disappears.
Hormone levels
Molecular characteristics of HCG

hCG is a glycoprotein composed of two subunits, alpha and beta, held together by ionic and hydrophobic forces. The alpha subunit is a glycopeptide of 92 aminoacids stabilized by five disulfide linkages. The aminoacid sequence of this subunit is identical to that of the pituitary glycoprotein hormones, luteinizing, follicle stimulating and thyroid stimulating hormones.
The beta subunit is a glycopeptide of 145 amino acids stabilized by six disulfide linkages. The beta subunits of the glycoprotein hormones are unique and give them their different biological characteristics.


Lapthorn, A J, Harris, D.C., Littlejohn, A, Lustbader, J W, Canfield, R E, Machin, K J,  Morgan, F J, Isaacs, N W:
Crystal structure of human chorionic gonadotropin.
Nature, 369, 455-461, 1994


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