The earliest manifestation of gonadogenesis in vertebrates is the formation of the genital ridges. The genital ridges form through the transformation of monolayer coelomic epithelium into a cluster of somatic cells. This process depends on increased proliferation of coelomic epithelium and disintegration of its basement membrane, which is foreshadowed by the expression of series of regulatory genes. The earliest expressed gene is Gata4, followed by Sf1, Lhx9, Emx2, and Cbx2. The early genital ridge is a mass of somatic SF1-positive cells (gonadal precursor cells) that derive from proliferating coelomic epithelium. Primordial germ cells (PGCs) immigrate to the coelomic epithelium even in the absence of genital ridges, e.g., in mouse null mutants for Gata4. And conversely, the PGCs are not required for the formation of the genital ridges. After reaching genital ridges, the PGCs become enclosed by somatic cells derived from coelomic epithelium. Subsequently, the expression of sex-determining genes begins and the bipotential gonads differentiate into either testes or ovaries. Gonadal precursor cells, derived from coelomic epithelium, give rise to the somatic supporting cells such as Sertoli cells, follicular cells, and probably also peritubular myoid and steroidogenic cells.

The aim of this study was to investigate consecutive stages of gonadal development of the yellow-bellied toad (Bombina variegata) with particular emphasis on the origin of somatic and germ cell lineages as well as the timing of gonial cell migration. Changes in gonadal basal lamina distribution helped to explain the exceptional mode of gonadal differentiation in this species. Atypical and rapid differentiation of the male gonad in B. variegata is the result of the ability of gonial cells to migrate into the center of the gonad relatively early. Thus, the testis medulla contains germ cells from the onset of gonadal differentiation into cortex and medulla, whereas in other anurans a sterile medulla is characteristic of both future testes and ovaries; germ cells translocate into the medulla during the subsequent stage of testis development. This atypical testiculogenesis is probably the result of an acceleration of the sex determination period, indicating a contribution of sex determination heterochrony to the course of gonadogenesis. The results also suggest that medullar cells are derived from proliferating coelomic epithelial cells. Moreover, Sertoli cells constitute an integral part of the germinal epithelium in B. variegata, as in other vertebrates. Spermatids do not contact Sertoli cells just before spermiation and do not form bundles.

Genetic control of gonadal development proceeds through either the male or female molecular pathways, driving bipotential gonadal anlage differentiation into a testis or ovary. Antagonistic interactions between the 2 pathways determine the gonadal sex. Essentially sex determination is the enhancement of one of the 2 pathways according to genetic sex. Initially, Sry with other factors upregulates Sox9 expression in XY individuals. Afterwards the expression of Sox9 is maintained by a positive feedback loop with Fgf9 and prostaglandin D2 as well as by autoregulative ability of Sox9. If these factors reach high concentrations, then Sox9 and/or Fgf9 may inhibit the female pathway. Surprisingly, splicing, nuclear transport, and extramatrix proteins may be involved in sex determination. The male sex determination pathway switches on the expression of genes driving Sertoli cell differentiation. Sertoli cells orchestrate testicular differentiation. In the absence of Sry, the predomination of the female pathway results in the realization of a robust genetic program that drives ovarian differentiation.

The vitamin A (retinol) and its metabolites such as retinoic acid (RA) affect vertebrate gametogenesis. The level of RA in cells relies on the balance between its synthesis and degradation. The sex-dependent equilibrium is reached in different ways in various species. It is known that RA induces meiosis in developing gonads in mouse, chicken and urodel amphibians, but its role in anuran amphibians has not been studied. Here we show in six anuran species (Xenopus laevis, Bombina bombina, Hyla arborea, Bufo viridis, Rana arvalis and Rana temporaria) that cultured undifferentiated gonads were insensitive to RA treatment, but the RA induced ectopic meiosis in cultured larval testes. In larval testes of all studied species, the exogenous RA induced leptotene phase of I meiotic prophase in gonia, but only in H. arborea and B. viridis gonia progressed to zygotene phase. In the cultured developing ovaries, exogenous RA led to increase in the number of oocytes as compared to the control. Inhibition of either RA synthesis or RA-receptors prevented meiotic entry in larval gonads of all species. Exogenous RA rescued this inhibitory effect demonstrating that the balance in RA homeostasis plays a key role in meiotic entry in anuran gonads. The localization of two enzymes, Raldh2 and Cyp26b1, which antagonistically control RA levels and whose abundance suggests the sites of RA synthesis and degradation respectively, showed two distinct expression patterns specific for (i) X. laevis, H. arborea, R. arvalis, R. temporaria and (ii) B. bombina, B. viridis. Thus, RA, in correlation with specific expression patterns of Raldh2 and Cyp26b, induces meiosis during gonad development in anurans. In addition, in B. viridis, RA signalling seems important for development of the Bidder's organ containing oocytes both in males and females.

The normal course of gonad development is critical for the sexual development and reproductive capacity of the individual. During development, an incipient bipotential gonad which consists of unorganized aggregate of cells, must differentiate into highly structured testis or ovary. Cell adhesion molecules (CAMs) are a group of proteins crucial for segregation and aggregation of different cell types to form different tissues. E-cadherin (Cdh1) is one of the CAMs expressed in the developing gonads. We used tissue-specific knockout of Cdh1 gene in OCT4+ germ cells and, separately, in SF1+ somatic cells of developing gonads. The knockout of E-cadherin in somatic cells caused decrease in the number of germ cells, while the knockout in the germ cells caused their almost complete loss. Thus, the presence of E-cadherin in both the germ and somatic cells is necessary for the survival of germ cells. Although the lack of E-cadherin did not impair cell proliferation, it enhanced apoptosis, which was a possible cause of germ cell loss. However, the somatic cells of the gonad differentiated normally into Sertoli cells in the testis cords, and into follicular cells in the ovaries. The testis and ovigerous cords maintained their integrity; they were covered by continuous basement membranes. The testicular interstitium with steroidogenic fetal Leydig cells did not show any noticeable changes. However, in the female gonads, because of the lack of germ cells, the ovarian follicles were absent. The sex determination and sexual differentiation of the gonad were not impaired. These results underscore an important role of E-cadherin in germ cell survival and gonad development.