A woman is born with all the eggs she will ever produce. She uses most of them up throughout her reproductive life and when the number of remaining eggs falls below a “critical threshold” her reproductive potential declines over a period of six or eight years whereupon it ceases completely. It is interesting that a woman already has 4-5 million eggs stored in her ovaries 3-4 months after conception. Then, during the next 5-6 months before birth, loses them at a furious rate, such that by the time she is born, more than half are already gone. This process of egg attrition continues after birth, but at a slower rate.
By the time the woman reaches puberty and begins to menstruate (the menarche) and ovulate, the egg population has already dropped to less than 2 million. The number of eggs remaining at the time of the menarche when the woman’s reproductive potential is launched is genetically determined.
The primitive eggs (oogonia) are stored in the ovaries surrounded by a thin layer of early granulosa cells. These units are referred to as primordial follicles. Groups of primordial follicles are genetically chosen (recruited) each ovulation cycle to embark upon a developmental journey of approximately 4 months, during which time they undergo highly complex (as yet poorly understood) developmental changes designed to prepare them for use during a single, designated ovulation cycle.
During this 4 month “recruitment journey” many primordial follicles are culled. Those that survive, arrive at the “starting gates” of a preordained cycle having grown to a few millimeters in diameter and are referred to as antral follicles. Each antral follicle contains a central collection of fluid surrounded (walled in) by a few layers of hormonally active granulosa cells in which the egg (now called a primary oocyte) resides.
The number of antral follicles made available for use during each ovulation cycle varies from woman to woman, and is largely genetically preordained. This is referred to as her “recruitment potential”. A woman’s recruitment potential also declines with age, starting at a high number (perhaps, 20-50 per cycle), declining progressively after the age of 30 or so, and then precipitously as she gets older and her egg reserve declines further .
As the woman’s recruitment declines below a certain threshold number, the brain (hypothalamus) recognizing that fewer eggs are being made available, signals the pituitary gland to increase its output of follicle stimulating hormone (FSH) in a futile attempt to increase her recruitment potential. This is followed some years later by a similar rise in basal luteinizing hormone (LH). The steady rise in basal FSH output heralds the onset of the climacteric, a 4-8 year period of time during which there is progressive decline in the woman’s recruitment potential (also referred to as her “ovarian reserve"). The climacteric also heralds the onset of premenopausal symptoms such as menstrual irregularities, hot flashes, dryness of the vagina and often profound emotional changes.
The declining ovarian reserve is also accompanied by a progressive reduction in the woman’s fertility potential as well as a steady reduction in her ability to respond to fertility drugs. Ultimately, when the woman’s recruitment potential is almost depleted, ovarian hormone cyclicity comes to an abrupt halt, and menstrual function ceases (i.e. advent of menopause) along with her reproductive potential.
Antral follicles ultimately will form ovarian Graafian follicles which, under the influence of FSH, grow progressively during the first half of the cycle. These follicles produce estrogen that causes the uterine lining to proliferate (thicken). Upon ovulation, at least one follicle becomes a corpus luteum (CL) which produces both progesterone and estrogen to prepare the endometrium (uterine lining) for. and in anticipation of embryo implantation. If implantation fails to occur, the CL starts to undergo a process of natural attrition starting about 4-6 days prior to menstruation. This is accompanied by a decrease in its progesterone output, which in turn results in an increase in the release of FSH by the pituitary gland. This process initiates the final and critical phase during which antral follicles undergo preparation. It results in their “lining up” at the starting line in readiness for use in the upcoming cycle.
There is a sustained rise in FSH output as the new cycle commences and with it, progressive growth and development of the recruited antral follicles. Within 5-6 days, one (and sometimes two or even three) of these follicles differentiate by growing faster than the rest. These are to be the “dominant follicles”, that have been selected and will continue their growth and will ultimately, ovulate. The remaining follicles will undergo attrition and ultimately disappear. This 5-6 day period is referred to as the “selection phase.”
