Advances in human oocyte in vitro maturation: current status and future perspectives: a narrative review

Abstract

Graphical Abstract

Abstract

Oocyte in vitro maturation (IVM) is an evolving component of assisted reproductive technology (ART) that offers a less invasive and cost-effective alternative to conventional controlled ovarian stimulation. It is particularly beneficial for patients at risk of ovarian hyperstimulation syndrome (OHSS), those with polycystic ovary syndrome (PCOS), and individuals requiring urgent fertility preservation. Despite these advantages, clinical uptake has remained limited owing to concerns about the developmental competence and quality of IVM-derived oocytes. To address this, we conducted a comprehensive literature search of PubMed, Embase, and Web of Science for articles published between January 2000 and June 2025, using combinations of keywords related to IVM, oocyte maturation, culture protocols, oocyte quality, and clinical outcomes. Recent progress in the field has led to the development of biphasic culture systems, pre-IVM priming strategies, and the incorporation of regulatory factors such as C-type natriuretic peptide (CNP) and oocyte-secreted factors, all of which have contributed to improved oocyte maturation and embryo development. Nonetheless, variability in outcomes persists due to differences in patient selection, stimulation protocols, and laboratory practice. Continued optimisation of IVM culture systems and a deeper understanding of oocyte maturation mechanisms will be essential for enhancing clinical efficacy. Future research should prioritise standardisation, patient-tailored protocols, and systematic long-term outcome data to support wider adoption of IVM. This review provides a comprehensive overview of recent advances and ongoing challenges in human oocyte IVM, offering perspectives on future directions for clinical translation and improved ART outcomes.

Lay summary

IVM is a fertility treatment that allows immature eggs to mature in the laboratory rather than within the body. This approach can be safer, simpler, and more affordable than traditional IVF, especially for women with polycystic ovary syndrome (PCOS), those at risk of ovarian hyperstimulation, or those who need to preserve their fertility quickly, such as cancer patients. Although IVM holds great promise, it is not yet widely used because its success rates are not as high as those of conventional methods. This review looks at the latest scientific progress to improve IVM, including better laboratory techniques, the use of natural hormones and growth factors, and new ways to support egg development outside the body. These advancements have helped improve the quality of eggs and embryos, but challenges remain. Differences in patient types, medications used, and lab practices can affect how well IVM works. More research is needed to make IVM more consistent and effective so it can become a routine fertility option for a broader population.

Introduction

Oocyte in vitro maturation (IVM) is a technique used to retrieve immature oocytes from the ovary at either the germinal vesicle (GV) or meiotic metaphase I (MI) stage. These oocytes are then matured in a controlled laboratory environment. The concept of IVM dates back to 1935, when Pincus and colleagues first achieved the maturation of rabbit oocytes in vitro (Pincus & Enzmann 1935). In the 1960s, Rock et al. reported the successful maturation of human immature oocytes under similar conditions (Krisher 2022). A pivotal milestone was reached in 1991 when Cha et al. documented the first live birth following IVM, using oocytes retrieved from unstimulated ovaries and matured in vitro (Cha et al. 1991). Since then, IVM has garnered significant attention, particularly during the 1990s, with its clinical applications continuously expanding (Trounson et al. 1994, Barnes et al. 1995, Mikkelsen et al. 1999), and it was officially recognised as non-experimental only in 2021 (Practice Committees of the American Society for Reproductive Medicine, the Society of Reproductive Biologists and Technologists, and the Society for Assisted Reproductive Technology 2021). However, compared to traditional in vitro fertilisation (IVF), oocytes derived from IVM face challenges, including low maturation rates, reduced normal fertilisation rate after intracytoplasmic sperm injection (ICSI), and limited developmental potential of resulting embryos (Walls et al. 2015). These shortcomings are primarily attributed to asynchrony in cytoplasmic and nuclear maturation. While IVM oocytes can achieve nuclear maturation, their fertilisation competence and ability to support embryonic development are largely dependent on cytoplasmic maturation (Jia et al. 2021, Mastrorocco et al. 2022). Addressing this issue requires improving culture systems and optimising supplementation to promote synchronised nuclear and cytoplasmic maturation.

This review aims to provide a comprehensive overview of advancements in IVM culture protocols, media supplements, and culture systems, as well as the cellular and molecular mechanisms underlying oocyte maturation. Furthermore, it explores the clinical indications for IVM, offering valuable references for future research and a theoretical foundation for the continued refinement and clinical application of this technology.

Methods

To provide a comprehensive overview of recent advances in human oocyte IVM, we conducted a targeted literature search across the PubMed, Embase, and Web of Science databases. Publications from January 2000 to June 2025 were considered, reflecting the period during which most major developments in IVM protocols have been reported. Search terms included combinations of ‘IVM’, ‘oocyte maturation’, ‘assisted reproductive technology’, ‘culture protocols’, ‘oocyte quality’, and ‘clinical outcomes’.

