Sex Determination. Reproductive Embryology. Secondary article. Testicular differentiation. Ovarian differentiation. Ductal and genital differentiation

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1 Joe Leigh Simpson, Baylor College of Medicine, Houston, Texas, USA Sex determination is the process by which genes direct male and female embryos to become distinguishable from each other. Reproductive Embryology Primordial germ cells originate in the endoderm of the yolk sac and migrate to the genital ridge to form the indifferent gonad. Initially, 46,XY and 46,XX gonads are indistinguishable. Indifferent gonads develop into testes if the embryo, or more specifically the gonadal stroma, is 46,XY. This process begins about 43 days after conception. Testes become morphologically identifiable 7 8 weeks after conception (9 10 weeks gestational or menstrual weeks). Secondary article Article Contents. Reproductive Embryology. Sex Determination in Males: Genes and Chromosomes Influencing Testicular Differentiation. Sex Determination in Females: Genes and Chromosomes Influencing Ovarian Differentiation. True Hermaphroditism: An Autosomal Disorder of Gonadal Differentiation. Selected Disordersof External Genital Development in 46,XX: Female Pseudohermaphroditism. Selected Disordersof External Genital Development in 46,XY: Male Pseudohermaphroditism. Klinefelter Syndrome (Seminiferous Tubule Dysgenesis) Testicular differentiation Sertoli cells are the first cells to become recognizable in testicular differentiation, organizing the surrounding cells into tubules. Leydig cells and Sertoli cells exert their function in dissociation from testicular morphogenesis; thus, these cells direct gonadal development, rather than the converse. These two cell types secrete different hormones, which in aggregate direct the embryo to develop into a male (Figure 1). Fetal Leydig cells produce testosterone, a hormone that stabilizes wolffian ducts and permits differentiation of the vasa deferentia, epididymides and seminal vesicles. Testosterone is then converted by 5a-reductase to dihydrotestosterone (DHT), and it is this hormone that is responsible for external genitalia virilization. These actions can be mimicked by the administration of testosterone to female or castrated male embryos. Fetal Sertoli cells produce the nonandrogenic glycoprotein antimu llerian hormone (AMH), also called mu llerian inhibitory substance (MIS); AMH diffuses locally to cause regression of mu llerian derivatives (uterus and fallopian tubes). This hormone may have functions related to gonadal development as well, given that when AMH is chronically expressed in XX transgenic mice oocytes fail to persist. Tubule-like structures develop in gonads, and mu llerian differentiation is abnormal. 5α-Reductase Indifferent gonad Embryonal testis Testosterone Wolffian stabilization Persistence of seminal vesicles Vasa deferentia Epididymides Dihydrotestosterone Genital virilization Y chromosome Product(s) of Y testicular determinant Müllerian inhibitory factor Müllerian inhibition Regresssion of uterus, fallopian tubes and upper vagina Penis Scrotum Labioscrotal fusion Figure 1 Embryonic differentiation in the normal male. Modified from Simpson JL (ed.) (1976) Disorders of Sexual Differentiation. New York: Academic Press. Ovarian differentiation In the absence of a Y chromosome, the indifferent gonad develops into an ovary. Transformation into fetal ovaries begins at days of embryonic development. Germ cells are initially present in 45,X embryos (Jirasek, 1976), but undergo atresia at a rate more rapid than that occurring in normal 46,XX embryos. Ductal and genital differentiation Ductal and external genital development occurs independently of gonadal differentiation. In the absence of testosterone and AMH, external genitalia develop in female fashion. Mu llerian ducts form the uterus and fallopian tubes; wolffian ducts regress. These changes occur in normal XX mammalian embryos and XY mammals that were castrated (embryonically) before testicular differentiation. 1

