Human ovarian folliculogenesis is a highly regulated and complex process. Characterization of follicular cell signatures during this dynamic process is important to understand follicle fate (to grow, become dominant, or undergo atresia). The transcriptional signature of human oocytes and granulosa cells (GCs) in early-growing and ovulatory follicles have been previously described; however, that of oocytes with surrounding GCs in small antral follicles have not been studied yet. Here, we have generated a unique dataset of single-cell transcriptomics (SmartSeq2) consisting of the oocyte with surrounding GCs from several individual (non-dominant) small antral follicles isolated from adult human ovaries. We have identified two main types of (healthy) follicles, with a distinct oocyte and GC signature. Using the CellphoneDB algorithm, we then investigated the bi-directional ligand–receptor interactions regarding the transforming growth factor-β (TGFβ)/bone morphogenetic protein (BMP), wingless-type (MMTV)-integration site (WNT), NOTCH, and receptor tyrosine kinases (RTK) signaling pathways between oocyte and GCs within each antral follicle type. Our work not only revealed the diversity of small antral follicles, but also contributes to fill the gap in mapping the molecular landscape of human folliculogenesis and oogenesis.

Current strategies for fertility preservation include the cryopreservation of embryos, mature oocytes or ovarian cortical tissue for autologous transplantation. However, not all patients that could benefit from fertility preservation can use the currently available technology. In this regard, obtaining functional mature oocytes from ovarian cortical tissue in vitro would represent a major breakthrough in fertility preservation as well as in human medically assisted reproduction. In this study, we have used a microfluidics platform to culture cryopreserved-thawed human cortical tissue for a period of 8 days and evaluated the effect of two different flow rates in follicular activation and growth. The results showed that this dynamic system supported follicular development up to the secondary stage within 8 days, albeit with low efficiency. Surprisingly, the stromal cells in the ovarian cortical tissue were highly sensitive to flow and showed high levels of apoptosis when cultured under high flow rate. Moreover, after 8 days in culture, the stromal compartment showed increase levels of collagen deposition, in particular in static culture. Although microfluidics dynamic platforms have great potential to simulate tissue-level physiology, this system still needs optimization to meet the requirements for an efficient in vitro early follicular growth.

Ovarian dysfunction poses significant threats to the health of female individuals. Ovarian failure can lead to infertility due to the lack or inefficient production of fertilizable eggs. In addition, the ovary produces hormones, such as estrogen and progesterone, that play crucial roles not only during pregnancy, but also in maintaining cardiovascular, bone, and cognitive health. Decline in estrogen and progesterone production due to ovarian dysfunction can result in menopausal-associated syndromes and lead to conditions, such as osteoporosis, cardiovascular disease, and Alzheimer’s disease. Recent advances in the design of bioengineered three-dimensional (3D) ovarian models, such as ovarian organoids or artificial ovaries, have made it possible to mimic aspects of the cellular heterogeneity and functional characteristics of the ovary in vitro. These novel technologies are emerging as valuable tools for studying ovarian physiology and pathology and may provide alternatives for fertility preservation. Moreover, they may have the potential to restore aspects of ovarian function, improving the quality of life of the (aging) female population. This review focuses on the state of the art of 3D ovarian platforms, including the latest advances modeling female reproduction, female physiology, ovarian cancer, and drug screening.

Study question Can we obtain secondary follicles from cultured cryopreserved-thawed human cortical tissue? Summary answer We obtained a comparable percentage of secondary follicles after culture of cryopreserved-thawed human cortical tissue to that reported after culture of fresh human cortical tissue. What is known already The complete in vitro maturation of oocytes starting from primary oocytes (present in unilaminar follicles) until mature MII oocytes has been achieved previously by two different multi-step culture protocols. These protocols were applied to culture fresh ovarian cortical tissue from adult cisgender donors. However, the efficiency of these two protocols for growing unilaminar follicles to secondary follicles starting from cryopreserved cortical tissue from cisgender patients has not been investigated. As the ovarian tissue available for fertility preservation is cryopreserved, it is important to understand the potential of cryopreserved unilaminar follicles to mature in vitro. Study design, size, duration Cryopreserved ovarian cortical tissue from 4 cisgender adult donors was used for in vitro culture. According to the existing culture protocols, cortical ovarian tissue was cultured either for 8 days using the first step medium (Telfer’s medium) reported by M.McLaughlin 2018 (doi: 10.1093/molehr/gay002) or for 7 and 21 days using the first step medium (Xu’s medium) reported by Xu 2021 (doi: 10.1093/humrep/deab003). Participants/materials, setting, methods Ovarian cortical tissue obtained from adult cisgender donors undergoing oophorectomy for fertility preservation purposes was cryopreserved before chemotherapy. The cryopreserved ovarian cortical tissue was thawed and cut into small pieces. Several pieces were fixed immediately (day 0), the others were either cultured in Telfer’s medium or Xu’s medium. After culture, follicle survival, growth and morphology were assessed by histology and immunofluorescence. Main results and the role of chance We performed quantification of the different follicular stages (primordial, primary, secondary and atretic) after culture and observed that the percentage of secondary follicles increased independently of the culture media used. However, the ovarian cortical tissue cultured using Telfer’s protocol resulted in a higher percentage of secondary follicles and lower percentage of atretic follicles compared to that using Xu’s protocol. After culture, the ovarian cortical tissue was further immunostained for TUNEL, PCNA, COLLIV, STAR, AMH and KRT19. We observed that secondary follicles present in ovarian cortical tissue cultured in Telfer’s medium showed more proliferative (PCNA+) FOXL2+ granulosa cells and less apoptotic (TUNEL+) stromal cells. By contrast, ovarian cortical tissue cultured in Xu’s medium showed less proliferative (PCNA+) FOXL2+ granulosa cells and more apoptotic (TUNEL+) stromal cells. Moreover, the expression of AMH was high in granulosa cells of secondary follicles present in ovarian cortical tissue cultured in Telfer’s medium and low in Xu’s medium. Finally, in both Telfer’s and Xu’s medium cultures the expression of KRT19 was low in granulosa cells of secondary follicles present in ovarian cortical tissue. Limitations, reasons for caution The number of donors was limited and the study lacked fresh ovarian cortical tissue as control. Only the first step in Telfers’ and Xu’s protocol was performed, hence further culture is required to determine whether complete maturation of oocytes in vitro is possible starting from cryopreserved human cortical tissue. Wider implications of the findings Our study showed evidence of follicular growth in cryopreserved-thawed human ovarian cortical tissue after a period of in vitro culture. This is an important first step to achieve in vitro maturation of oocytes from cryopreserved-thawed human ovarian cortical tissue that could be used for clinical applications. Trial registration number NOT APPLICABLE

