Introducing the Oocyte
In a fetus, there is a pool of around 7 million primordial follicles in the developing ovaries. At birth, this amount drops to 300 – 400,000 and at puberty the size of the pool plateaus around 200,000. At this stage in life, the primordial follicles will start to be recruited into a growing follicular pool that stay in a rested early Meiosis 1 stage of development.
The transition of primordial follicles into the growing follicular pool is regulated by AMH (anti-müllerian hormone). It is a growth factor secreted by granulosa cells in the developing follicles. AMH peaks at puberty and declines until menopause. It is a good indicator of ‘ovarian reserve’ since it reflects the size of the growing follicular pool. The AMH test is often ordered during infertility investigations and can be used to help diagnose someone’s infertility and/or predict how well individuals will respond to ovarian hyper-stimulation. The results are categorized into high, medium, low, and very low ovarian reserve.
The Menstrual Cycle
At the onset of puberty, cohorts of antral follicles are recruited from this growing follicular pool. During each menstrual cycle around 5-10 antral follicles are recruited in response to increasing FSH (follicle stimulating hormone) levels. Out of this cohort, one is selected to be the dominant follicle; the rest will undergo atresia (die). Around day 14 of the cycle, there is a surge of LH (luteinizing hormone). This causes the ovulatory (dominant) follicle to ovulate and expel one mature egg from the ovary. The remaining follicle structure (granulosa and theca cells) forms a corpus luteum that secretes progesterone to maintain a potential pregnancy by preparing the endometrium. Approximately 400 eggs will be ovulated during a reproductive lifetime.
Antral follicles are present in the ovary and will continue to grow when sufficiently stimulated with FSH. Therefore, by administering continual doses of gonadotropin medications (e.g GonalF/Puregon/Rekovelle) starting on day 3 of a cycle, the selection of one dominant follicle can be disrupted and multiple follicles can be stimulated to grow instead. This is known as controlled ovarian hyperstimulation.
Once follicles reach a size of 1.7mm or higher, a hCG (human chorionic gonadotropin) trigger can be administered. This medication is used since it shares the same receptors as LH and thus mimics the natural LH surge. The number of expected eggs to be retrieved can be estimated by counting how many follicles are measuring 1.5mm or higher on an ultrasound scan on the day of trigger. Estrogen levels measured on this day can also act as a predictor for egg number; the optimal estrogen:egg ratio is 734-1097 pmol/L per mature follicle.
Three stages of oocyte maturation are commonly seen in the IVF lab: MII (metaphase II), MI (metaphase I), and GV (germinal vesicle).
A GV is an oocyte where the nucleus is still visible. Through the process of germinal vesicle break down (GVBD), the oocyte becomes an MI stage with no visible nucleus. Maturation continues as the oocyte releases the 1st polar body (a small package of excess DNA) and becomes an MII stage oocyte, resting at the end of Meiosis I. This is the fully mature stage and the oocyte is now ready for fertilization with a sperm cell.
Transition occurs from GV (immature) to MI (partially mature) to MII (fully mature) in response to the mid-cycle surge of LH or the hCG trigger. Timing is important, if a retrieval happens too soon after the trigger shot, the eggs may not have enough time to make it to the MII stage!
Not all eggs retrieved will be mature; on average 85% will be at the appropriate MII stage, 4% will be MI, and 11% will be GVs. This does vary person-to-person depending on their own physiology and the response to the medications.
Prior to the ICSI (intra-cytoplasmic sperm injection) procedure, oocytes are denuded or “stripped” which removes the support cells (cumulus) that surrounds them and allows the embryologists to assess the maturation of the eggs. Following that, mature oocytes are injected directly with a single sperm cell. Preferably MIIs are used as they are fully mature and have a higher fertilization rate, but MI may also be inseminated if the number of MIIs retrieved is low. GVs are not injected. Injection occurs approximately 3-4 hours after retrieval to allow the oocytes to “rest” following the stressful collection process and stripping procedure.
For standard IVF, the oocytes are not stripped prior to insemination. Instead, they are put in a culture dish with a specified amount of prepared sperm and allowed to inseminate overnight. This means that the maturation status of the eggs remain unknown. The cumulus cells are left intact as they assist in fertilization since they attract swimming sperm. The following morning, the possible fertilized eggs are stripped of the cumulus cells and the fertilization status is assessed under the microscope. Oocytes that did not successfully fertilize can be assessed for maturity.
Signs of Fertilization
Fertilization assessments take place 18 hours after the time of insemination (IVF or ICSI). Fertilization is successful when the presence of two pro-nuclei (2PN) are seen and two polar bodies are present. The 2nd polar body is released after the sperm penetrates the egg and the oocyte completes the Meiosis 2 stage, removing the final bit of unnecessary DNA before the two genomes fuse. PNs are formed when the egg and sperm DNA combine to create the correct chromosomal compliment (46 chromosomes). On occasion, the fertilized egg (zygote) may display one pronuclei or three pronuclei; any zygotes showing 1 or 3PNs are abnormal and will be discarded as they have a higher risk of genetic abnormalities.
Following IVF or ICSI, approximately 70% and 80% of MII oocytes will successfully fertilize, respectively. These success rates will decrease if there is any reduced quality of the sperm or eggs (e.g. increased age, poor response to medications, abnormal sperm motility/morphology etc.). Around 40% of these fertilized eggs (zygotes) will go on to develop to the blastocyst stage. Of these blastocysts 25–35% will be of good quality (A/B grade) and will be suitable for transfer or freezing for future use.