2 Sex Determination in Males: Genes and Chromosomes Influencing Testicular Differentiation Sex chromosomes (X and Y) as well as the autosomes contain loci that must remain intact for normal testicular development. Y chromosome In mammals a single Y can direct male sex differentiation, irrespective of normal X chromosomes (e.g. 47,XXY or 48,XXXY). Thus, sex determination in mammals differs fundamentally from that in Drosophila, a species in which the ratio of the X chromosomes to autosomes determines sex. The major testicular determinants (testis-determining factor) in humans were localized to the Y short arm (Yp) in the 1960s. Since the early 1990s it has become clear that sexdetermining region Y (SRY) is the testicular determinant (Sinclair et al., 1990). SRY was identified as result of mapping that took advantage of the syndrome of phenotypic males who are 46,XX and phenotypic females who are 46,XY. Phenotypic males with a 46,XX complement usually (80%) arise following interchange of not only the obligatory pseudoautosomal regions of Xp and Yp, but also the contiguous nonpseudoautosomal region that contains the testis determinants. In these cases SRY is mapped to the smallest translocated region compatible with male differentiation. Some sporadic XY females also show point mutations within SRY. SRY is composed of two open reading frames consisting of 99 and 273 amino acids, respectively. The pivotal sequence involves a high-mobility group (HMG) box that shares features in common with other DNA-binding sequences. SRY is expressed before testicular differentiation is manifested and transgenic XX mice with SRY show testicular differentiation (Koopman et al., 1991). Y chromosome and spermatogenesis Deletions of Y long arm (Yq) may be associated with azoospermia. About 10 15% of azoospermic men have deletions in DAZ (Deleted in AZoospermia), and about 5 10% of oligospermic men have deletions. Several loci exist, but their exact number and interrelationship are uncertain. One popular model assumes three loci: AZFa, the rarest and whose phenotype is associated with absence of spermatogenesis and stem cell; AZFb, whose phenotype shows maturational arrest and corresponds to a locus called RNA-Binding Motif (RBM); and AZFc, associated with both azoospermia and oligospermia and considered to contain the locus DAZ. Autosomal genes are also important for spermatogenesis. One well-known locus is DAZLA (Deleted in AZoospermia-Like Autosomal homologue), located on human chromosome 3. X chromosome and testicular development In addition to genes on the Y chromosome, various clinical disorders indicate that testicular differentiation also requires loci on X. The importance of genes on the X chromosome has long been evidenced by an X-linked recessive form of XY gonadal dysgenesis (Simpson et al., 1971; German et al., 1978). Of more recent interest is the demonstration of a region on the X short arm (Xp) that suppresses testicular development when duplicated in 46,XY individuals. This Dose-Sensitive Sex reversal (DSS) phenomenon involves a region that contains the locus for adrenal hypoplasia (AHC). Its murine homologue is Ahch. We shall allude later to the role this gene has been purported to play in primary ovarian differentiation. Autosomes and testicular development Several different autosomal regions are pivotal for testicular differentiation. Based somewhat on circumstantial reasons, it has been postulated that several genes (Sf-1, WT-1) are necessary for the indifferent gonad to differentiate into the testes. That is, they act upstream of SRY. Other genes are presumed to act downstream, i.e. after SRY exerts its action. Among the latter is the locus responsible for camptomelia dysplasia and XY gonadal dysgenesis (sex reversal), located on 17q24.3!q25.1. This region encompasses SOX-9, which, like SRY, is a DNAbinding protein. Other autosomal regions that preclude testicular development when deleted include 9p and 10q. Further evidence of autosomal control over testicular development is the existence of testicular differentiation in 46,XX true hermaphrodites, almost all of whom lack SRY. The responsible loci must therefore be autosomal. Furthermore, in mice the Mus pociavinus Y is not always capable of directing testicular differentiation. When placed on a predominately C57 autosomal background murine, true hermaphrodites result. Thus, murine autosomes play a role in preserving testicular development. The number of autosomal genes exerting actions both downstream and upstream from SRY is uncertain. The review of Ottolenghi et al. (1998) constitutes a good synthesis. 2