Study question Can cryopreserved-thawed human ovarian cortex be cultured in vitro to produce growing follicles as efficiently as from fresh samples? Summary answer In contrast to fresh samples, cryopreserved-thawed ovarian cortex did not develop morphologically normal growing follicles, even when adding follicular growth stimulators. What is known already A multistep culture system has been shown to produce mature oocytes from immature follicles present in the human cortex, without the addition of follicular growth stimulators. However, these promising results were achieved using fresh human ovarian cortex tissue donated from cisgender women, which is not available for the purpose of fertility preservation. It remains unclear whether this multistep culture system can be used to mature follicles from cryopreserved-thawed human ovarian cortex. In addition, the applicability of the culture system to mature follicles in vitro has not been investigated using ovarian tissue from transmasculine people. Study design, size, duration Fresh and cryopreserved ovarian cortex tissue isolated from the same transmasculine people were cultured during 8 days (first step of the culture system) with or without the addition of a PI3K/Akt promotor, Sphingosine-1-Phosphate (S1P). The ovarian cortex fragments were collected a day 0 and day 8 for downstream morphological and quantitative analysis of the follicular population. This study also included data of in vitro culture of cryopreserved samples from oncological cisgender women. Participants/materials, setting, methods Ovarian cortex tissue was collected from six testosterone user transmasculine people (25,8 ± 4,9 years) and three oncological cisgender women (24,0 ± 6,0 years). Fresh cortex was either directly cultured or previously cryopreserved. All ovarian fragments were cultured in the same conditions for 8 days. After the culture period, follicular population was quantified on histological sections (haematoxylin-eosin) and compared to samples of day 0. Follicular cell types, cell proliferation and apoptosis were assessed by immunofluorescence. Main results and the role of chance Fresh ovarian cortex tissue isolated from transmasculine people showed morphologically normal primary follicles (PFs) and secondary follicles (SFs) after 8 days of culture and no antral follicles were observed. Moreover, the addition of the follicular growth stimulator, S1P, to the culture medium significantly increased the number of growing follicles (PFs, P < 0.05; SFs, P < 0.01). The addition of S1P stimulated the emergence of SFs that showed expression of anti-mullerian hormone (AMH), contained proliferative (PCNA+) granulosa cells and showed low levels of apoptotic cells (TUNEL assay). Structurally, these in vitro grown SFs showed an intact basement membrane assessed by the expression of collagen IV. By contrast, using the same culture protocol but starting from cryopreserved-thawed cortex tissue from the same patients resulted in a very low number of viable growing follicles (PFs and SFs) even in the presence of S1P. Lastly, when using the same culture protocol on cryopreserved-thawed samples from oncological cisgender women, we observed a low efficiency of follicular growth, comparable to that observed in cryopreserved ovarian tissue from (age-matched) transmasculine people. Limitations, reasons for caution Limitations of this study are: small sample size and lack of fresh cortex tissue from oncological cisgender women. Due to the high variability of follicular density among patients, the use of a larger number of samples is recommended. Wider implications of the findings Optimization of culture systems to grow and mature follicles in the ovarian cortex of cryopreserved samples (both from cis and trans persons) is urgently needed. Current culture protocols can be applied to fresh samples, but further characterization is needed to consider this as an alternative in fertility preservation treatment. Trial registration number Not applicable