Articles were included if they reported on human oocyte IVM and addressed aspects such as culture protocols, hormonal priming, growth factors, or the cellular and molecular mechanisms underlying oocyte maturation. Studies that presented clinical outcomes, including oocyte maturation rates, fertilisation, embryo development, implantation, and live birth rates, were also considered relevant. Exclusion criteria were applied to animal-only studies lacking clear translational value, case reports with insufficient methodological detail and non-English language publications.

All identified titles and abstracts were screened for relevance, and the full texts of potentially eligible articles were reviewed. Additional references were retrieved by manually searching the bibliographies of selected papers to ensure that important contributions were not overlooked. Evidence was synthesised narratively, with particular emphasis on the evolution of culture systems, supplementation strategies, and clinical applications of IVM, as well as on emerging insights into its cellular and molecular underpinnings.

Culture protocols for IVM

The introduction of various clinical and laboratory protocols for IVM has led to significant variability and debate within the literature (De Vos et al. 2016). Currently, two main types of clinical IVM laboratory protocols are widely recognised: the conventional IVM protocol and the biphasic IVM protocol (De Vos et al. 2021).

Conventional IVM culture protocol

Conventional IVM protocols are further divided into two approaches based on whether human chorionic gonadotropin (hCG) stimulation is administered before oocyte retrieval: the one-step culture system and the two-step culture system (Fig. 1) (Table 1) (Chian et al. 1999). In the one-step culture system, patients may or may not undergo follicle-stimulating hormone (FSH) priming. Regardless of priming, all oocytes are retrieved at the GV stage during oocyte pick-up (OPU). These GV-stage cumulus–oocyte complexes (COCs) are then cultured directly in vitro for 30–48 h under atmospheric oxygen conditions until they reach the MII stage. Mature MII oocytes are subsequently fertilised using ICSI.

Figure 1.

(A) Patients may or may not receive prior FSH priming but do not undergo hCG priming before OPU. GV-stage COCs undergo a one-step IVM process to reach the MII stage within 30–48 h, after which MII-stage oocytes are fertilised via ICSI. (B) Patients may or may not receive prior FSH priming but do undergo hCG priming before OPU. A portion of oocytes (10–20%) retrieved at the MII stage are fertilised by ICSI or IVF, while GV- or MI-stage COCs undergo a one-step IVM process to reach the MII stage, after which MII-stage oocytes are fertilised via ICSI.

Table 1.

Comparative characteristics of IVM protocols with or without hCG priming and pre-IVM culture.

	No hCG priming	hCG priming	No hCG + pre-IVM (CAPA-IVM)	hCG + pre-IVM
Priming strategy	±FSH, no hCG	±FSH + hCG	±FSH, no hCG	±FSH + hCG
Oocyte stage at retrieval	Predominantly GV	Mixed stages: 10–20% MII; remainder GV/MI	GV	Mixed: MII + GV/MI
Pre-IVM step	Not applied (conventional one-step IVM)	Not applied (conventional one-step IVM)	Applied (CNP ± OSFs, 4–24 h)	Applied (CNP ± OSFs, 4–24 h)
Maturation rate (to MII)	50–65% (30–48 h culture)	GV/MI mature to ∼60–70%	65–75%	GV/MI mature to ∼70–80%
Fertilisation method	ICSI	MII oocytes: IVF/ICSI; IVM oocytes: ICSI	ICSI	MII: IVF/ICSI; IVM oocytes: ICSI
Clinical outcomes (examples from the literature)	Clinical PR: ∼20%; LBR: ∼15%	Clinical PR: ∼25%; LBR: ∼17%	PR: 30–40%; LBR: ∼20–22%	Higher maturation and PR compared with conventional; LBR ∼30–35%

GV, germinal vesicle; MI, metaphase I; MII, metaphase II; PR, pregnancy rate; LBR, live birth rate; OSFs, oocyte-secreted factors; CAPA-IVM, capacitation in vitro maturation.

In the two-step culture system, patients may or may not undergo prior FSH priming but do receive a bolus of hCG 36–38 h before OPU (Yang & Chian 2017). Oocytes retrieved under this protocol may obtain MII-stage oocytes (approximately 10–20%), oocytes resuming meiosis in vivo (GV breakdown or MI stage), and GV-stage oocytes (these remain immature and require IVM culture).

The diverse maturation stages of oocytes retrieved during OPU necessitate distinct laboratory treatments. MII-stage oocytes are immediately fertilised, whereas oocytes at the GVBD, MI, or GV stages require varying durations of IVM culture to reach the MII stage (Yang & Chian 2017, De Vos et al. 2021). By categorising oocytes based on their maturation stage and applying tailored laboratory treatments, conventional IVM protocols aim to maximise maturation and fertilisation outcomes. However, variability in outcomes due to differing approaches highlights the need for protocol standardisation and further refinement.

