What does the "Trigger Shot" do?
Trigger shot replaces the LH (Luteinizing Hormone) surge; it is the most critical element of IVF.
A trigger shot, or LH (Luteinizing Hormone) surge in the natural setting, “loosens” eggs within the follicles, making them retrievable by the fertility doctors. Trigger shots also initiate the process to reduce the number of chromosomes from 46 to 23 (meiosis), so that one cell embryo (a fertilized egg) has normal (46) chromosomes.
Ms. Thompson, who is in a new relationship after having tubal ligation, came to discuss her IVF treatment. She was curious about the role of the trigger shot.
A trigger shot accomplishes tasks carried out by the LH surge in the natural settings. Overwhelming majority of IVF cycles use human chorionic gonadotropin (hCG) as trigger shot, because it activates LH receptors and, compared with LH, it is abundant - hCG is extracted from the urine of pregnant women.
The first role of trigger shot is to “loosen” the eggs from its surrounding. Eggs do not float in the follicular fluid. Instead, they are surrounded by granulosa cells (corona radiata) and anchored to one corner of the inner wall of the follicles. These cells nourish and protect the developing eggs. Some of them leave the follicle with the escaping egg at ovulation, but most remain in the follicles. Thus, loosening the binding between the granulosa cells facilitates the escape of eggs by ovulation (or egg retrieval).
One of the genes activated by a trigger shot (or LH surge) produces an enzyme to digest the inter-cellular matrix, a protein that binds cells together. Without the activation of such a gene, spontaneous ovulation does not happen, and egg retrievals are rarely successful. The former is obvious; the latter because the negative pressure (100 - 130 mm Hg) of the retrieval needle is not sufficient to break loose the egg (from the surrounding granulosa cells).
There are reports describing how a second trigger shot salvages eggs from the contra-lateral ovary when a planned egg retrieval yields nothing after the first trigger shot. Thus, some IVF clinics routinely check blood hCG level the day after trigger shot.
The second role of a trigger shot is to resume the meiosis (the reduction division). Meiosis reduces the number of chromosomes from 46 to 23. So that one-cell embryos (fertilized eggs) have the normal 46 chromosome. It is interesting to point out that the trigger shot does not start the meiosis - it resumes the meiosis.
Meiosis is a lengthy process, including meiosis I and meiosis II. Meiosis I begins in cells destined to become eggs (but stops at the diplotene stage) before a girl is born. These cells stay at the diplotene stage of meiosis I until LH surge (or trigger shot).
After a trigger shot (or LH surge), these cells resume meiosis I, proceed to meiosis II, but stops at metaphase II of meiosis II. Meiosis II does not complete until after fertilization when the chromosome number is 23.