3 Sex Determination in Females: Genes and Chromosomes Influencing Ovarian Differentiation DAX 1 and the potential existence of a primary ovarian determinant In the absence of a Y chromosome, the indifferent gonad develops into an ovary. Given that germ cells exist in 45,X human fetuses (Jirasek, 1976) and 39,X mice, the pathogenesis of germ cell failure must involve increased germ cell attrition, not failure of formation. If two intact X chromosomes are not present, 45,X ovarian follicles usually degenerate by birth. The second X chromosome is therefore accepted as responsible for ovarian maintenance, as opposed to primary ovarian differentiation. It is unclear whether primary ovarian differentiation requires a specific gene, or rather occurs constitutively as the default pathway in the absence of SRY and the other testicular determinants. Some have focused on the Xp region that, when duplicated, directs 46,XY embryos into females. Could the region play a primary role in ovarian differentiation in 46,XX individuals? The relevant region in humans contains AHC (adrenal hypoplasia congenita) or DAX 1 (dosage-sensitive sex reversal/adrenal hypoplasia critical region X). Its mouse homologue is Ahch. Ahch is upregulated in the XX mouse ovary, and transgenic XY mice overexpressing Ahch develop as females, at least in the presence of a relatively weak Sry. However, if XX mice lose Ahch (knockout) ovarian development is not impaired and ovulation and fertility are normal (Yu et al., 1998). Furthermore, XY mice mutant for Ahch show testicular germ cell defects. Thus, Ahch is clearly not a primary ovarian differentiation in mice, and presumably neither is human DAX 1. There remains no evidence that primary ovarian differentiation is other than passive (constitutive). X-ovarian maintenance genes Irrespective of whether a primary ovarian-determining gene exists, regions of the X chromosome are important for ovarian maintenance. The location and role of these ovarian maintenance determinants traditionally have been deduced by phenotypic karyotypic correlations of individuals with absence of the X short arm, the X long arm, or the entire X (monosomy X). Each arm of the X has several distinct regions of differential importance for ovarian maintenance. Pinpointing the location molecularly has proceeded more slowly than delineation of the Y. Considering phenotype as a function of region of the X deleted is genetically and clinically informative. Monosomy X The chromosomal abnormality most frequently associated with ovarian dysgenesis is absence of one X (monosomy X), also referred to as Turner syndrome. In most 45,X adults with gonadal dysgenesis, the normal gonad is replaced by a white fibrous streak, located in the position ordinarily occupied by the ovary. That germ cells are usually absent in 45,X adults despite being present in 45,X embryos is the basis for the belief that the pathogenesis of germ cell failure is increased atresia, not failure of initial germ cell formation. Oestrogen levels are low; gonadotropins (follicle-stimulating hormone, FSH, and luteinizing hormone, LH) are increased. Short stature and various somatic anomalies may occur skeletal, cardiac, renal and auditory. Verbal IQ is greater than performance IQ, but overt mental retardation is uncommon. That 45,X adults lack germ cells as adults is not so predictable as one might expect. Relatively normal ovarian development occurs in many other monosomy X mammals (e.g. mice). The likely explanation is that in humans not all loci on the normal heterochromatic (inactive) X are inactivated. Indeed, about 15 20% of the human X-linked genes escape X-inactivation. Loci on Xp are far more likely to escape X-inactivation than those of Xq (perhaps 20 30% versus 1 2%). Genes that escape X-inactivation appear to be clustered, and it is in these regions that key ovarian maintenance determinants are likely to occur. In addition, X-inactivation never exists in oocytes, X- reactivation of germ cells