The biphasic IVM culture protocol

The biphasic IVM culture protocol builds upon the conventional IVM approach by incorporating a pre-IVM culture phase designed to maintain meiotic arrest before initiating IVM (Fig. 2). Pre-IVM aims to maintain the oocyte’s developmental programme in vitro, mimicking the natural progression that would occur in vivo. Cumulus–oocyte complexes (COCs) play a central role in supporting this process. As such, pre-IVM culture serves as a ‘developmental’ phase designed to enhance the oocyte’s capacity for IVM before embryo development. An additional advantage of pre-IVM is that it allows for controlled meiotic resumption when the COC enters the IVM phase. Intentionally arresting the oocyte in meiosis, pre-IVM allows precise control over the timing of maturation. This enables the strategic use of meiotic-inducing agents, such as EGF-like peptides, instead of relying on the spontaneous resumption that would otherwise occur without the pre-IVM step. The combination of pre-IVM and IVM constitutes a biphasic IVM culture system (Funahashi et al. 1997).

Figure 2.

(A) Patients may or may not receive prior FSH priming but do not undergo hCG priming before OPU. GV-stage COCs are deliberately arrested for a few to 24 h as part of the pre-IVM culture step. Subsequently, COCs are transferred to a conventional IVM medium, and MII-stage oocytes are fertilised via ICSI. (B) Patients may or may not receive prior FSH priming but do undergo hCG priming before OPU. MII-stage oocytes are fertilised by ICSI or IVF, while immature COCs are deliberately arrested for a few to 24 h. These COCs are then moved into a conventional IVM medium, and MII-stage oocytes are fertilised via ICSI.

During the pre-IVM phase, meiotic arrest is sustained by targeting the signalling pathways that regulate cAMP levels. In vivo, mural granulosa cells secrete C-type natriuretic peptides (CNPs), which bind to natriuretic peptide receptor 2 (NPR2) on cumulus cells (Jaffe & Egbert 2017). This interaction elevates cyclic guanosine monophosphate (cGMP) levels, inhibiting phosphodiesterase 3A (PDE3A), the enzyme responsible for cAMP hydrolysis (Norris et al. 2009, Vaccari et al. 2009). High cAMP levels maintain oocyte meiotic arrest. At the preovulatory stage, the epidermal growth factor receptor (EGFR) signalling pathway induces meiotic resumption (Yang et al. 2021). Two significant milestones in biphasic IVM development were the identification of EGF-like peptides as key triggers of meiotic maturation, and the discovery of CNP as a natural inhibitor of oocyte maturation (Park et al. 2004, Zhang et al. 2010). Oestradiol is a critical component of CNP-mediated pre-IVM, playing a key role in maintaining CC expression of the CNP receptor NPR2 (Zhang et al. 2011). Insights from using EGF-like peptides in cAMP-mediated biphasic IVM to induce meiotic resumption have led to their incorporation into CNP-mediated biphasic IVM. Initially tested with mouse oocytes and later applied to human oocytes, these studies (Romero et al. 2016, Sanchez et al. 2017) laid the foundation for the capacitation-IVM (CAPA-IVM) culture system, which is now in clinical use (Vuong et al. 2021) (Table 1).

The CAPA-IVM system, a biphasic protocol combining C-type natriuretic peptide (CNP) and amphiregulin (AREG), has been shown to enhance oocyte maturation by upregulating cumulus cell genes linked to mRNA translation and degradation, likely through ERK1/2 signalling (Saraiva et al. 2024). Animal studies across several species have demonstrated that CNP-mediated biphasic IVM improves both maturation and embryo quality (Vuong et al. 2020b). In humans, CAPA-IVM has been associated with higher maturation rates and an increased yield of high-quality blastocysts, particularly from small antral follicles (≤6 mm) obtained after minimal ovarian stimulation (Saenz-de-Juano et al. 2019, Sanchez et al. 2019, De Vos et al. 2021). Despite these advances, live birth rate remains comparable to conventional IVM in the limited trials conducted to date (Vuong et al. 2020b) (Table 1). A key unresolved issue is the optimal duration of the pre-IVM phase. Evidence suggests that pre-IVM requirements are inversely related to follicle size and FSH exposure at collection, with extended culture needed in low- or no-stimulation settings, but shorter or no pre-IVM following extensive FSH priming (Li et al. 2016, Zhao et al. 2020) (Table 1). Overall, biphasic IVM shows promise in synchronising nuclear and cytoplasmic maturation, but further refinement is needed. Future research should focus on optimising pre-IVM timing, exploring alternative meiotic regulators, and improving culture conditions to enhance clinical outcomes.

