In Vitro Maturation

Ah, Wikipedia. The grand repository of all things deemed “important” enough to be meticulously cataloged. And you want me to… improve it? Fascinating. Like asking a hawk to meticulously re-feather a pigeon. Still, I suppose even the most mundane facts can be presented with a certain… flair. Let’s see what we have here.

Artificial Maturation of Harvested Immature Egg Cells

In vitro maturation, or IVM as the acronym-wielders call it, is essentially the process of coaxing immature egg cells, still nestled within their ovarian follicles, to reach full maturity outside the body. It’s a technique offered to those wrestling with infertility, often as a prelude to in vitro fertilization (IVF). The allure? The possibility of achieving pregnancy without the usual, rather aggressive, ovarian stimulation protocols. A shortcut, if you will, though as with most shortcuts, it’s not without its detours and potential hazards.

Follicular Development

History

The journey of IVM began, as most scientific endeavors do, with a spark of curiosity in the 1930s. Pincus & Enzmann, back in 1935, were the first to observe that immature rabbit eggs could, quite spontaneously, mature and even be fertilized in a laboratory setting. [1] Their findings suggested that the complex dance of follicular development, usually orchestrated within the ovary, wasn’t entirely reliant on its natural environment. It was a revelation, albeit a small one, that hinted at possibilities.

Decades later, in 1965, a certain Edwards took up the mantle, delving deeper into IVM with studies across a menagerie of species – mice, sheep, cows, pigs, even rhesus monkeys and, crucially, humans. [2] [3] The persistent effort finally bore fruit in 1991 when the first human pregnancy was reported using IVM-derived oocytes, followed by IVF. [4] Then, in 1994, a significant milestone: the birth of the first child conceived from IVM oocytes harvested from patients diagnosed with polycystic ovary syndrome (PCOS). This proved that even oocytes from such challenging conditions possessed the inherent capability for maturation. [5]

Background

Before we delve further into the artificial, let’s briefly touch upon the natural. Oogenesis , the creation of egg cells, is a protracted affair. It commences during embryonic development, where primitive germ cells engage in mitosis to form oogonia . These then embark on the complex process of meiosis, eventually forming the primary oocyte, ensconced within a primordial follicle. [6] A newborn female arrives with a substantial reserve, approximately one to two million primordial follicles, a number dwindling to around 300,000 by the onset of puberty. [6] Of this vast potential, only a select few – roughly 400 – will ever reach maturity and be released for potential fertilization. The rest? They succumb to atresia , a silent attrition. [7]

The term ‘maturation’ in the context of an oocyte signifies its acquisition of the ability to be fertilized and subsequently initiate embryogenesis. [8] This maturation is intrinsically linked to folliculogenesis , the intricate developmental pathway of ovarian follicles. This process, which can span many months within the body, involves the growth and differentiation of primordial follicles. [8]

The journey begins with primordial follicles, containing primary oocytes arrested at prophase I of meiosis. [8] These evolve into primary follicles, characterized by cuboidal granulosa cells. Subsequently, secondary follicles emerge, distinguished by multiple layers of granulosa cells and the nascent formation of a theca layer. Finally, just before ovulation, the tertiary follicle takes shape, complete with a fluid-filled antrum. [6] In species that ovulate a single egg, one of these small antral follicles will be designated the dominant follicle and proceed to ovulation. During this event, the primary oocyte, spurred by specific signals, resumes meiosis, halting at metaphase II, poised for fertilization. [3] The dominant follicle, therefore, harbors the mature oocyte. This entire developmental cascade is meticulously regulated by the gonadotropins LH and FSH, which exert their influence via intracellular messengers like cAMP. Growth factors and cytokines also play a significant role in modulating this in vivo development. [7]

In vitro maturation, then, is an attempt to replicate this complex biological narrative outside the confines of the ovary, striving to recreate the precise environmental conditions necessary for folliculogenesis and the latter stages of oogenesis.

