Browsing by Subject "Ovary"

A unique feature of the fetal gonad is its ability to form two distinct organs, the testis and the ovary, from a single bipotential primordium. The outcome of this decision, which is made by a population of somatic cells known as the bipotential supporting cell precursors, determines whether an embryo will develop as a phenotypic male or female. Though several molecular pathways have been shown to be required for female fate determination in vertebrates, the intricacies of ovarian morphogenesis are not well understood. A key event in ovarian development occurs around birth, when meiotic germ cells and somatic granulosa cells organize into primordial follicles, the structures that generate mature oocytes for ovulation in adult females. We investigated the embryonic origins and proliferative properties of granulosa cells in the fetal mouse ovary and found that the precursors emerge from the ovarian surface epithelium and then enter mitotic arrest in a specification process that extends from the bipotential stage to the end of the postnatal follicle assembly period. Maintenance of cell cycle arrest in granulosa cell precursors appears to be regulated by Wnt signaling. The first granulosa cells to be specified were exclusively incorporated into the subset of follicles that begin to grow immediately upon assembly. We show that this first group of granulosa progenitors derives from the supporting cell precursors present in the bipotential gonad. Interestingly, both XX and XY supporting cell precursors were mitotically arrested towards the end of the bipotential period, indicating that adoption of supporting cell fate might be regulated by the cell cycle. We also show that antagonism of Notch signaling may be required for these precursor cells to exit the cell cycle and differentiate.

In Witschi's classic model of vertebrate gonad development, the cortex and medulla of the undifferentiated gonad expand and differentiate in a mutually exclusive manner to yield the mature ovary and testis (Witschi 1951). Estrogen acts on both the cortex and medulla to promote female fate determination and ovary development in non-mammalian vertebrates. However, the downstream receptors and targets through which estrogen exerts its effects on the gonad have not yet been elucidated. We selected the red-eared slider turtle Trachemys scripta as a model with which to address this question. We first characterized the cellular composition of the turtle gonad before and after sex determination, identifying four populations of somatic cells distinguishable by their location within the gonad as well as the complement of transcription factors expressed. This information was then applied to an investigation of estrogen signaling pathways in the turtle ovary. We show that i) estrogen likely acts through its canonical receptors rather than a non-canonical pathway involving ERK signaling; ii) early exposure to estrogen resulted in the premature downregulation of a testis-specific gene, SOX9, in the medulla; iii) less estrogen is needed to promote ovarian differentiation in the cortex of the gonad than to repress testicular differentiation of the medulla, consistent with the localized production of estrogen in the medulla; and iv) estrogen's repressive effect on SOX9 expression may be mediated by Wnt signaling.

Our findings add complexity to the standard model of how the male and female supporting cell lineages are established in mice, reveal evolutionary conservation between mice and turtles in the timing of granulosa cell specification relative to sex determination., and refine our understanding of how estrogen acts to promote ovarian development in non-mammalian species.

The mouse gonad begins its development as a bipotential primordia, capable of developing into a testis or ovary depending on the presence of the sex-determining gene, Sry. In the XY gonad, opposing pro-testis and pro-ovary pathways compete in gonadal supporting cells. While the individual cellular decision process is well understood, the higher-level process of coordination of cell fates across the gonad remains to be explained. The testis and ovary exhibit distinct patterns of differentiation, suggesting that either development of each organ requires a particular organizing principle or bipotentiality requires regional separation for fate specification or stabilization. The overall goal of this work is to improve characterizations of the spatiotemporal features of sex determination and gonadogenesis, including cell fate organization, morphogenic processes, and system context.Though several hypotheses have connected gonad morphogenesis to sex determination, the morphogenic processes that occur in the gonad have not been sufficiently characterized for formulating testable hypotheses. To capture and analyze the complexity of genital ridge morphogenesis, we generated a 3-dimensional time course of gonad development in native form and context using whole embryo tissue clearing and light sheet microscopy. Analysis revealed that the early gonad exhibits anterior-to-posterior patterns as well as increased rates of growth, rotation, and separation in the central domain. In extending characterization to the neighboring nephric ducts, we found a close alignment of gonad and mesonephric duct movements as well as delayed duct development in Cbx2 mutants, which undergo XY sex reversal and gonad dysgenesis. These data support mechanical integration of gonad and mesonephric duct morphogenesis. In investigating the mechanisms underlying the center-to-pole pattern of testis differentiation, we performed anteroposterior axis analyses and ex vivo gonad reconstruction cultures. These experiments allowed us to rule out two commonly accepted theories in the field: paracrine relay and center-first Sry expression. After searching for patterns in other cellular processes during gonadogenesis, including cell cycle arrest and coelomic epithelium proliferation, we uncovered a center-biased pattern of supporting cell precursor ingression. The updated model indicates that differences between the patterns of differentiation in the testis and ovary are due to features of their respective regulatory networks connecting their fate dynamics to different general gonadal organizing principles acting upstream of supporting cell differentiation. Following recent work on the rete testis and rete ovarii suggesting these structures contribute to gonadal supporting cell populations, we characterized early development of the rete and adjacent tissues in both sexes. Comparison of the GATA4+/PAX8+ presumptive rete with mesonephric and gonadal cells led to the identification of undescribed patterns in mesonephros development which may play a role in sexual dimorphism of the rete. Cells of the rete may derive from mesonephric condensates in a process similar to kidney nephron development. Cell cycle analysis revealed the mesonephric tubules and early rete to be a largely non-proliferating population of cells, suggesting expansion through recruitment of new cells. These results were used to establish preliminary theories for lineage relationships in early urogenital development. Initial attempts at lineage tracing to test the theory were unsuccessful. The findings presented here contribute to a more comprehensive and systems level understanding of sex determination and gonad development. In particular, the incorporation of high-resolution spatial information into theories of sex determination serves to connect individual cell fate decisions to organ level patterns of differentiation in space and time. These results will be useful for novel hypothesis generation as well as for designing more detailed models and simulations of sex determination and gonadogenesis.