occurring before entry in meiotic oogenesis. Clinically, 45,X women should be counselled to anticipate primary amenorrhoea and sterility. With hormone therapy uterine size becomes normal, and 45,X women can carry pregnancies in their own uterus after receipt of donor embryos or donor oocytes. The latter could be fertilized in vitro with their husband s sperm (assisted reproduction technology) and resulting embryos transferred to the hormonally synchronized 45,X patient. Success rates per cycle are 20 40%. Genes on the X short arm Deletions of the short arm [46,X,del(Xp)] show variable phenotype, depending upon the amount of Xp persisting. Approximately half of 46,X,del(Xp)(p11) individuals show primary amenorrhoea and gonadal dysgenesis (Simpson, 1998; Figure 2). The others menstruate and show breast development, or show premature ovarian failure. Molecular analysis has somewhat refined the key region, but still only to proximal mid Xp. No candidate gene has been proposed. Women with more distal deletions [del(x)(p21.1 to p )] menstruate more often, but some are infertile or have secondary amenorrhoea. This distal locus [Xpter!p21] thus plays a less important role in ovarian maintenance than loci on Xp11 (Simpson, 1998). 3

4 Genes on the X long arm Primary amenorrhoea Secondary amenorrhoea oligomenorrhoea Fertility or regular menses Figure 2 The X chromosome showing ovarian function as a function of terminal deletion. In familial cases involving Xq deletions, each individual is counted. From Lobo RA (ed.) (1998) Perimenopause, Serono Symposium USA, Norwell, MA. New York: Springer-Verlag. Almost all terminal deletions originating at Xq13 are associated with primary amenorrhoea, lack of breast development, and ovarian failure (Figure 2). Xq13 is thus a key region for germ cell (ovarian) maintenance. Loci could lie in proximal Xq21, but not more distal given that del(x)(q21!q24) individuals menstruate far more often. In more distal Xq deletions (Xq25 28), the usual phenotype is not complete ovarian failure, but premature ovarian failure (i.e. menopause under age 40 years) (Simpson, 1998). Distal Xq thus seems less important for ovarian maintenance than proximal Xq, but the former still plays a role in ovarian maintenance. One candidate gene has been proposed: Diaphanous, the human homologue of the Drosophila melanogaster gene diaphanous. In Drosophila this gene causes sterility in both males and females. Human DIA maps to Xq21, and in one Xq21; autosomal translocation characterized by sterility, DIA was perturbed (Bione et al., 1998). However, in other Xq21; autosomal translocations conferring sterility, DIA is not perturbed; nor is Xq21 the pivotal region. More distal Xq deletions [del(x) (q25)] are far more likely to be associated with premature ovarian failure (POF), or to show no abnormalities at all. Ovarian maintenance genes on autosomes Ovarian failure histologically similar to that occurring in individuals with an abnormal sex chromosomal complement may be present in 46,XX individuals. Mosaicism has been excluded in affected individuals, although mosaicism restricted to the embryo can never be excluded. The mechanism underlying failure of germ cell persistence in most forms of 46,XX gonadal dysgenesis is unknown, but several general hypotheses seem reasonable. One possibility is a disturbance of meiosis, a mechanism that can also be invoked to explain occurrence of germ cell breakdown in both monosomy X and balanced chromosomal translocations. In plants and lower mammals, meiosis is under genetic control, and it is likely that this is true in humans as well. Other pathogenic possibilities include interference with germ cell migration, abnormal connective tissue, failure of DNA repair mechanisms, disturbance of cell cycle check points, heat-shock proteins (the chaperone proteins that accompany steroid receptors), and gonadotropin receptor defects. Many autosomal genes in mice and Drosophila deleteriously affect germ cell development or gametogenesis, and are thus attractive candidate genes for human XX gonadal dysgenesis. Often the phenotype of these murine knockout models is restricted to germ cell abnormalities in the ovary or testes, genes being predicted to