Gonads are very peculiar organs given their bipotential competence. Indeed, early differentiating genital ridges evolve into either of 2 very distinct organs: the testis or the ovary. Accumulating evidence now demonstrates that both genetic pathways must repress the other in order for the organs to differentiate properly, meaning that if this repression is disrupted or attenuated, the other pathway may completely or partially be expressed, leading to disorders of sex development. Among these disorders are the cases of XY male-to-female and XX female-to-male sex reversals as well as true hermaphrodites, in which there is a discrepancy between the chromosomal and gonadal sex. Here, we review known cases of XY and XX sex reversals described in mammals, focusing mostly on domestic animals where sex reversal pathologies occur and on wild species in which deviations from the usual XX/XY system have been documented.

The steroidogenic acute regulatory (StAR) protein is responsible for the movement of cholesterol across mitochondrial membranes and is therefore a key factor in regulating the timing and rate of steroidogenesis. In this study, we characterized the coding region of the star gene in the ranid Lithobates sylvaticus and studied star mRNA levels in steroidogenic tissues during development and under natural conditions. Our results support previous research showing that the StAR sequence is well conserved. We determined that star is expressed in both the interrenal and gonadal tissues of adults and in the tadpole gonad-mesonephros complex (GMC). The mRNA levels of star in the GMC were found to increase during tadpole development, reaching a maximum between Gosner stages (Gs) 32 and 38. We observed a significant drop in star mRNA levels at the end of prometamorphosis (Gs40-41), just before the start of the metamorphic climax. Significant differences in star levels between females and males, with males presenting higher levels than females, were detected at Gs36-38. To our knowledge, this is the first study that reports transitory star sex differences in tadpoles' developing GMC. Our results suggest an involvement of StAR in anuran late male GMC formation and development that requires further investigation.

Cryptorchidism is the most common cause of non-obstructive azoospermia in man. In contrast to the general belief that temperature-dependent effects on the undescended gonad damage cryptorchid testes before sexual maturation is complete, molecular pathology strongly supports the theory that impaired mini-puberty is responsible for azoospermia and infertility in cryptorchidism. Molecular biological observations favor LH deficiency, with EGR4 as a master regulatory gene in Leydig cell dysgenesis, as the reason for impaired mini-puberty, and recent evidence supports the idea that infertility in cryptorchidism is a consequence of alterations in the Piwi pathway.

In 4 complete androgen insensitivity syndrome (CAIS) members of one family, 2 presented extreme and unusual clinical features: male gender identity disorder (case 1) and female precocious central puberty (case 2). The AR gene carried the mutation c.1752C>G, p.Phe584Leu. Gender dysphoria in CAIS may be considered as a true transgender and has been described in 3 other cases. Central precocious puberty has only been described in 1 case; Müllerian ducts in case 2 permitted menarche. Despite the common CAIS phenotype, there was a familial disparity for gender identity adequacy and timing and type of puberty.

Little is known about gonadotropins and sex steroid levels in postpubertal women with complete androgen insensitivity syndrome (CAIS). In order to define reproductive hormone profiles in women with CAIS and intact gonads, 42 postpubertal females with proven CAIS (age range 14-50 years) with testes in situ were examined. Reproductive hormone values [testosterone (T), estradiol (E2), sex hormone-binding globulin (SHBG), luteinizing hormone (LH), follicle-stimulating hormone (FSH)] were assessed by commercially available immunoassays. In women with CAIS, LH levels (median 18.5 IU/l, range 5.5-51.1 IU/l) were elevated above the usual adult reference ranges, whereas FSH values (3.5 IU/l, 0.4-16.3 IU/l) were not. Basal T (20 nmol/l, 6-52 nmol/l) and E2 values (113 pmol/l; 18-257 pmol/l) were found in the usual adult male reference ranges; SHBG levels (53 nmol/l, 15-180 nmol/l) were in the adult female reference range. Calculated free androgen indices (Tx10³/SHBG: 380, 114-863) and aromatization indices (E2/T: 0.052, 0.020-0.196) did not differ from the reference ranges for adult men given in the literature (Tx10³/SHBG: 315-936; E2/T: 0.03-0.07). Reproductive hormone profiles in women with CAIS do not follow the usual male/female pattern, suggesting a specific postpubertal hormone milieu. Albeit calculation of CAIS-specific reference ranges requires larger series and standardization of laboratory methods, these results may be a prerequisite for the identification of pathologic hormone patterns in women with CAIS and gonads in situ. The present data will also be useful to monitor hormone replacement therapy in individuals with removed gonads.

Androgen receptor (AR) mutations are associated with androgen insensitivity syndrome (AIS). Missense mutations identified in the AR-N-terminal domain (AR-NTD) are rare, and clinical phenotypes are typically mild. We investigated 7 missense mutations and 2 insertion/deletions located in the AR-NTD. This study aimed to elucidate the pathogenic role of AR-NTD mutants in AIS and to use this knowledge to further define AR-NTD function. AR-NTD mutations (Q120E, A159T, G216R, N235K, G248V, L272F, and P380R) were introduced into AR-expression plasmids. Stably expressing cell lines were established for del57L and ins58L. Transactivation was measured using luciferase reporter constructs under the control of GRE and Pem promoters. Intrinsic fluorescence spectroscopy and partial proteolysis studies were performed for mutations which showed reduced activities by using a purified AR-AF1 protein. Pem-luciferase reporter activation was reduced for A159T, N235K, and G248V but not the GRE-luciferase reporter. Protein structure analysis detected no significant change in the AR-AF1 region for these mutations. Reduced cellular expression and transactivation activity were observed for ins58L. The mutations Q120E, G216R, L272F, P380R, and del57L showed small or no detectable changes in function. Thus, clinical and experimental analyses have identified novel AR-signalling defects associated with mutations in the structurally disordered AR-NTD domain in patients with AIS.

In teleosts, the expression of steroidogenic enzymes and related transcription factor genes occurs in a stage- and tissue-specific manner causing sexual development. The role of sox3, an evolutionary ancestor of SRY, has not been studied in detail. Therefore, the full-length cDNA of sox3 (1,197 kb) was cloned from catfish testis, and mRNA expression was analyzed during gonadal development, during the seasonal reproductive cycle, and after human chorionic gonadotropin (hCG) induction. Tissue distribution analysis showed that sox3 expression was higher in testis, ovary, and brain compared to other tissues analyzed. Developing and mature testis showed higher sox3 expression than ovary of corresponding stages, and more sox3 transcripts were found during the spawning phase of the seasonal reproductive cycle. Expression of sox3 was upregulated by hCG after in vivo and in vitro induction, suggesting that gonadotropins might stimulate it. In situ hybridization and immunohistochemistry showed the presence of sox3 mRNA and protein in somatic and interstitial cell layers of the testis. Sox3 could also be found in the zona radiata of developing and mature oocytes. Exposure of methyltestosterone (1 µg/l) and ethinylestradiol (1 µg/l) for 21 days during testicular development showed lower sox3 expression levels in the testis and brain, indicating a certain feedback intervention. These results suggest a possible role for Sox3 in the regulation of testicular development and function.

Mammalian sex determination is the process by which the bipotential embryonic genital ridge commits to either the testicular or ovarian fate. The existence of a gonad primordium with 2 possible developmental outcomes, depending on the presence or absence of the Y chromosome, remains a fascinating paradigm for examining the manner in which genes cooperate to establish cell fate and regulate differentiation. Development of the mouse gonad offers a unique model for the investigation of sex determination, characterised by powerful transgenic, genomic and other experimental resources for the study of the molecular control of organogenesis. This review focuses on recent progress in our understanding of mouse testis determination, with an emphasis on multi-locus studies of mutant alleles aimed at constructing pathways of interacting gene products. Studies in 2 broad areas have been especially revealing: (i) identification of pathways required for the appropriate expression of Sry, and (ii) characterisation of the antagonistic interactions between the core testis- (SRY-SOX9-FGF9) and ovary- (RSPO1-WNT4-CTNNB1-FOXL2) determining gene regulatory networks. We discuss these advances with an eye on emerging themes in mammalian sex determination.