During fertility treatments like in vitro fertilization (IVF), eggs are retrieved from the ovaries. While the aim is to collect mature, fertilizable eggs, immature eggs are also often retrieved. This article explores how eggs can mature outside the body after retrieval, detailing the biological requirements for egg maturity and a specialized laboratory technique.
Egg Maturation: The Biological Need
For successful reproduction, an egg must reach a specific stage of maturity before fertilization. A mature egg (metaphase II or MII oocyte) has completed its first meiotic division, reducing chromosomes by half to combine with sperm’s genetic material.
A key indicator of a mature egg is the presence of a first polar body, a small cell expelled during this process. Immature eggs have not completed these cellular divisions and cannot be conventionally fertilized. These may be in earlier stages, such as the germinal vesicle (GV) stage (visible nucleus) or metaphase I (MI) stage (nucleus absent, polar body not extruded).
In Vitro Maturation (IVM) Explained
In vitro maturation (IVM) is a laboratory technique allowing immature eggs to complete development outside the body. It serves as an alternative or supplementary method to conventional IVF, where eggs typically mature within the ovaries using hormonal stimulation. IVM’s purpose is to “ripen” these eggs in a controlled environment, making them competent for fertilization.
The process involves culturing retrieved immature eggs in specialized media under specific laboratory conditions. By maturing eggs in vitro, IVM aims to reduce or eliminate extensive hormonal stimulation, a common component of traditional IVF protocols. This offers a gentler approach to fertility treatment for certain patients.
The IVM Process and Candidate Profile
The IVM process begins with retrieving immature eggs, often from smaller follicles that wouldn’t typically yield mature eggs in a conventional IVF cycle. These eggs, usually in germinal vesicle (GV) or metaphase I (MI) stages, are cultured in a specific maturation medium for 24 to 48 hours to reach the mature metaphase II (MII) stage. Once maturation is confirmed, these eggs can be fertilized, often using intracytoplasmic sperm injection (ICSI) for improved success rates.
IVM is suitable for specific patient profiles. Women with polycystic ovary syndrome (PCOS) are ideal candidates, as they face a higher risk of ovarian hyperstimulation syndrome (OHSS) with conventional IVF due to their increased number of small follicles. IVM minimizes or avoids the high doses of hormones that can trigger OHSS. It is also used for fertility preservation, especially for cancer patients needing rapid egg retrieval before gonadotoxic treatments, who cannot undergo ovarian stimulation.
Effectiveness and Considerations of IVM
The effectiveness of IVM varies, with reported maturation rates of retrieved immature eggs typically ranging from 36% to 65%. Once matured, fertilization rates for IVM-matured eggs, particularly with ICSI, range from 60% to 80%. Embryos derived from IVM-matured eggs can then be transferred or cryopreserved.
Pregnancy rates with IVM are encouraging, with some studies reporting clinical pregnancy rates per transfer ranging from 21.8% to 44% in specific patient groups like those with PCOS. While IVM has advanced, some studies indicate cumulative live birth rates might be lower compared to conventional IVF for certain populations, especially across multiple cycles. Factors influencing IVM success include the quality of retrieved immature eggs and the expertise of the laboratory performing the maturation process. IVM offers a valuable alternative for patients for whom traditional IVF is unsuitable or unsafe, such as those at risk of OHSS or needing urgent fertility preservation.