act in ways disparate from germ cells deficiency or errors of gametogenesis. Several distinct forms of XX gonadal dysgenesis exist. These genes include various pleiotropic genes that cause ovarian failure and various somatic anomalies, galactosaemia, 17a-hydroxylase deficiency, aromatase deficiency and FSH or LH receptor defects. True Hermaphroditism: An Autosomal Disorder of Gonadal Differentiation True hermaphrodites have both ovarian and testicular tissue. They may have a separate ovary and a separate testis, or, more often, one or more ovotestes. Most true hermaphrodites (60%) have a 46,XX chromosomal complement; others have 46,XX/46,XY, 46/XY, 46,XX/ 47,XXY, or rarer complements. Phenotype may reflect karyotype, but it is generally preferable merely to generalize about the phenotype of all true hermaphrodites. If no medical intervention were to occur (in modern societies a rarity), two-thirds of true hermaphrodites would be raised as males. By contrast, external genitalia are usually ambiguous or predominantly female. Breast development usually occurs at puberty, despite the predominantly male external genitalia. Gonadal tissue may be located in the ovarian, inguinal or labioscrotal regions. A testis or an ovotestis is more likely to be present on the right than on the left. Spermatozoa are rarely present; however, apparently normal oocytes are often observed, even in ovotestes. A few 46,XX true hermaphrodites have even become pregnant, usually but not always after removal of testicular tissue. A uterus is usually present, although sometimes bicornuate or unicornuate. Absence of a uterine horn usually indicates ipsilateral testis or ovotestis. 4

5 Acetate Cholesterol A C Pregnenolone B C Progesterone F 11-Deoxycorticosterone G Corticosterone 17α-OH Pregnenolone D Dehydroepiandrosterone B B 17α-OH Progesterone D E Androstenedione Testosterone F E 11-Deoxycortisol Oestrone Oestradiol G Cortisol Aldosterone Figure 3 Important adrenal and gonadal biosynthetic pathways. Letters designate enzymes required for the appropriate conversions. A, 20ahydroxylase, 22a-hydroxylase and 20,22-desmolase; B, 3b-ol-dehydrogenase; C, 17a-hydroxylase; D, 17,20-desmolase; E, 17-ketosteroid reductase; F, 21-hydroxylase; and G, 11-hydroxylase. From Simpson JL (1976) Disorders of Sexual Differentiation: Etiology and Clinical Delineation. New York: Academic Press. 46,XX/46,XY and 46,XY true hermaphroditism 46,XX/46,XY true hermaphrodites are usually chimaeras. In a single individual there are two or more cell lines, each derived from different zygotes. 46,XY cases may be unrecognized chimaeras. However, chimaerism is an unlikely explanation for 46,XX true hermaphrodites. Explanations for the presence of testes in individuals who ostensibly lack a Y include: (1) translocation of SRY from the paternal Y to the paternal X during meiosis; (2) translocation of SRY from the paternal Y to a paternal autosome; (3) undetected mosaicism or chimaerism; and (4) autosomal sex-reversal genes. 46,XX true hermaphroditism 46,XX true hermaphrodites almost never show SRY or DNA sequences from their father s Y. Genes seem more likely explanations, given existence of sibships showing XX true hermaphroditism, or occurrence of both 46,XX males and 46,XX true hermaphrodites. In these kindreds 46,XX males usually show genital ambiguity, unlike the typical 46,XX male (Simpson, 2000). Selected Disorders of External Genital Development in 46,XX: Female Pseudohermaphroditism expected in 46,XX individuals. In male pseudohermaphroditism external genital development is at odds with that expected in 46,XY individuals. The most common cause of female pseudohermaphroditism is congenital adrenal hyperplasia, resulting from deficiencies of the various enzymes required for steroid biosynthesis (Figure 3): 21-hydroxylase, 11b-hydroxylase, and 3b-ol-dehydrogenase. In each disorder inheritance is autosomal recessive. These first two genes are mitochondrial P-450 enzymes, located on chromosomes 6 and 8, respectively. 