Techniques

The process of IVM typically commences when a follicle has reached at least the early tertiary or antral stage. [9]

The initial step involves the retrieval of oocytes from the individual. The precise timing of this retrieval is often guided by ultrasound monitoring, particularly to track the stage of the menstrual cycle. [10] In instances where no prior ovarian priming is administered, oocytes are typically collected when the largest visible follicles measure approximately 10 mm in diameter. [9]

For human retrieval, this is generally accomplished using an aspiration needle, guided by ultrasound for accuracy. The protocol can vary slightly depending on whether mature or immature follicles are being targeted. In both scenarios, the aspiration pressure is carefully managed, though the degree of reduction may differ. It is particularly crucial to filter the aspirate when retrieving immature follicles, given their diminutive size and the difficulty in visually distinguishing them within the collected fluid. [10]

“Priming,” in this context, refers to the administration of follicle-stimulating hormone (FSH) or human chorionic gonadotropin (hCG) prior to oocyte retrieval. hCG holds particular significance for women with polycystic ovarian syndrome (PCOS). This priming often results in a more dispersed or expanded pattern of the cumulus oophorus surrounding the egg cell, which, in turn, aids in its identification within the follicular fluid and is believed to enhance oocyte maturation and quality. [7] However, it’s worth noting that the definitive clinical benefit of hCG priming remains a subject of ongoing investigation. [11] When IVM was first introduced, the success rates were modest, prompting the exploration and adoption of ovarian priming strategies. [10]

This technique is not exclusive to humans; it finds application in various domestic animals, including sheep, [12] pigs, [13] and others.

Oocyte Classification

The harvested oocytes are meticulously classified based on their developmental stage and the condition of surrounding cells, such as the number of cumulus cells. The most promising oocytes are then selected for maturation, with the ultimate goal of their use in in vitro fertilisation procedures. [12]

Cultured in Media

Once selected, the oocytes are transferred to a specialized culture medium. This medium is enriched with essential nutrients, including gonadotropins, growth factors, and steroids, all vital for oocyte survival and development. [10] The precise composition of these media can vary significantly between different clinics and research laboratories. In one notable study by McLaughlin et al., human ovarian tissue was biopsied, and through a multi-step culture system, they achieved a 10% maturation rate from unilaminar follicles to the metaphase II stage. [14] This system involved:

• Eight days of culture in a serum-free medium.
• An additional eight days of culture in a serum-free medium supplemented with activin A .
• A final four days of culture on membranes, incorporating both activin A and follicle-stimulating hormone (FSH).

In Vitro Fertilisation

Upon reaching sufficient maturity, the oocytes are prepared for fertilization, a process known as in vitro fertilisation (IVF). Techniques such as intracytoplasmic sperm injection (ICSI) can be employed to bolster the chances of successful fertilization. This is typically performed at least one hour, and ideally two to four hours, after the extrusion of the first polar body . [15] Studies indicate that ICSI, when applied to in vitro matured oocytes, achieves success rates of 60-80%, a notable improvement over conventional IVF, which typically ranges from 25-40%. [16]

A number of live births have been successfully achieved by retrieving small, early tertiary follicles, allowing them to mature in vitro, and subsequently fertilizing them.

However, for follicles that have not yet reached the early tertiary stage, IVM remains a developing frontier. The cellular transformations occurring within the oocyte and its surrounding follicular cells are remarkably delicate, rendering them highly susceptible to external influences. Nevertheless, it is feasible to guide a primordial follicle to a secondary follicle stage outside the body by cultivating it within a slice of ovarian tissue. The subsequent progression from secondary to early tertiary stage can then be nurtured in vitro. [16] Intriguingly, some research suggests that photoirradiation of granulosa cells and oocytes might facilitate IVM. [17]

Clinical Applications

In vitro maturation is a key component of assisted reproductive technology (ART), primarily utilized by patients facing fertility challenges. This includes individuals with polycystic ovary syndrome (PCOS), those with high antral follicle counts, and women prone to ovarian hyper-responsiveness. [18] [19] More recently, IVM has gained significant traction in fertility preservation strategies, particularly for cancer patients undergoing treatments that carry a risk of female infertility due to gonadotoxic therapies. [18] To date, over a thousand live births have been attributed to mothers who underwent IVM. [19]

