Follicle development as an orchestrated signaling network in a 3D organoid

Abstract

The ovarian follicle is the structural and functional unit of the ovary, composed of the female gamete (the oocyte) and supportive somatic cells. Follicles are not only the source of a female's germ cell supply, but also secrete important hormones necessary for proper endocrine function. Folliculogenesis, the growth and maturation of the follicular unit, is a complex process governed by both intrafollicular crosstalk and pituitary-secreted hormones. While the later stages of this process are gonadotropin-dependent, early folliculogenesis appears to be controlled by the ovarian microenvironment and intrafollicular paracrine and autocrine signaling. In vitro follicle culture remains challenging because of the limited knowledge of growth factors and other cytokines influencing early follicle growth. Here we discuss the current state of knowledge on paracrine and autocrine signaling influencing primary follicles as they develop into the antral stage. Given the importance of intrafollicular signaling and the ovarian microenvironment, we reviewed the current engineering approaches for in vitro follicle culture, including 3D systems using natural hydrogels such as alginate and synthetic hydrogels such as poly(ethylene glycol). Our discussion is focused on what drives the proliferation of granulosa cells, development of the thecal layer, and antrum formation-three processes integral to follicle growth up to the antral stage. Further research in this area may reveal the mechanisms behind these complex signaling relationships within the follicle, leading to more successful and physiologically-relevant in vitro culture methods that will translate well to clinical applications.

Keywords: 3D culture; Biomaterials; Crosstalk; Cytokines; Ovarian folliculogenesis.

Fig. 1

Schematic detailing the stages of mammalian folliculogenesis taken from Edson et al., “The mammalian ovary from genesis to revelation,” Endocrine Reviews, 2009, 30, 6, pp. 624–712 by permission of Oxford University Press [2]

Fig. 2

a) Oocyte-secreted BMP-15 promotes mitosis and kit ligand expression in granulosa cells and b) granulosa cell-secreted kit ligand promotes oocyte growth but is suppressed by oocyte-secreted GDF-9

Fig. 3

Growth factors and nutrients are more capable of promoting follicle growth when the structure is cultured in 3D (b) versus in 2D culture (a), especially given the complex crosstalk between cells within the follicle (c)

Fig. 4

a) Co-culture with feeder cells can provide follicles with secreted factors in order to enhance growth and b) multiple follicle culture can also enhance growth and development via interfollicular crosstalk