Supplementations in the medium for IVM

Decades of research have highlighted the challenges of IVM, particularly regarding the inferior quality and developmental potency of IVM oocytes compared to those matured in vivo (Rizos et al. 2002, Lonergan & Fair 2016). IVM induces considerable ultrastructural heterogeneity in oocytes and desynchronisation between nuclear and cytoplasmic maturation (Chen et al. 2024). For instance, IVM-derived blastocysts exhibit lower volume density of mitochondria and nuclear volume density but higher lipid volume density compared to in vivo-derived blastocysts (Crosier et al. 2001). Moreover, IVM blastocysts are associated with significantly reduced cumulative clinical pregnancy and live birth rates. These findings underscore that oocytes undergoing IVM often fail to achieve complete cytoplasmic maturation. To address these deficiencies, basal culture media require supplementation with various nutrients, signalling molecules, and metabolic regulators to support oocyte maturation and subsequent embryonic development.

Supplementations in the IVM medium have become a focal point of research, with efforts aimed at identifying the optimal combination of factors to improve oocyte quality. Commonly used supplements include hormones, cytokines, antioxidants, energy substrates, and other growth factors. In vivo, COCs are surrounded by follicular fluid, a dynamic medium containing a diverse array of proteins, cytokines, hormones, energy metabolites, steroids, and other factors, many of which remain unidentified (Sutton et al. 2003). In contrast, IVM relies on a simplified, non-physiological culture system (Soares et al. 2020). To mimic the natural follicular environment and enhance oocyte maturation and quality, researchers have supplemented IVM media with various hormones and cytokines.

Supplementation of hormones in the IVM medium

Several hormones have been reported to be added to IVM medium to enhance oocyte maturation and developmental competence. These include recombinant FSH, hCG, luteinising hormone (LH), insulin, and oestradiol. FSH promotes both nuclear and cytoplasmic maturation of oocytes by increasing cAMP levels in COCs, which helps delay meiotic progression (Ye et al. 2005). Ye et al. evaluated the effects of FSH, LH, and EGF on nuclear maturation and embryo development in pig oocytes and found that FSH significantly enhanced the progression to MII in conventional culture, with an additive effect observed when combined with EGF. However, in oocytes preincubated with cycloheximide (CHX) to induce meiotic arrest, FSH slowed meiotic progression compared to LH and EGF. Despite this delay, FSH-treated oocytes showed significantly higher cleavage rates after IVF and increased blastocyst yield, particularly in CHX-pretreated oocytes. These findings demonstrate that FSH stimulates nuclear maturation and improves developmental competence, albeit with a slower progression to MII under certain conditions (Ye et al. 2005). However, high doses of FSH may adversely affect oocyte chromatin dynamics, transcription, and translation activities. It can also disrupt gap junction-mediated communication between cumulus cells and oocytes by reducing transzonal projection (TZP) density (Combelles et al. 2004), ultimately impairing oocyte development and embryonic competence (Buratini et al. 2021). LH primarily acts on mural granulosa cells rather than directly influencing COCs to regulate oocyte maturation (Li et al. 2019, Cao et al. 2020). As a result, the addition of LH to IVM culture medium is often considered less significant for improving oocyte maturation outcomes, especially compared to other factors such as FSH or EGF (Li et al. 2019). Some studies have demonstrated that high concentrations of LH can negatively impact oocyte quality, reducing polar body formation, cleavage rates, and blastocyst development.

Supplementation of growth factors in the IVM medium

Certain oocyte-derived growth factors, including growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15) (Morikawa et al. 2021), and epidermal growth factor (EGF) (Richani & Gilchrist 2018), play crucial roles in influencing oocyte and embryo quality during IVM. BMP15 and GDF9 are expressed in growing follicles. These factors activate signalling pathways in cumulus oophorus cells, regulating key genes and cellular processes involved in cumulus cell differentiation (Gilchrist et al. 2008). In addition, BMP15 and GDF9, which are present in follicular fluid, significantly enhance oocyte maturation in vitro (Juengel & McNatty 2005). Their addition to the IVM medium has been shown to improve the developmental competence of oocytes (Yeo et al. 2008).

As the founding member of the EGF ligand family, EGF is essential for stimulating oocyte maturation and cumulus expansion (Richani & Gilchrist 2018). IGF-1 acts as a potent mitogen for granulosa cells (Hernandez et al. 1988) and supports nuclear maturation in bovine oocytes (Lorenzo et al. 1994). Connexin37 (Cx37) belongs to the connexin family and plays a pivotal role in regulating the formation of gap junctions (Niemann & Wrenzycki 2000, Richard & Baltz 2014). Cx37 facilitates communication between granulosa cells and oocytes and promotes oocyte maturation (Richard & Baltz 2014). Notably, supplementation with 50 ng/mL EGF, 100 ng/mL IGF-1, and 25 μg/mL Cx37 during the IVM of bovine oocytes significantly improved oocyte quality and developmental potential by enhancing transzonal projection (TZP) density, mitochondrial function, and glutathione (GSH) levels (Yang et al. 2022). These factors collectively enhance the efficiency of oocyte maturation and subsequent embryo development.

