Sertoli cell differentiation and Y-chromosome activity: a developmental study of X-linked transgene activity in sex-reversed X/XSxra mouse embryos

Mouse germ cell development: from specification to sex determination

Primordial germ cells (PGCs) are embryonic progenitors for the gametes. In the gastrulating mouse embryo, a small group of cells begin expressing a unique set of genes and so commit to the germline. Over the next 3-5 days, these PGCs migrate anteriorly and increase rapidly in number via mitotic division before colonizing the newly formed gonads. PGCs then express a different set of unique genes, their inherited epigenetic imprint is erased and an individual methylation imprint is established, and for female PGCs, the silent X chromosome is reactivated. At this point, germ cells (GCs) commit to either a female or male sexual lineage, denoted by meiosis entry and mitotic arrest, respectively. This developmental program is determined by cues emanating from the somatic environment.

Sexual dimorphic expression of DMRT1 and Sox9a during gonadal differentiation and hormone-induced sex reversal in the teleost fish Nile tilapia (Oreochromis niloticus)

We examined the expression profiles of tDMRT1 and Sox9a during gonadal sex differentiation and hormone-induced sex reversal. tDMRT1 was detected in the gonial germ-cell-surrounding cells in XY fry specifically before the appearance of any signs of morphological sex differentiation, that is, sex differences in germ cell number and histogenesis, such as differentiation into intratesticular efferent duct or ovarian cavity. The signals became localized in the Sertoli and epithelial cells comprising the efferent duct during gonadal differentiation. After the induction of XY sex reversal with estrogen, tDMRT1 decreased and then disappeared completely. In contrast, tDMRT1 was expressed in the germ-cell-surrounding cells in XX sex reversal with androgen. On the other hand, Sox9a did not show sexual dimorphism before the appearance of sex differences in histogenesis and was not expressed in the efferent duct in the testis. These results suggest that tDMRT1 is a superior testicular differentiation marker in tilapia.

Balancing the bipotential gonad between alternative organ fates: a new perspective on an old problem

The embryonic gonads give rise to one of two morphologically and functionally different organs, a testis or an ovary. Sex determination is the embryonic process that determines the developmental fate of the gonad. In mammals, sex determination is regulated by a DNA binding protein encoded on the Y chromosome, Sry, and it's downstream mediator, Sox9, which trigger testis determination in the bipotential gonad. However, evidence suggests that the extracellular signals. Fgf9 and Wnt4, are also required to establish divergent organogenesis of the gonad. In this review, we discuss how these extracellular signals interface with cell-autonomous factors to determine the fate of the mammalian gonad, and we derive a model that could provide a molecular explanation for testis determination in vertebrates where Sry is absent.

Genomic imprinting of XX spermatogonia and XX oocytes recovered from XX<-->XY chimeric testes

We produced XX<-->XY chimeras by using embryos whose X chromosomes were tagged with EGFP (X*), making the fluorescent green female (XX*) germ cells easily distinguishable from their nonfluorescent male (XY) counterparts. Taking advantage of tagging with EGFP, the XX* "prospermatogonia" were isolated from the testes, and the status of their genomic imprinting was examined. It was shown that these XX cells underwent a paternal imprinting, despite their chromosomal constitution. As previously indicated in sex-reversal XXsxr testes, we also found a few green XX* germ cells developed as "eggs" within the seminiferous tubules of XX*<-->XY chimeric testes. These cells were indistinguishable from XX* prospermatogonia at birth but resumed oogenesis in a testicular environment. The biological nature of the "testicular eggs" was examined by recovering the eggs from chimeric testes. The testicular eggs not only formed an egg-specific structure, the zona pellucida, but also were able to fuse with sperm. The collected testicular eggs were indicated to undergo maternal imprinting, despite the testicular environment. The genomic imprinting did not always follow the environmental conditions of where the germ cells resided; rather, it was defined by the sex that was chosen by the germ cells at early embryonic stage.

Molecular characterization of three gonad cell lines

To facilitate the study of the regulation and downstream interactions of genes involved in gonad development it is important to have a suitable cell culture model. We therefore aimed to characterize molecularly three different mouse gonad cell lines. TM3 and TM4 cells were originally isolated from prepubertal mouse gonads and were tentatively identified as being of Leydig cell and Sertoli cell origin, respectively, based upon their morphology and hormonal responses. The third line is a conditionally immortalized cell line, derived from 10.5-11.5 days post-coitum (dpc) male gonads of transgenic embryos carrying a temperature-sensitive SV40 large T-antigen. We studied by reverse transcription-polymerase chain reaction (RT-PCR) the expression profiles of a number of genes known to be important for early gonad development. Moreover, we assessed these cell lines for their capacity to induce SOX9 transcription upon expression of SRY, a key molecular event occurring during sex determination. We found that all three cell lines were unable to upregulate SOX9 expression upon transfection of SRY-expression constructs, even though these cells express many of the studied embryonic gonad genes. These observations point to a requirement for SRY cofactors for direct or indirect upregulation of SOX9 expression during testis determination.

