Comprehensive Review of In Vitro Human Follicle Development for Fertility Restoration: Recent Achievements, Current Challenges, and Future Optimization Strategies

Abstract

Ovarian tissue cryopreservation (OTC) and subsequent transplantation (OTT) is a fertility preservation technique widely offered to prepubertal girls and young fertile women who need to undergo oncological treatment but are at a high risk of infertility. However, OTT is not considered safe in patients with certain diseases like leukemia, Burkitt's lymphoma, and ovarian cancer because of the associated risk of malignant cell reintroduction. In vitro follicle development has therefore emerged as a promising means of obtaining mature metaphase II (MII) oocytes from the primordial follicle (PMF) pool contained within cryopreserved ovarian tissue, without the need for transplantation. Despite its significant potential, this novel approach remains highly challenging, as it requires replication of the intricate process of intraovarian folliculogenesis. Recent advances in multi-step in vitro culture (IVC) systems, tailored to the specific needs of each follicle stage, have demonstrated the feasibility of generating mature oocytes (MII) from early-stage human follicles. While significant progress has been made, there is still room for improvement in terms of efficiency and productivity, and a long way to go before this IVC approach can be implemented in a clinical setting. This comprehensive review outlines the most significant improvements in recent years, current limitations, and future optimization strategies.

Keywords: bovine ovarian tissue; follicle activation; folliculogenesis; human ovarian tissue; in vitro culture; in vitro growth; in vitro maturation.

Figure 4

(A) Schematic representation of morphological parameters used to assess oocyte competence. Created with BioRender.com. (B) Bright field image showing a MII oocyte with and enlarged abnormal polar body. Reproduced with permission from [10]. Confocal images displaying (C) equatorially aligned chromosomes (blue) and meiotic spindles (green), and (D) chromosomal misalignment. Reproduced with permission from [128].

Figure 1

Illustrative representation of the multi-step IVC system supporting in vitro follicle development from PMFs contained within the ovarian cortex to mature MII oocytes, as described by McLaughlin et al., 2018 [10]. Step 1: PMF activation. Step 2: Isolation of secondary follicles (a) and subsequent individual culture in V-shaped wells until the antral follicle stage (b). Step 3: Mechanical dissection of COCs. Step 4: Oocyte IVM until reaching the MII stage. Created with BioRender.com, accessed on 1 January 2024.

Figure 2

When the Hippo pathway is active (left), SAV1 and MST1/2 complex phosphorylates LATS1/2 and MOB1. Activated LATS1/2 subsequently phosphorylates the YAP/TAZ complex, resulting in cytoplasmic retention and no DNA transcription. Conversely, when the Hippo pathway is disrupted (right) during ovarian tissue fragmentation, dephosphorylated YAP1/TAZ translocates to the nucleus to bind with TEAD, leading to transcriptional activation of genes associated with cell growth and survival. Created with BioRender.com. Abbreviations: LATS1/2 (large tumor suppressor kinase 1/2); MOB1 (Mps one binder 1); MST1/2 (mammalian Ste20-like serine/threonine kinases 1/2); P (phosphorylated); SAV1 (protein salvador homolog 1; TEAD (TEA domain family members).

Figure 3

The PI3K/AKT pathway is activated following binding of several growth factors to tyrosine-kinase receptors on cell membranes. This interaction leads to PIP2 transformation into PIP3. AKT is then phosphorylated and translocated to the nucleus where it phosphorylates FOXO1, resulting in its export into the cytoplasm. After translocation, inactive FOXO1 ceases its inhibitory effect over transcriptional factors, enhancing follicle activation and growth. mTOR, another AKT downstream effector regulates protein synthesis and cell growth through ribosomal biosynthesis, also promoting follicle activation. PTEN, on the other hand, counteracts the conversion of PIP2 into PIP3, inhibiting the pathway. Activators are represented in green and inhibitors in red. Created with BioRender.com. Abbreviations: AKT (protein kinase B); bFGF (basic fibroblast growth factor); EGF (epidermal growth factor); FOXO1 (forkhead box O1); mTOR (mammalian target of rapamycin); P (phosphorylated); PDGF (platelet-derived growth factor); PDK1 (phosphoinositide-dependent kinase-1); PI3K (phosphatidylinositol 3-kinase); PIP2 (phosphatidylinositol 4,5-bisphosphate); PIP3 (phosphatidylinositol 3,4,5-trisphosphate); PTEN (phosphatase and tensin homolog); rpS6 (ribosomal protein S6); S6K1 (S6 kinase 1); TSC1 and TSC2 (tuberous sclerosis complex 1 and 2); VEGF (vascular endothelial growth factor).