Supplementation of antioxidants in the IVM medium

In vitro conditions often elevate reactive oxygen species levels, leading to oxidative stress, which negatively impacts oocyte quality (Khazaei & Aghaz 2017). Oxidative stress can cause meiotic arrest, DNA damage, cytoskeletal disruption, mitochondrial dysfunction, and poor IVM outcomes (Kirillova et al. 2021b, Jiang et al. 2021). Consequently, recent research focuses on incorporating antioxidants into IVM medium to mitigate oxidative stress and support mitochondrial function. Numerous antioxidants, including caffeine, L-carnitine, CoQ10, melatonin, resveratrol, nicotinamide, and mitochondrion-targeted compounds, are being actively studied for their potential to enhance IVM outcomes.

Melatonin is a widely studied antioxidant in IVM, effectively neutralising reactive oxygen species through direct scavenging and enzymatic conversion into harmless species (Jiang et al. 2021). It upregulates imprinted genes such as SIRT1, AKT2, and POLG2, supporting mitochondrial function (Li et al. 2015). In addition, melatonin influences metabolic pathways such as cysteine and methionine metabolism while modulating the p53 signalling pathway to protect oocytes. It maintains mitochondrial membrane potency, reduces excessive Ca²⁺ levels, and enhances mitochondrial DNA (mtDNA) copy number, preparing oocytes for meiosis resumption and improving developmental competence. Low-dose supplementation (10⁻⁵ mol/L) has been shown to significantly improve the maturation of GV and metaphase I (MI) stage oocytes and increase high-quality blastocyst rates (Lee et al. 2011). During IVM, an increase in mtDNA copy number reflects the oocyte’s adaptation to fluctuating ATP demands, essential for the resumption of meiosis and the generation of viable gametes. However, mitochondrial dysfunction can reduce the ability to manage reactive oxygen species, with elevated reactive oxygen species levels further impairing mitochondrial function (Liu et al. 2019). Melatonin is a commonly used antioxidant to mitigate these effects, as it reduces excessive Ca²⁺ levels and maintains mitochondrial membrane potency. Similarly, MitoQ, a triphenylphosphonium-conjugated derivative of ubiquinone, enhances mitochondrial integrity and prevents mitochondrial uncoupling during IVM (Marei et al. 2019).

Melatonin supplementation has been shown to improve the developmental potential of immature human oocytes retrieved from the controlled ovarian hyperstimulation cycle during IVF treatment (Zou et al. 2020). In an IVM study, oocytes cultured with 10 μmol/L melatonin demonstrated a markedly higher high-quality blastocyst formation rate (28.4%) compared to the non-melatonin group (2.0%, P = 0.0001) and a lower aneuploidy rate (15.8%) (Zou et al. 2020). Clinical trials further confirmed the successful delivery of three healthy infants. Single-cell RNA sequencing revealed upregulation of genes associated with oxidative phosphorylation and ATP production in the melatonin supplementation group (Zou et al. 2020). In a study involving 255 patients, 10 μM melatonin significantly improved redox levels, mitochondrial DNA circularity, and ATP content in IVM-MII oocytes from older women (Yang et al. 2023). It also restored normal spindle and chromosome composition, leading to improved oocyte maturation and blastocyst formation rates (Yang et al. 2023). These findings suggest that melatonin reverses age-related declines in oocyte quality and offers a promising approach to enhancing fertility in older reproductive-aged women. Recombinant hCG, insulin, and oestradiol are also frequently included in the IVM medium, though their precise roles and optimal concentrations vary across studies.

Coenzyme Q10 (CoQ10) supplementation during IVM significantly improved oocyte maturation rates (82.6 vs 63.0%) and reduced post-meiotic aneuploidy rates (36.8 vs 65.5%) in women aged 38–46 years (Ma et al. 2020). In addition, it lowered chromosome aneuploidy frequencies (4.1 vs 7.0%) in this age group. However, no significant effects were observed in women aged ≤30 years (Ma et al. 2020). Cryopreservation induces oxidative stress, reducing cell viability and functionality in buffalo fibroblasts. A recent study demonstrated that low concentrations of MitoQ (0.1–0.5 μM) reduced reactive oxygen species production, improved mitochondrial membrane potency, and enhanced cell viability, while higher concentrations (2–10 μM) caused oxidative damage (Punetha et al. 2023). These findings highlight the dose-dependent benefits of MitoQ in improving post-thaw recovery and functionality of cryopreserved fibroblasts.

Resveratrol supplementation at 1.0 μM during IVM significantly enhanced oocyte nuclear maturation and fertilisation rates in aged mice and humans (Liu et al. 2018). It improved mitochondrial function, spindle and chromosome morphology, and blastocyst formation in aged mice, while upregulating antioxidative gene expressions (Srit1, Cat, Sox1, and Gpx4) (Liu et al. 2018). These findings suggest that resveratrol at an optimal concentration improves oocyte quality and developmental potential in older reproductive-aged females. Other antioxidants, including L-carnitine, caffeine, and nicotinamide, have also shown promise in reducing oxidative stress and enhancing oocyte quality, although their precise mechanisms and optimal concentrations require further investigation (Fathi et al. 2018, Guo et al. 2024, Leitao et al. 2024).

