A lot of times the lab hears feedback from the IVF coordinators that patients were surprised, or upset, that when performing ICSI, the embryologist was not able to inject every single egg that was retrieved. The main cause of this is due to the maturity stage of the egg itself.
Females are born with the total number of eggs that they will ever have in a lifetime. Prior to puberty, these eggs are arrested at an early stage of meiosis I, or at an “immature stage”. At this immature stage, the egg has a full set of chromosomes. As a female enters into puberty, luteinizing hormone (LH) surges occur during the menstrual cycle. These LH surges stimulate the resumption of the meiosis process in the egg and prepare the egg for ovulation and fertilization. Meiosis is an essential process that must occur within the egg because it reduces the number of chromosomes of the egg by half. When this meiosis process is complete, the egg is considered to be in the “mature” stage and is now ready to be fertilized. When fertilization occurs, the egg will once again have the proper number of chromosomes, one set coming from the egg and one set coming from the sperm, to form the embryo with a full component of genetic material.
During the IVF cycle when the follicles reach a certain size, an HCG injection is given. This HCG injection functions similarly to the role of luteinizing hormone (LH) in the menstrual cycle; it attempts to induce the eggs to undergo that final maturation step (or resumption of the meiosis process). You are instructed to give this HCG injection at a specific time prior to the egg retrieval so that the eggs have sufficient time and exposure to the HCG in hopes of yielding mature eggs from the follicles.
So, what does the lab look for when it is time to perform ICSI?
In preparation for fertilization, the egg undergoes the meiosis phases to reduce its number of chromosomes by half. The excess DNA or chromosomes are extruded out of the egg as a small cytoplasmic body known as a polar body. When extruded, this polar body sits in the perivitelline space between the oocyte and the zona pellucida and is visible under the microscope. When the embryologist sees the presence of this polar body they consider the egg to be mature and ready to be injected with the sperm to attempt fertilization. If the embryologist does not see this polar body under the microscope they consider the egg to be immature. Immature eggs are not capable of fertilization.
It is possible, however, that immature eggs can mature in the lab. This is called in vitro maturation. For optimal success, it is critical that the ICSI procedure be performed in the IVF lab within a specified time window following HCG injection and egg retrieval. When immature eggs are observed at time of ICSI the embryologist will wait the full allotted fertilization time window to see if the egg will undergo in vitro maturation in the lab. If the embryologist sees the polar body within the allotted time window, ICSI will be performed. If the embryologist does not see the polar body within the allotted time window, ICSI will not be performed and it is considered that the egg has arrested development and will not mature. As per lab protocol, immature eggs are discarded if they do not reach the appropriate stage for ICSI.
It is common to have immature eggs obtained during IVF. Typically about 10-15% of the eggs retrieved are immature. There are times in the IVF lab when all eggs are mature at the time of ICSI. There are times in the IVF lab when in vitro maturation of the eggs occurs and the embryologist is able to perform ICSI. There are times in the lab when ICSI does not occur as the eggs remain immature. Just know that our embryologists do make every attempt to give each egg a chance to mature and a chance to fertilize. It is a challenge that the embryologists are more than willing to undertake especially since sometimes the result is worth every effort in the lab!