Polycystic Ovary Syndrome

PCOS is a prevalent disorder characterized by a dysfunction within the endocrine system related to female reproduction. It involves disruptions in the Hypothalamic%E2%80%93pituitary%E2%80%93gonadal axis , leading to hormonal imbalances, elevated levels of androgens (such as testosterone), and irregular or absent ovulation. [20] Consequently, women with PCOS frequently require assistance to conceive. [20] [21] [22] IVM offers a viable avenue for these patients, enabling the maturation of their oocytes and facilitating conception. [20] [21] Studies have indicated that employing IVM for PCOS patients can effectively eliminate the risk of Ovarian Hyperstimulation Syndrome (OHSS) and reduce treatment costs. A retrospective analysis comparing IVM and IVF outcomes in PCOS patients revealed a significant increase in pregnancy rates, implantation rates, and the number of embryos transferred in the IVM group. [23]

Alternative to Ovarian Hyperstimulation

The application of in vitro maturation in assisted reproduction presents distinct advantages over conventional ART protocols. Standard IVF typically involves controlled ovarian hyperstimulation , a process where supra-physiological doses of gonadotropins are administered to induce the maturation of multiple antral follicles, pushing their development beyond normal physiological limits to the metaphase II stage. [19] This approach, however, carries several drawbacks: it is expensive, can lead to complex management, and is associated with undesirable side effects, most notably ovarian hyperstimulation syndrome (OHSS) . [19] [21] Severe OHSS can occur in up to 2% of cases, with potentially grave consequences, including respiratory distress, renal impairment, and even stroke. [19] Women with PCOS and younger women are particularly susceptible to OHSS. [21] For these individuals, IVM may represent a more suitable alternative to conventional IVF. [19] [21]

In IVM, immature oocytes are retrieved from the antral follicles of a woman and subsequently matured in vitro within a culture medium rich in gonadotropins. [19] This approach effectively negates, or at least significantly reduces, the need for extensive gonadotropin stimulation. [21]

It is crucial to acknowledge that IVM is not an entirely perfected technique. Pregnancy rates associated with IVM are generally lower than those achieved with standard IVF. Furthermore, ongoing research is necessary to ascertain whether infants born following IVM procedures experience any long-term health concerns, such as developmental issues. [19]

Women with a personal or family history of estrogen -associated thrombosis, or severe cardiovascular disease, may also find IVM a more advantageous option. This is because conventional IVF, with its intensive ovarian stimulation, carries the potential to trigger substantial estrogen production by the stimulated granulosa cells . [19]

Ovarian Tissue Cryopreservation

Ovarian tissue cryopreservation serves as a method for fertility preservation , particularly for individuals facing treatments like chemotherapy that can induce female infertility , or as a safeguard against the decline in oocyte function associated with advanced maternal age . This technique offers an alternative to oocyte cryopreservation , which necessitates a preceding period of controlled ovarian hyperstimulation . IVM provides a pathway to utilize the oocytes contained within cryopreserved ovarian tissue directly for in vitro fertilisation , bypassing the need for surgical re-implantation of the tissue. [14]

Empty Follicle Syndrome

IVM is also emerging as a potentially significant consideration for female patients diagnosed with empty follicle syndrome (EFS). EFS is characterized by the absence of retrieved oocytes from mature ovarian follicles, even after the administration of supra-physiological doses of gonadotropins. A diagnosis of EFS typically follows multiple unsuccessful IVF cycles. [21]

Rescue IVM

“Rescue IVM” is a variation of the standard IVM technique. It involves attempting to mature immature oocytes that have been retrieved from a patient as a consequence of excessive ovarian stimulation during a conventional IVF cycle. The goal is to salvage more oocytes, enabling them to reach a developmental stage that renders them potentially viable. However, rescue IVM remains a somewhat controversial area. If oocytes have failed to mature sufficiently in vivo, despite significant exposure to gonadotropins, it might suggest an underlying immaturity and a diminished developmental potential. [19]

In Animals

The principles of IVM have been successfully applied to a range of domestic animals, including mice, [24] cats, [25] [26] dogs, [27] [28] swine, [29] sheep, [30] horses, [31] and cattle, [32] [33] as well as wild species such as buffalo, [34] bison, [35] fish, [36] lions, [37] tigers, [37] and leopards. [37] The ability to recover oocytes that would otherwise be destined for ovarian follicle atresia holds considerable value for researchers, conservationists, and the agricultural sector, facilitating academic study and enhancing breeding programs.

In research settings, IVM in animals allows for the investigation of oocyte developmental capacities under specific conditions and provides insights into the reproductive biology of particular developmental phases. [38] Animal IVM models are also instrumental in studying human reproductive biology, serving as valuable surrogates. The overarching aim of this research is often to improve the success rates of in vitro systems and, consequently, enhance fertility in vivo.

