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Review
. 2022 Mar;157(3):333-345.
doi: 10.1007/s00418-021-02058-w. Epub 2021 Nov 30.

Insights into in vivo follicle formation: a review of in vitro systems

Ren Tanimoto  1 , Kyota Yoshida  1 , Shinya Ikeda  1 , Yayoi Obata  2
Affiliations
Review

Insights into in vivo follicle formation: a review of in vitro systems

Ren Tanimoto et al. Histochem Cell Biol. 2022 Mar.

Abstract

In vitro systems capable of reconstituting the process of mouse oogenesis are now being established to help develop further understanding of the mechanisms underlying oocyte/follicle development and differentiation. These systems could also help increase the production of useful livestock or genetically modified animals, and aid in identifying the causes of infertility in humans. Recently, we revealed, using an in vitro system for recapitulating oogenesis, that the activation of the estrogen signaling pathway induces abnormal follicle formation, that blocking estrogen-induced expression of anti-Müllerian hormone is crucial for normal follicle formation, and that the production of α-fetoprotein in fetal liver tissue is involved in normal in vivo follicle formation. In mouse fetuses, follicle formation is not carried out by factors within the ovaries but is instead orchestrated by distal endocrine factors. This review outlines findings from genetics, endocrinology, and in vitro studies regarding the factors that can affect the formation of primordial follicles in mammals.

Keywords: Anti-Müllerian hormone; Estrogen; Oogenesis; Ovarian culture; Primordial follicle formation; α-fetoprotein.

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Conflict of interest statement

The authors declare that there are no conflicts of interests.

Figures

Fig. 1
Fig. 1
Failure in secondary follicle formation in in vitro-cultured mouse fetal ovary. a Fetus-derived ovary on day 17 of culture. Ovaries were cultured with a conventional medium containing fetal bovine serum (FBS). This ovary contains numerous growing oocytes, but they have not formed a clearly visible follicle structure. b Newborn-derived ovary on day 10 under the same culture conditions. Growing oocytes are enclosed within secondary follicles. c Fetus-derived ovary on day 17 of culture with a medium containing FBS and estrogen receptor antagonist ICI 182,780. Hypoplastic follicles were recovered by inhibiting estrogen receptors. Bars, 200 µm
Fig. 2
Fig. 2
Abnormal arrangement of laminin basement membrane in in vitro-cultured mouse fetal ovary. Follicular basement membrane is labeled in red and oocytes in green using anti-Laminin and anti-DDX4 antibodies, respectively. The areas surrounded by white dashed lines are examples of a follicle area that should be enclosed with laminin. Arrow heads indicate absence of laminin arrangement. a Fetus-derived ovary cultured with a conventional medium containing FBS on day 17 of culture. The wrapping an individual follicle by laminin envelope is interrupted. b Fetus-derived ovary cultured with a medium containing SPS instead of FBS on day 17 of culture. If FBS that possibly provides estrogen and/or estrogen-like substances is excluded in culture during primordial follicle assembly, laminin envelope completely covers single follicle. c Fetus-derived ovary cultured with medium supplemented with exogenous AMH under the same conditions as in b on day 17 of culture. AMH is ectopically expressed by activation of estrogen signaling. Even if FBS-free medium is used, laminin envelope again fails in enclosing individual follicles in the cultured ovary by AMH addition to the medium. Bars, 100 µm
Fig. 3
Fig. 3
Premature expression of AMH in in vitro-cultured mouse fetal ovary. a AMH expression in in vivo-derived ovaries. AMH is not expressed in an ovary from 2-day-old mouse (left), but detected in granulosa cells in primary or greater follicles in an ovary from 6-day-old mouse (right). b AMH expression in an ovary cultured with a conventional medium containing FBS. AMH is prematurely expressed in an ovary on day 9, which corresponds to 2-day-old mouse in vivo (left). Arrow heads indicate premature AMH expression in ovarian somatic cells. c AMH expression in an ovary cultured with a medium containing FBS and ICI. Repression of the ectopic expression of AMH in an ovary on day 9 is achieved by blocking of estrogen signaling (left). In contrast, AMH expression is prominent in primary follicles that are formed in an ovary on day 13 (right). AMH is labeled in green and nuclei in magenta using DAPI. Reuse and modify the images published in Development (Tanimoto et al. 2021). Bars, 50 µm
Fig. 4
Fig. 4
Follicles formed in a cultured ovary by blocking estrogen receptors. Ovarian sections of 10-day-old mice (a) and fetus-derived ovaries cultured in a medium containing FBS and ICI on day 17 (b). This follicle is enclosed with only a few theca cells (arrow heads). Bars, 25 µm
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