The ovary has two well defined hormone producing “compartments”. The first is the follicle, which houses the egg and has an inner lining of granulosa cells. These cells produce estrogen in response to FSH. The second is the stroma or theca, connective tissue that surrounds the follicles and produces androgens or male hormones (predominantly testosterone). Testosterone is the building block from which granulosa cells that line the follicle manufacture estrogen. It is carried in a “bucket brigade fashion” from the stroma/theca to the granulose cells of the follicle. LH promotes production of testosterone by the stroma/theca.
Neither follicle growth with estrogen production, nor proper egg development and maturation would be possible without delivery of some testosterone to the follicle granulosa cells. Some LH is thus essential. However, too much LH results in overproduction of testosterone, which is damaging. It reduces follicle growth, as often evidenced by a decline in estrogen production. When too much testosterone gets into the follicular fluid it interferes with egg development, compromises egg maturation and ultimately affects embryo "competence". When such damaged eggs fertilize (if indeed they are able), the embryos that they propagate are highly unlikely to be “competent.”
There are situations such as advancing age (above 40 years), diminishing ovarian reserve, and polycystic ovarian syndrome (PCOS) which are characterized by overgrowth of ovarian stroma/theca (stromal hyperplasia or hyperthecosis). In such cases, an increased ovarian output of testosterone is almost inevitable. These also happen to be the very patients who tend to produce poorly developed eggs and "incompetent" embryos. It follows that when conducting controlled ovarian hyperstimulation (COH) in such cases, the protocols must strike a balance between optimizing follicle growth and development while avoiding excessive ovarian LH-induced testosterone production.
With the availability of GnRH agonist (GnRHa) and GnRH antagonist drugs, it is possible to strategically suppress pituitary release of LH and so regulate follicular exposure to ovarian testosterone. We also have access to pure DNA-based recombinant FSH (FSHr) and LH (LHr) preparations. Accordingly, there is no longer any need to administer fertility drugs that contain LH or hCG, especially not to women that are likely to have ovarian hyperthecosis. It also makes little sense to me in such cases to use protocols such as the GnRHa-flare protocol that result in excessive pituitary gland release of LH early on in the stimulation cycle, when the eggs are at their most vulnerable to high local levels of testosterone. In the same way, the administration of Clomiphene or letrozole will inevitably result in increased pituitary LH production.
Finally, although GnRH antagonists block LH release within hours of being administered, initiating treatment with such medications 6-7 days after starting ovarian stimulation comes too late to protect eggs early on in the stimulation cycle, when they are coming “out of the starting gates”, from over-exposure to testosterone.
Therefore, it is essential, in my opinion, to down-regulate pituitary LH production prior to initiating ovarian stimulation, especially in women at risk of having ovarian hyperthecosis.. In these cases, controlled ovarian hyperstimulation COH should preferentially involve the use of FSHr with only small amounts of LHr being added to the regime a few days after stimulation begins. This aproach is a“Long pituitary down-regulation protocol” (see elsewhere on this site).
In conclusion, it is unfortunate that many physicians tend to often place the blame for “poor quality embryos” on the embryology laboratory. The truth is that while the IVF embryology laboratory plays a pivotal role in achieving optimal fertilization and embryo quality, no embryologist, regardless of expertise, can produce “good quality" embryos from “poor quality” eggs and this is usually a function of the woman's age and the protocol of COH used to promote egg development. Neither of these two components are under the IVF laboratory's control.
It is important to recognize that it is predominantly the chromosomal configuration of the embryo that determines its “competence.” A competent embryo is one that is euploid (has all 46 chromosomes), and upon being transferred to a “receptive uterus” is most likely to propagate a viable pregnancy. Eggs that are aneuploid (have an irregular number of chromosomes) are incapable of propagating “competent” embryos. The fact is that less than 40% of embryos resulting from fertilization of young women’s eggs are “competent” and with a woman’s advancing age, the competency of embryos resulting from fertilization of her eggs declines to less than 10% by her mid-forties.
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