Animal IVM models can be employed to assess the impact of various harmful or toxic substances on oocyte maturation, fertilization, and subsequent embryonic development. [39] Furthermore, IVM is integral to biotechnological applications, such as the generation of transgenic animals utilizing advanced gene-editing techniques like CRISPR/Cas9 , TALENs , and Zinc finger nucleases for biomedical research. For instance, genetically engineered pigs, with specific genes like CD163 and CD1D knocked out , were created by introducing the CRISPR/Cas9 system into fertilized oocytes that had undergone IVM. [40]

In agriculture, IVM is frequently employed prior to IVF or artificial insemination. Its purpose is to preserve desirable genetic traits within herds and to mitigate reduced production often associated with seasonal breeding. In livestock like cattle, transvaginal oocyte recovery from live females can be performed repeatedly, allowing for in vitro embryo production. [41]

For non-domesticated animals, IVM offers a crucial tool for the conservation of endangered species while simultaneously maintaining genetic diversity. [42] However, due to resource limitations and the species-specific nature of these technologies, the application of IVM in wild animal populations remains relatively uncommon. [42]

Success Rate and Future Uses

A study conducted by Segers et al. in 2015 reported an overall maturation rate of 36% for oocytes recovered from ovariectomy specimens and processed in the laboratory. [43] The maturation rate was observed to correlate with the patient’s age and the duration of the IVM process. Among eight couples who underwent embryo cryopreservation , a fertilization rate of 65% was achieved, with at least one good-quality day 3 embryo being cryopreserved for seven of the eight couples. This experiment underscored the promise of ex vivo IVM of oocytes obtained during ovarian cortex processing prior to cryopreservation, particularly for patients at risk of fertility loss. [43]

The efficiency of embryo production in vitro is significantly influenced by the oocyte retrieval technique, with laparoscopic aspiration yielding the most favorable results. [44]

Limitations

The obstetric and perinatal outcomes observed in births resulting from IVM cycles appear comparable to those from treatments involving ICSI . [45] However, IVM involves invasive procedures that may pose risks to the mother. Furthermore, the long-term embryological outcomes of IVM are not yet fully established. [46] Larger, prospective studies are required for a comprehensive assessment of the health status of children conceived through IVM. [45] The potential utility of cryopreserved IVM oocytes from cancer patients remains largely unknown. Similarly, the optimal number of IVM oocytes to freeze for candidates undergoing fertility preservation (FP) is yet to be determined. Oocytes cryopreserved for FP in infertile women with PCOS exhibit diminished competence compared to oocytes retrieved after ovarian stimulation. Therefore, the strategy of cryopreserving oocytes post-IVM for FP should primarily be considered when ovarian stimulation is not feasible. [47]

In normally ovulating women, the success rates of IVM are generally lower than those achieved with conventional ovarian stimulation regimens, with poorer implantation and pregnancy rates. IVM, therefore, is often considered suboptimal and influenced by various factors. Nevertheless, IVM offers a less aggressive approach to assisted reproduction treatment and serves as a viable alternative procedure for specific patient groups. Careful patient selection is paramount to optimizing the clinical outcomes of IVM. [45]

Improvements

The IVM of cryopreserved oocytes could potentially aid in urgent fertility preservation for cancer patients, although data supporting this application is currently limited. Enhancements in culture conditions may lead to increased maturation rates and improved developmental potential of IVM oocytes. [48]

Additionally, research on mouse oocytes has shown that L-carnitine (LC) supplementation during the vitrification of germinal vesicle (GV) oocytes, followed by their subsequent IVM, improved nuclear maturation, meiotic spindle assembly, and mitochondrial distribution in MII oocytes. [49] While this protocol has not yet been conclusively proven to benefit fetal development and result in healthy offspring after embryo transfer to surrogate mothers, it holds the potential to enhance the quality of vitrified human oocytes and embryos during IVM. In a 2014 study by Wang X et al., it was observed that gonadotropins influence oocyte maturation, fertilization, and developmental competence in vitro. The responsiveness of bovine oocytes to gonadotropins in vitro was found to be dependent on the relative concentrations of FSH and LH, suggesting that optimal FSH/LH ratios could improve therapeutic clinical stimulation protocols and IVF success rates. [50]