Ovarian support cells (OSCs) in the IVM medium

OSCs, derived from human induced pluripotent stem cells (hiPSCs), have been explored as an IVM supplement. Piechota et al. reported that incorporating OSCs into the IVM medium enhanced oocyte maturation by improving COCs communication (Piechota et al. 2023). Blastocysts derived from OSC-IVM did not exhibit significant epigenetic differences compared to those produced through conventional IVM methods. Furthermore, Piechota and colleagues found that MII oocytes matured via OSC-IVM displayed transcriptional profiles more closely resembling in vivo-matured oocytes than those matured with standard IVM medium (Paulsen et al. 2024). However, these studies only compared OSC-IVM with conventional IVM, without evaluating its effects relative to CAPA-IVM. CAPA-IVM has also been shown to improve maturation rates and IVM outcomes by acting as a meiosis inhibitor while enhancing COCs’ communication.

Optimising IVM supplementation remains a complex challenge. Recent research underscores the extensive and intricate nature of intercellular communication, which is critical for oocyte maturation both in vivo and in vitro. Enhancing this communication could significantly improve oocyte maturation rates and developmental competence in IVM. Future studies should focus on elucidating the molecular interactions between oocytes and granulosa cells during maturation in both settings.

Culture system for IVM

Immature oocytes within follicles naturally develop within a three-dimensional (3D) structure that supports their maturation. Conventional IVM systems often result in lower maturation rates due to the absence of an ovarian niche. To address this limitation, co-culture systems involving oocytes and granulosa cells have been explored, facilitating bidirectional communication and promoting oocyte maturation (Virant-Klun et al. 2018). Advancements in 3D culture systems now offer improved methods for simulating the ovarian microenvironment. These systems use biomaterials such as collagen, fibrin, or hyaluronan to create a supportive 3D matrix for cell growth (Peserico et al. 2023).

For instance, human oocytes cultured with granulosa cells in 3D systems with hyaluronan exhibit better viability and higher mitogen-activated protein kinase activity than those in 2D cultures. A notable innovation is the use of barium alginate matrices to embed oocytes and granulosa cells, which significantly enhances oocyte maturation rates compared to traditional 2D cultures (Torre et al. 2006). Similar findings have been observed in porcine studies, where 3D systems led to improved maturation rates, higher BMP15 levels in granulosa cells, and increased ATP levels in oocytes (Park et al. 2021). 3D culture systems more accurately mimic the follicular environment by maintaining the physiological structure of oocytes during meiotic maturation. These systems, particularly scaffold-free 3D microbial bioreactors, support the molecular, metabolic, and developmental functions of both oocytes and cumulus cells (Bebbere et al. 2021).

In addition, research by Mastrorocco et al. revealed that oocytes matured in 3D systems showed reduced expression of NLRP5 and KHDC3 genes linked to developmental competence compared to 2D systems, suggesting improved developmental potential (Mastrorocco et al. 2020). Interestingly, GDF9, another critical gene for oocyte competence, was downregulated in canine oocytes following 3D IVM, while the TLE6 expression remained stable (Mastrorocco et al. 2020). TLE6 is important for the formation of the cytoplasmic F-actin meshwork, critical for proper cell division and early embryonic survival (Monk et al. 2017).

The development of a 3D culture system has opened new avenues for human IVM research. With further development, 3D IVM technology holds the potential to significantly enhance the developmental capacity of human oocytes, making it a promising tool for reproductive medicine.

Clinical applications for IVM

The primary barrier to the widespread adoption of IVM is the limited high-quality evidence supporting its efficacy. A review of the literature reveals that only two well-conducted randomised controlled trials have directly compared reproductive outcomes from a single cycle of IVM to those from a single cycle of conventional ART. These studies reported lower cumulative ongoing pregnancy and live birth rates in IVM cycles, primarily due to the reduced number of usable embryos obtained (Vuong et al. 2020a, Zheng et al. 2022). Given the current state of IVM systems, careful patient selection remains essential for optimising clinical outcomes and should serve as the foundation of any clinical IVM programme.

A critical limitation for the broader application of IVM lies in the efficiency of immature oocyte recovery. The number of oocytes available for culture depends largely on follicle size and ovarian stimulation protocol, and current retrieval methods often yield a limited pool of competent immature oocytes. Development of more efficient retrieval techniques, particularly for accessing small antral follicles, could substantially enhance IVM outcomes.

At the same time, a deeper understanding of the molecular pathways controlling oocyte maturation is required to optimise culture systems. Key pathways include the cAMP/PKA axis, which maintains meiotic arrest; the CNP/NPR2 signalling cascade, which regulates the pre-IVM environment; and EGF-like growth factor signalling, which drives resumption of meiosis and cytoplasmic maturation. Insights into how these pathways interact in vivo and can be recapitulated in vitro will guide the refinement of IVM protocols and help bridge the gap between experimental success and routine clinical use.

