Identification of cells migrating from the thecal layer of ovarian follicles

Two-Decade Experience of Royan Institute in Obtaining Mature Oocyte from Cryopreserved Ovarian Tissue: In Vitro and In Vivo Approaches

Ovarian tissue cryopreservation (OTC) holds promise for preservation of fertility among women who have lost their fertility due to diseases such as cancer. OTC has significantly assisted such cases by helping them maintain normal hormonal levels and menstrual cycles. Appropriate strategies to develop and extract mature oocytes from OTC could overcome a range of complications that are associated with ovarian dysfunction, caused by aging, and primary or secondary ovarian insufficiency. Scientists from different departments at The Royan Institute (Tehran, Iran) have been conducting studies to find the best way to extract mature oocytes from animal and human cryopreserved ovarian tissues. The various studies conducted in this area in the past 20 years, by researchers of the Royan Institute, are collated and provided in this review, in order to provide an idea of where we are now in the area of fertility preservation.

In vitro and in vivo mouse follicle development in ovaries and reaggregated ovaries

Follicle development requires complex and coordinated interactions between both the oocyte and its associated somatic cells. In ovarian dysfunction, follicle development may be abnormal due to defective somatic cell function; for example, premature ovarian insufficiency or malignancies. Replacing defective somatic cells, using the reaggregated ovary (RO) technique, may 'rescue' follicle development. ROs containing mature follicles have been generated when transplanted to a host mouse to develop. We have developed a RO culture technique and the aims were to determine how follicle development differed between transplanted and cultured ROs, and the influence of ovarian age (P2 vs P6). Mouse ROs were cultured for 14 days; P2 and P6 ovaries cultured as Controls. Follicle development was compared to ROs transplanted for 14 days and ovaries from P16 and P20 mice. ROs generated from either P2 or P6 exhibited similar follicle development in culture whereas in vivo follicle development was more advanced in P6 ROs. Follicles were more developed in cultured ROs than transplanted ROs. However, follicles in cultured ROs and ovaries had smaller oocytes with fewer theca and granulosa cells than in vivo counterparts. Our results demonstrate the fluidity of follicle development despite ovary dissociation and that environment is more important to basal lamina formation and theca cell development. Furthermore, follicle development within cultured ROs appears to be independent of oocyte nest breakdown and primordial follicle formation in source ovaries. Our results highlight the need for understanding follicle development in vitro, particularly in the development of the RO technique as a potential fertility treatment.

Formation of multiple-oocyte follicles in culture

Basement membranes are found in every organ of the body. They provide structure and a selective filter for molecules. The ovary is no different with the follicular basal lamina (FBL) separating the granulosa and theca cells, facilitating regulation of the changing follicular environment providing appropriate conditions for the developing oocyte. The FBL is modified in C1galt1 Mutant mice (C1galt1 ^FF:ZP3Cre) resulting from oocyte-specific deletion of C1galt1. Changes in the FBL lead to follicles joining to generate multiple-oocyte follicles (MOFs); where two or more oocytes are contained within a single follicle. This study aimed to determine if single-oocyte follicles could join in culture to become MOFs by co-culturing preantral follicles from Control or Mutant mice. Co-cultured follicles from both Control and Mutant follicles could superficially fuse (73% of Control follicle pairs; 84% of Mutant). Confocal microscopy revealed alterations in the organization of the space between follicles but was unable to discern MOFs. When co-cultured follicle pairs were embedded, sectioned and stained with haematoxylin, it was revealed that MOFs had formed from 50% of Mutant follicle pairs but none from Control follicle pairs. In conclusion, MOFs can form from C1galt1 Mutant follicles in culture and this model is a useful tool to elucidate the role of the oocyte in follicle development and the generation and function of the FBL. Furthermore, understanding the relationship between oocyte function and FBL generation will likely provide insight into optimizing conditions for follicle culture, which is important for fertility treatments and ART.

Comparison of two related lines of tauGFP transgenic mice designed for lineage tracing

Background

The tauGFP reporter fusion protein is produced nearly ubiquitously by the TgTP6.3 transgene in TP6.3 mice and its localisation to microtubules offers some advantages over soluble GFP as a lineage marker. However, TgTP6.3^Tg/Tg homozygotes are not viable and TgTP6.3^Tg/− hemizygotes are smaller than wild-type. TP6.4 mice carry the TgTP6.4 transgene, which was produced with the same construct used to generate TgTP6.3, so we investigated whether TgTP6.4 had any advantages over TgTP6.3.

Results

Although TgTP6.4^Tg/Tg homozygotes died before weaning, TgTP6.4^Tg/− hemizygotes were viable and fertile and only males were significantly lighter than wild-type. The TgTP6.4 transgene produced the tauGFP fusion protein by the 2-cell stage and it was widely expressed in adults but tauGFP fluorescence was weak or absent in several tissues, including some neural tissues. The TgTP6.4 transgene expression pattern changed over several years of breeding and mosaic transgene expression became increasingly common in all expressing tissues. This mosaicism was used to visualise clonal lineages in the adrenal cortex of TgTP6.4^Tg/− hemizygotes and these were qualitatively and quantitatively comparable to lineages reported previously for other mosaic transgenic mice, X-inactivation mosaics and chimaeras. Mosaicism occurred less frequently in TP6.3 than TP6.4 mice and was only observed in the corneal epithelium and adrenal cortex.

Conclusions

Mosaic expression makes the TgTP6.4 transgene unsuitable for use as a conventional cell lineage marker but such mosaicism provides a useful system for visualising clonal lineages that arise during development or maintenance of adult tissues. Differences in the occurrence of mosaicism between related transgenic lines, such as that described for lines TP6.3 and TP6.4, might provide a useful system for investigating the mechanism of transgene silencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0149-x) contains supplementary material, which is available to authorized users.

Culture and co-culture of mouse ovaries and ovarian follicles

The mammalian ovary is composed of ovarian follicles, each follicle consisting of a single oocyte surrounded by somatic granulosa cells, enclosed together within a basement membrane. A finite pool of follicles is laid down during embryonic development, when oocytes in meiotic arrest form a close association with flattened granulosa cells, forming primordial follicles. By or shortly after birth, mammalian ovaries contain their lifetime's supply of primordial follicles, from which point onwards there is a steady release of follicles into the growing follicular pool. The ovary is particularly amenable to development in vitro, with follicles growing in a highly physiological manner in culture. This work describes the culture of whole neonatal ovaries containing primordial follicles, and the culture of individual ovarian follicles, a method which can support the development of follicles from an immature through to the preovulatory stage, after which their oocytes are able to undergo fertilization in vitro. The work outlined here uses culture systems to determine how the ovary is affected by exposure to external compounds. We also describe a co-culture system, which allows investigation of the interactions that occur between growing follicles and the non-growing pool of primordial follicles.