Turning on the male – SRY, SOX9 and sex determination in mammals

The decision of the bi-potential gonad to develop into either a testis or ovary is determined by the presence or absence of the Sex-determining Region gene on the Y chromosome (SRY). Since its discovery, almost 13 years ago, the molecular role that SRY plays in initiating the male sexual development cascade has proven difficult to ascertain. While biochemical studies of clinical mutants and mouse genetic models have helped in our understanding of SRY function, no direct downstream targets of SRY have yet been identified. There are, however, a number of other genes of equal importance in determining sexual phenotype, expressed before and after expression of SRY. Of these, one has proven of central importance to mammals and vertebrates, SOX9. This review describes our current knowledge of SRY and SOX9 structure and function in the light of recent key developments.

Sexual differentiation of germ cells in XX mouse gonads occurs in an anterior-to-posterior wave

Differentiation of mouse embryonic germ cells as male or female is dependent on the somatic environment of the gonad rather than the sex chromosome constitution of the germ cell. However, little is known about the initiation of germ cell sexual differentiation. Here, we traced the initiation of germ cell sexual differentiation in XX gonads using the Stra8 gene, which we demonstrate is an early molecular marker of female germ cell development. Stra8 is upregulated in embryonic germ cells of XX gonads prior to meiotic entry and is not expressed in male embryonic germ cells. A developmental time course of Stra8 expression in germ cells of XX gonads has revealed an anterior-to-posterior wave of differentiation that lasts approximately 4 days, from embryonic days 12.5 to 16.5. Consistent with these results, we find that embryonic ovarian germ cells upregulate the meiotic gene Dmc1 and downregulate the Oct4 transcription factor in an anterior-to-posterior wave. In complementary experiments, we find that embryonic XX gonads upregulate certain gene markers of somatic female differentiation in an anterior-to-posterior pattern, while others display a center-to-pole pattern of regulation. Thus, sexual differentiation and meiotic entry of germ cells in embryonic XX gonads progress in an anterior-to-posterior pattern that may reflect local environmental cues that are present in the embryonic XX gonad.

Regulation of male sexual development by Sry and Sox9

Sry, a gene from the Y chromosome, is known to initiate testis formation and subsequent male differentiation in mammals. A related gene, Sox9, also plays a critical role in testis determination, possibly in all vertebrates. A number of models have been presented regarding the molecular modes of action of these two genes. However, details regarding their regulation, regulatory target genes, and interacting protein factors and co-factors have not been established with any certainty. In this review, we examine new evidence and re-examine existing evidence bearing on these issues, in an effort to build up an integrative model of the network of gene activity centred around Sry and Sox9. J. Exp. Zool. 290:463-474, 2001.

Sry, Sox9 and mammalian sex determination

Sry is the Y-chromosomal gene that acts as a trigger for male development in mammalian embryos. This gene encodes a high mobility group (HMG) box transcription factor that is known to bind to specific target sequences in DNA and to cause a bend in the chromatin. DNA bending appears to be part of the mechanism by which Sry influences transcription of genes downstream in a cascade of gene regulation leading to maleness, but the factors that cooperate with, and the direct targets of, Sry remain to be identified. One gene known to be downstream from Sry in this cascade in Sox9, which encodes a transcription factor related to Sry by the HMG box. Like Sry, mutations in Sox9 disrupt male development, but unlike Sry, the role of Sox9 is not limited to mammals. This review focuses on what is known about the two genes and their likely modes of action, and draws together recent data relating to how they might interconnect with the network of gene activity implicated in testis determination in mammals.

Germ cell development in the XXY mouse: evidence that X chromosome reactivation is independent of sexual differentiation

Prior to entry into meiosis, XX germ cells in the fetal ovary undergo X chromosome reactivation. The signal for reactivation is thought to emanate from the genital ridge, but it is unclear whether it is specific to the developing ovary. To determine whether the signals are present in the developing testis as well as the ovary, we examined the expression of X-linked genes in germ cells from XXY male mice. To facilitate this analysis, we generated XXY and XX fetuses carrying X chromosomes that were differentially marked and subject to nonrandom inactivation. This pattern of nonrandom inactivation was maintained in somatic cells but, in XX as well as XXY fetuses, both parental alleles were expressed in germ cell-enriched cell populations. Because testis differentiation is temporally and morphologically normal in the XXY testis and because all germ cells embark upon a male pathway of development, these results provide compelling evidence that X chromosome reactivation in fetal germ cells is independent of the somatic events of sexual differentiation. Proper X chromosome dosage is essential for the normal fertility of male mammals, and abnormalities in germ cell development are apparent in the XXY testis within several days of X reactivation. Studies of exceptional germ cells that survive in the postnatal XXY testis demonstrated that surviving germ cells are exclusively XY and result from rare nondisjunctional events that give rise to clones of XY cells.