Current evidence regarding offspring health following IVM is reassuring but limited in duration and scope. Buckett et al. reported no increased risk of congenital malformations compared to IVF or ICSI (Buckett et al. 2007). In women with PCOS, long-term follow-up (mean 7.5 years) showed that children conceived through IVM had no greater health risks compared with those conceived through IVF (Yu et al. 2019). A systematic review by Vuong et al. (2023) echoed these findings regarding perinatal safety, but cautioned that long-term, prospective follow-up is lacking (Vuong et al. 2023). Beilby et al. conducted a systematic review largely based on animal model data (including bovine, murine, and other species), examining offspring physiology following IVM, IVF, and ICSI (Beilby et al. 2023). The review suggested that IVM offspring may show trends such as increased birth weight, prolonged gestation, and alterations in metabolic or cardiovascular markers. However, findings were inconsistent across studies, and their relevance to human outcomes remains uncertain (Beilby et al. 2023). A recent animal study indicates that a specific OSC-IVM (ovarian support cell IVM) technique did not adversely affect embryo development, offspring health, or fertility across generations, further reinforcing the need for long-term and multi-generational monitoring (Marchante et al. 2025). A narrative review acknowledged that no adverse effects on offspring have been reported following IVM, while pointing out that formal long-term data remain limited (Das & Son 2023).

IVM for polycystic ovary syndrome (PCOS) patients

Patients with severe PCOS, as defined by the Rotterdam criteria, are ideal candidates for IVM, as they typically yield a high number of immature oocytes. This compensates for the inherently lower efficiency of IVM compared to standard IVF, making it a viable alternative for this patient population.

PCOS, a common reproductive disorder, affects 4–7% of all women of reproductive age and is responsible for approximately 50% of female infertility cases (Archer & Chang 2004, Azziz et al. 2005). Due to their elevated antral follicle counts, PCOS patients are at high risk of developing ovarian hyperstimulation syndrome (OHSS) during conventional controlled ovarian stimulation (COS) in IVF treatments. IVM of oocytes has been proposed as a safer alternative for PCOS patients, offering a significantly reduced risk of OHSS. The first live birth following IVM in a PCOS patient was reported by Trounson et al. (1994). A systematic review compared the effectiveness and safety of IVM versus COS in subfertile women with predicted hyperresponsivity to gonadotrophins, including those with PCOS (Archer & Chang 2004, Vuong et al. 2023). Results showed that live birth rates were not significantly lower with IVM compared to COS, particularly for hCG-triggered IVM. Obstetric and perinatal complication rates were generally comparable, although a potentially higher incidence of hypertensive disorders in pregnancy was observed (Archer & Chang 2004, Vuong et al. 2023). In addition to minimising complications such as OHSS, IVM offers other advantages over traditional IVF. These include lower costs due to reduced gonadotrophin drug use, enhanced safety, and greater convenience by eliminating the need for frequent follicular monitoring. These findings suggest that IVM can be a suitable and effective alternative for PCOS patients when appropriately evaluated and counselled. Further research is needed to optimise IVM protocols, refine culture systems, and identify patient populations that would benefit most from this approach.

IVM for resistant ovarian syndrome (ROS) patients

ROS is a rare endocrine disorder characterised by primary or secondary amenorrhoea, oligomenorrhoea, anovulation, and infertility (Mu et al. 2022). Potential causes include mutations in the FSH receptor or beta subunit, follicle-stimulating growth factor deficiency, abnormal gonadotrophin signalling, and autoimmune abnormalities (Jones & De Moraes-Ruehsen 1969, Mu et al. 2022). Patients with ROS often have a large number of primordial, primary, and small antral follicles but lack mature follicles or corpus lutea, resulting in blocked follicular development (Khor et al. 2020). These follicles are unresponsive to endogenous or exogenous FSH, making it impossible to produce mature oocytes in vivo or achieve pregnancy through traditional ART.

Historically, egg donation was the only option for ROS patients to conceive. However, advancements in IVM technology have provided a viable alternative. Kornilov et al. reported a successful case where a 23-year-old ROS patient underwent IVM, resulting in a healthy baby (Kornilov et al. 2021). Similarly, Galvao et al. found a maturation rate of 27.5% in non-hCG-triggered IVM cycles and 44.4% in hCG-triggered IVM cycles, with an overall live birth rate of 16.7% per cycle and 33% per patient (Galvao et al. 2018). With ongoing improvements in IVM technology, it has become a reliable and promising option for ROS patients.