3b-ol-Dehydrogenase is a microsomal enzyme coded by a gene on chromosome 1. In 21-hydroxylase deficiency molecular pathogenesis includes gene conversion involving a contiguous pseudogene, point mutations and deletions. In the other two enzyme deficiencies point mutations predominate and, as in 21-hydroxylase deficiency, no single nucleotide is consistently involved. The common pathogenesis involves decreased production of adrenal cortisol, a glucocorticoid that regulates secretion of adrenocorticotrophic hormone (ACTH) through negative feedback inhibition. If cortisol production is decreased, ACTH secretion increases. Elevated ACTH levels lead to increased quantities of steroid precursors, from which androgens can be synthesized. Because the fetal adrenal gland begins to function during the third month of embryogenesis, excessive production of adrenal androgens will virilize the external genitalia. Mu llerian and gonadal development are normal because neither is androgen dependent. In some disorders of sexual development, gonadal development is normal but abnormalities exist in external or internal genital development. In female pseudohermaphroditism external genital development is at odds with that 5

6 Selected Disorders of External Genital Development in 46,XY: Male Pseudohermaphroditism There are a number of different forms of male pseudohermaphroditism (Simpson, 2000). In male pseudohermaphroditism testes are present as expected for 46,XY individuals. However, external genitalia fail to develop as expected in males. Defects in testosterone biosynthesis Male pseudohermaphroditism (genital ambiguity) due to deficiencies in testosterone biosynthesis may result from deficiencies of 17a-hydroxylase, 17,20-desmolase, 3b-oldehydrogenase or 17-ketosteroid reductase (Figure 3). A mutation may also involve StAR, the protein responsible for transporting cholesterol to the nucleus in order that it can be converted to pregnenolone. Deficiencies of 21- or 11b-hydroxylase, the most common causes of female pseudohermaphroditism, do not cause male pseudohermaphroditism. Androgen insensitivity In complete androgen insensitivity (complete testicular feminization) 46,XY individuals show bilateral testes, female external genitalia, a blindly ending vagina and no mu llerian derivatives. These findings are entirely predictable given the pathogenesis: receptors that are unable to respond to testosterone. Antimu llerian hormone (AMH) is synthesized, as it is in the normal testis. Cells respond normally to AMH, for which reason müllerian derivatives regress as predicted. As also expected on the basis of the testes synthesizing oestrogens in unimpeded fashion, affected individuals manifest breast development and feminize at puberty. Partial androgen insensitivity results in genital ambiguity. A mild form affects only spermatogenesis. Both complete and partial androgen insensitivity result from mutations in the androgen receptor gene on Xq11 Xq12. This gene consists of eight exons; exons 2 and 3 are DNA-binding domains, whereas exons 4 to 8 are androgen-binding domains. Many different mutations have been reported, involving all exons. It is not always possible to predict phenotype on the basis of the mutation. 5a-Reductase deficiency These genetic males show ambiguous external genitalia at birth, but paradoxically virilize at puberty like normal males. They experience phallic enlargement, increased facial hair, muscular hypertrophy, and voice deepening, yet no breast development. Their external genitalia consist of a phallus that resembles a clitoris more than a penis, a perineal urethral orifice, and usually a separate, blindly ending, perineal orifice that resembles a vagina (pseudovagina). This disorder results from deficiency of the enzyme 5areductase, which is necessary to convert testosterone to dihydrotestosterone (DHT). That intracellular 5a-reductase deficiency results in this phenotype is consistent with virilization of the external genitalia during embryogenesis requiring dihydrotestosterone; wolffian differentiation requires only testosterone. Two 5a-reductase (SRD5) genes exist. The Type I gene is located on chromosome 5 (SRD5A1), the Type II gene (SRD5A2) on chromosome 2p23. Expressed in gonads, Type II mutations produce male pseudohermaphroditism. LH receptor defect (Leydig cell hypoplasia) In complete absence of Leydig cells 46,XY individuals show female external genitalia, no uterus and