The molecular and cellular basis of gonadal sex reversal in mice and humans

Abstract

The mammalian gonad is adapted for the production of germ cells and is an endocrine gland that controls sexual maturation and fertility. Gonadal sex reversal, namely, the development of ovaries in an XY individual or testes in an XX, has fascinated biologists for decades. The phenomenon suggests the existence of genetic suppressors of the male and female developmental pathways and molecular genetic studies, particularly in the mouse, have revealed controlled antagonism at the core of mammalian sex determination. Both testis and ovary determination represent design solutions to a number of problems: how to generate cells with the right properties to populate the organ primordium; how to produce distinct organs from an initially bipotential primordium; how to pattern an organ when the expression of key cell fate determinants is initiated only in a discrete region of the primordium and extends to other regions asynchronously; how to coordinate the interaction between distinct cell types in time and space and stabilize the resulting morphology; and how to maintain the differentiated state of the organ throughout the adult period. Some of these, and related problems, are common to organogenesis in general; some are distinctive to gonad development. In this review, we discuss recent studies of the molecular and cellular events underlying testis and ovary development, with an emphasis on the phenomenon of gonadal sex reversal and its causes in mice and humans. Finally, we discuss sex-determining loci and disorders of sex development in humans and the future of research in this important area.

FIGURE 1

Model of gene regulatory networks in mammalian sex determination. Sex-determining genetic activity in the XY pre-Sertoli cell (PSC) is dominated by SRY-SOX9-FGF9/FGFR2. This genetic pathway is stabilized by mutually positive (curved arrows) interactions between SOX9 and FGF9, resulting in continued expression of SOX9 in the Sertoli cell lineage, in combination with antagonism (hammered lines) of ovary-determining genes/proteins (shown in red). Only a short period of SRY expression is required to upregulate Sox9 from its basal levels in the bipotential gonad. FGF9 and PGD₂ support paracrine signaling (arrows exiting from lower cell), resulting in the recruitment of additional supporting cell precursors (upper PSC) that express SRY and/or SOX9 to the Sertoli cell lineage. The secreted molecules FGF9 and desert hedgehog (DHH) also function in masculinizing the germ cell and steroidogenic cell lineages, respectively. This testis-determining gene regulatory network and its downstream targets (‘testis genes’) initiate a variety of morphogenetic processes, including the migration of endothelial cells from the adjacent mesonephros, which are crucial to the formation of testis cords and the associated coelomic blood vessel. In the female (XX) gonad, the absence of SRY in pregranulosa cells (PGC) results in successful antagonism of SOX9/FGF9 by ovary-determining gene products. This causes commitment to the granulosa cell lineage and permits retinoic acid (RA) to drive ovarian germ cells into meiosis. Note the linear core of the testis-determining pathway and the somewhat more modular appearance of the ovary-determining network. Note also that NR5A1 functions at distinct stages—this is likely to be true of several molecules included. Question marks indicate missing components or uncertainty in terms of regulatory relationships. IR = INSR, INSRR, and IGF1R.

FIGURE 2

Cell lineages of the embryonic/fetal ovary and testis. (a) Color-coded diagram showing the arrangement of the bipotential somatic (supporting and steroidogenic) and germ cell lineages of the early gonad (around E11.5 in the mouse). The gonad forms on the ventromedial surface of the mesonephros. (b) In the female gonad, from around E13.5, germ cells and pregranulosa (supporting) cells are arranged into structures called ovigerous cords (oc), surrounded by interstitial mesenchymal cells (including steroidogenic precursors). These structures are not visible by light microscopy but can be revealed by immunostaining. In contrast, robust testis cords comprising germs cells and Sertoli cells (also supporting cells) are visible at a comparable stage in males, as is the prominent coelomic vessel (cv) that is formed from migratory mesonephric endothelial cells. Two interstitial cell types are prominent at this stage: peritubular myoid cells (not shown) that surround the testis cords and the steroidogenic Leydig cells, which produce androgens. (c) At around birth in the mouse, the ovigerous cords break down into primary follicles, containing a single oocyte surrounded by layers of granulosa cells and then theca (steroidogenic) cells.

FIGURE 3

Ovotestis development in embryos haploinsufficient for Map3k4. Ovotestis development (previously known as true hermaphroditism) is associated with partial disruption to the testis-determining pathway. The examples from the mouse shown here are caused by a combination of three genetic factors: haploinsufficiency (−/+) for the gene Map3k4, a C57BL/6J (B6)-derived set of autosomes and X chromosome, and a Y chromosome from the AKR strain (Y^AKR). The wild-type (+/+) B6 Y^AKR gonads have ordered stacks of testis cords (around 10), highlighted by the expression of the Sertoli cell marker, Sox9, and absence of the ovarian somatic marker, Wnt4. Ovotestes are characterized by regions of testicular tissue (brackets) that are immediately adjacent to ovarian tissue in the same gonad (revealed by marker gene expression). The ovarian regions of the ovotestis are usually at one or both poles. It is thought that the sensitivity of the gonadal poles to local sex reversal is caused by a delay in the receipt of the masculinizing signal (thought to include FGF9) that emanates from the gonad center earlier in development (see text).