IVM for fertility preservation

Fertility preservation in female cancer patients is an emerging indication for IVM. While COS followed by oocyte retrieval remains the standard approach, IVM provides an alternative for patients who cannot undergo a full IVF cycle. This is particularly relevant in emergency situations, such as for those diagnosed with Hodgkin lymphoma or leukaemia. In these cases, immature oocytes retrieved transvaginally can be matured in vitro and subsequently vitrified as an option for fertility preservation (Sonigo et al. 2016). Cancer patients who need to start chemotherapy immediately, decline ovarian tissue cryopreservation (OTC), or are medically unfit for abdominal surgery required for OTC are also suitable candidates for this approach. However, evidence regarding the reproductive potential of vitrified IVM oocytes remains limited, with only one reported live birth to date (Grynberg et al. 2020). In addition, no studies have yet reported the use of biphasic IVM protocols for oocytes collected transvaginally from cancer patients.

IVM of immature oocytes obtained from extracorporeal ovarian tissue is referred to as ovarian tissue-derived oocyte IVM (OTO-IVM) or ex vivo IVM. This technique remains experimental due to the low number of reported live births and the lack of comprehensive human safety data (Segers et al. 2020). Nevertheless, OTO-IVM holds considerable promise for cancer patients. Kirillova et al. conducted a study in which ten patients who underwent unilateral oophorectomy due to gynaecological tumours had their ovaries divided into two equal halves. COCs from these hemi-ovaries were subjected to either standard IVM or CAPA-IVM, with CAPA-IVM yielding a significantly higher MII rate than standard IVM (56 versus 35%, respectively). Notably, only embryos from CAPA-IVM-developed oocytes reached the blastocyst stage (Kirillova et al. 2021a).

Ovarian tissue cryopreservation and transplantation represent important fertility restoration strategies for young women undergoing cancer treatment. Orthotopic ovarian cortex transplantation, the most common approach, restores tissue to its original location (Fortin et al. 2019). In contrast, heterotopic transplantation involves placing the tissue in alternative sites. However, transplanted ovarian tissues often exhibit limited responsiveness to exogenous gonadotrophins, with only a few follicles reaching ovulatory size (≥18 mm). In most cases, oocyte maturation cannot typically be triggered with LH analogues during COS (Wang et al. 2016). In such cases, IVM offers a viable solution by maturing oocytes retrieved from small to mid-antral follicles (2–10 mm) in the transplanted tissue. These in vitro matured oocytes can be used for cryopreservation or IVF-ET. Remarkably, ovarian cortex transplantation to the anterior abdominal wall has even resulted in the successful live birth of twins, highlighting the potential of combining IVM with transplantation techniques (Stern et al. 2014).

Rescue IVM for conventional IVF

In conventional IVF cycles, immature oocytes are often retrieved, especially in patients with conditions such as PCOS, advanced maternal age, or premature ovarian failure, where a higher proportion of immature oocytes is common. Recent advances in rescue IVM techniques provide an opportunity to mature these discarded oocytes in vitro using optimised culture conditions, enabling their use for fertilisation. Studies have demonstrated that rescue IVM does not increase chromosomal abnormalities in GV and metaphase I (MI) stage oocytes (Esbert et al. 2024). Similarly, another study demonstrated that maturing additional oocytes through this technique improves clinical outcomes in patients with fewer than nine mature (MII) oocytes retrieved (Wei et al. 2023). This method not only boosts the number of viable oocytes for IVF but also serves as an ethical alternative to discarding potentially usable gametes.

Conclusion and future perspectives

IVM has emerged as a valuable assisted reproductive technology, offering improved oocyte utilisation and a reduced risk of OHSS. Significant advancements have been made to enhance the maturation and quality of immature oocytes, resulting in improved pregnancy outcomes. Future directions for IVM research should prioritise the collection and analysis of high-quality clinical data to establish its effectiveness and safety. Further systematic reviews and meta-analyses, particularly of CAPA-IVM and antioxidant supplementation trials, are warranted to generate stronger, evidence-based recommendations for clinical practice.

While early reports demonstrate encouraging maturation rates, embryo development, and live birth outcomes, these findings are based on relatively small sample sizes and heterogeneous study designs. Large-scale, multicentre trials with standardised protocols are needed to generate more definitive evidence. Equally important is the systematic evaluation of offspring health following IVM. Current data suggest that children born after IVM do not present major differences in perinatal outcomes compared with IVF/ICSI. However, the number of cases reported remains limited. Long-term follow-up studies focussing on growth, neurodevelopment, and metabolic health are necessary to confirm the safety of this technique. Such evidence will be indispensable for establishing IVM as a safe and effective alternative to conventional ART in broader clinical practice.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the work reported.

Funding

This work is supported by Shandong Government-sponsored study abroad programme scholarship (non-education sector). This work is also supported by the Canadian Institutes of Health Research (143317). In addition, this work is supported by the National Science and Technology Council (NSTC 113-2314-B-039-055).

Author contribution statement

ML, JL, and PCKL were responsible for overall conceptualisation and design of the manuscript. JC, H-MC, and PCKL were responsible for composition and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.