bilateral testes devoid of Leydig cells. The molecular basis is a mutation in the LH receptor gene, located on chromosome 2. Klinefelter Syndrome (Seminiferous Tubule Dysgenesis) Males with at least one Y chromosome and at least two X chromosomes have seminiferous tubule dysgenesis. Usually they are azoospermic or severely oligospermic. The clinical condition is called Klinefelter syndrome, the incidence of which is about 1 per 1000 (0.1%) liveborn males. As already noted, the demonstration in Klinefelter syndrome that the mammalian Y chromosome is capable of directing male differentiation irrespective of the number of X chromosomes was the first clear indication that sex determination was fundamentally different in mammals than in D. melanogaster. In the most common form of Klinefelter syndrome 47,XXY seminiferous tubules degenerate to be replaced with hyaline material. Spermatozoa are rare in semen analysis, but a few can usually be recovered at testicular biopsy for use in intracytoplasmic sperm injections (ICSI). External genitalia are usually well differentiated. In 80 90% of 47,XXY men penile size is in the normal range; however, after administration of androgens the penile length may still increase by 1 3 cm. The scrotum is usually well developed and vasa deferentia normal. The prostate is smaller than usual, presumably reflecting decreased androgen levels. Plasma testosterone is approximately half that of normal males. The decreased androgen production causes lack of normal secondary sexual development. 47,XXY patients are usually not retarded. 6

7 If mental retardation exists in a 47,XXY male, his IQ is usually Klinefelter syndrome may be associated with 46,XY/ 47,XXY mosaicism, the frequency of which is probably underestimated. 46,XY/47,XXY patients are less likely than 47,XXY patients to have azoospermia, small testes, or decreased facial or pubic hair. Mean plasma testosterone levels are also higher in 46,XY/47,XXY, and mature spermatozoa are more likely to be detected. The Klinefelter phenotype may also be associated with the complements 48,XXXY and 49,XXXY. In these forms mental retardation is consistently present. Somatic anomalies occur more often in 48,XXXY and 49,XXXXY than in 47,XXY. Mental retardation is often severe in the former two. 48,XXYY patients share some features with 47,XXY and other features with 47,XYY. Testicular hypoplasia results in poorly developed secondary sexual characteristics, and many 48,XXYY patients have mental retardation. Somatic anomalies present are reminiscent of those occurring in 48,XXXY and 49,XXXXY. References Bione S, Sala C, Manzini C et al. (1998) A human homologue of the Drosophila melanogaster diaphanous gene is disrupted in a patient with premature ovarian failure: evidence for conserved function in oogenesis and implications for human sterility. American Journal of Human Genetics 62: German J, Simpson JL, Chaganti RSK et al. (1978) Genetically determined sex-reversal in 46,XY humans. Science 205: Jirasek J (1976) Principles of reproductive embryology. In: Simpson JL (ed.) Disorders of Sexual Differentiation, pp New York: Academic Press. Koopman P, Gubbay J, Vivian N, Goodfellow P and Lovell-Badge R (1991) Male development of chromosomally female mice transgenic for SRY. Nature 351: Ottolenghi C, Veitia R, Nunes M, Souleyreau-Therville N and Marc Fellous (1998) Genetics of sex determination and its pathology in man. In: Kempers RD (ed.) Fertility and Reproductive Medicine, pp Amsterdam: Elsevier Science. Simpson JL (1998) Genetics of oocyte depletion. In: Lobo RA (ed.) Perimenopause, Serono Symposia USA, Norwell, MA, pp New York: Springer-Verlag. Simpson JL (2000) Genetics of sexual differentiation. In: Carpenter SEK and Rock J (eds) Pediatric and Adolescent Gynecology. Philadelphia: Lippincott Williams & Wilkins. Simpson JL, Christakos AC, Horwith M and Silverman FS (1971) Gonadal dysgenesis associated with apparently chromosomal complements. Birth Defects 7(6): Sinclair AH, Berta P, Palmer MS et al. (1990) A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif. Nature 346: Yu RN, Ito M, Saunders TL, Camper SA and Jameson JL (1998) Role of Ahch in gonadal development and gametogenesis. Nature Genetics 20: