The biology of human psychosexual differentiation

Most attempts to identify biological underpinnings of gender identity and sexual orientation in humans have investigated effects of sex steroids, so pivotal in the differentiation of the genitalia, showing strong parallels between animals and the human. The information on humans is derived from the so-called 'experiments of nature', clinical entities with a lesser-than-normal androgen exposure in XY subjects and a higher than normal androgen exposure in XX subjects. Prenatal androgenization appears to predispose to a male gender identity development, but apparently not decisively since 40-50% of 46,XY intersexed children with a history of prenatal androgen exposure do not develop a male gender identity. Obviously, male-to-female transsexuals, with a normal androgen exposure prenatally (there is no serious evidence to the contrary) develop a female gender identity, through unknown biological mechanisms apparently overriding the effects of prenatal androgens. The latest studies in 46, XX subjects exposed to prenatal androgens show that prenatal androgenization of 46,XX fetuses leads to marked masculinization of later gender-related behavior but does not lead to gender confusion/dysphoria. The example of female-to-male transsexuals, without evidence of prenatal androgen exposure, indicates that a male gender identity can develop without a significant androgen stimulus. So we are far away from any comprehensive understanding of hormonal imprinting on gender identity formation. Brain studies in homosexuals have not held up in replication studies or are in need of replication in transsexuals. Genetic studies and the fraternal birth order hypothesis provide indications of familial clustering of homosexuality but in many homosexuals these genetic patterns cannot be identified. The biological explanations advanced for the birth order hypothesis lack any experimental support.     

On the evocability of a positive oestrogen feedback action on LH secretion in transsexual men and women.

In transsexual men with homosexual behaviour and intact testicular function, as well as in homosexual men with normal gender identity, following a negative oestrogen feedback effect a delayed positive oestrogen feedback action on LH secretion was evoked. By contrast, in transsexual men with hypo- or asexuality and intact testes or hypergonadotrophic hypo- or agonadism, as well as in heterosexual men with normal gender identity, a negative oestrogen feedback effect was not followed by a positive feedback action on LH release. In transsexual women with homosexual behaviour and oligo- and/or hypomenorrhoea, only a weak or at best moderate positive oestrogen feedback action on LH release was evocable, similarly as in castrated and oestrogen-primed heterosexual men. By contrast, in a transsexual woman with bisexual behaviour and eumenorrhoea, a strong positive oestrogen feedback action on LH secretion was evocable, as well as in heterosexual women with normal gender identity.     

Influence of neonatally administered gonadotropin on the sexual function of adult rats]

Male and female rats were daily injected with 10 IU HCG plus 10 IU FSH from the 1st to 14th day of life in order to investigate the influence of neonatal gonadotrophin administration on the sex-specific differentiation of the brain. When adult, the males showed hypogonadism associated with approximately normal sexual activity. In the females, precocious puberty, indicated by premature vaginal opening and spontaneous estrus, occurred. Furthermore, bisexuality with a tendency towards more male behavioural patterns was observed, but no impairment of ovarian cyclicity. Thus, hypergonadotrophic hypergonadism during the hypothalamic differentiation phase gave rise to bisexual behaviour in adult female rats associated with normal ovarian cycles. The question of a direct or indirect influence of gonadotrophins on the sex-specific brain differentiation is discussed.     

Sequence variation in the androgen receptor gene is not a common determinant of male sexual orientation.

To test the hypothesis that DNA sequence variation in the androgen receptor gene plays a causal role in the development of male sexual orientation, we have (1) measured the degree of concordance of androgen receptor alleles in 36 pairs of homosexual brothers, (2) compared the lengths of polyglutamine and polyglycine tracts in the amino-terminal domain of the androgen receptor in a sample of 197 homosexual males and 213 unselected subjects, and (3) screened the the entire androgen receptor coding region for sequence variation by PCR and denaturing gradient-gel electrophoresis (DGGE) and/or single-strand conformation polymorphism analysis in 20 homosexual males with homosexual or bisexual brothers and one homosexual male with no homosexual brothers, and screened the amino-terminal domain of the receptor for sequence variation in an additional 44 homosexual males, 37 of whom had one or more first- or second-degree male relatives who were either homosexual or bisexual. These analyses show that (1) homosexual brothers are as likely to be discordant as concordant for androgen receptor alleles; (2) there are no large-scale differences between the distributions of polyglycine or polyglutamine tract lengths in the homosexual and control groups; and (3) coding region sequence variation is not commonly found within the androgen receptor gene of homosexual men. The DGGE screen identified two rare amino acid substitutions, ser205-to-arg and glu793-to-asp, the biological significance of which is unknown.     

What many transgender activists don't want you to know: and why you should know it anyway

Currently the predominant cultural understanding of male-to-female transsexualism is that all male-to-female (MtF) transsexuals are, essentially, women trapped in men's bodies. This understanding has little scientific basis, however, and is inconsistent with clinical observations. Ray Blanchard has shown that there are two distinct subtypes of MtF transsexuals. Members of one subtype, homosexual transsexuals, are best understood as a type of homosexual male. The other subtype, autogynephilic transsexuals, are motivated by the erotic desire to become women. The persistence of the predominant cultural understanding, while explicable, is damaging to science and to many transsexuals.     

Becoming what we love: autogynephilic transsexualism conceptualized as an expression of romantic love

The increasing prevalence of male-to-female (MtF) transsexualism in Western countries is largely due to the growing number of MtF transsexuals who have a history of sexual arousal with cross-dressing or cross-gender fantasy. Ray Blanchard proposed that these transsexuals have a paraphilia he called autogynephilia, which is the propensity to be sexually aroused by the thought or image of oneself as female. Autogynephilia defines a transsexual typology and provides a theory of transsexual motivation, in that Blanchard proposed that MtF transsexuals are either sexually attracted exclusively to men (homosexual) or are sexually attracted primarily to the thought or image of themselves as female (autogynephilic), and that autogynephilic transsexuals seek sex reassignment to actualize their autogynephilic desires. Despite growing professional acceptance, Blanchard's formulation is rejected by some MtF transsexuals as inconsistent with their experience. This rejection, I argue, results largely from the misconception that autogynephilia is a purely erotic phenomenon. Autogynephilia can more accurately be conceptualized as a type of sexual orientation and as a variety of romantic love, involving both erotic and affectional or attachment-based elements. This broader conception of autogynephilia addresses many of the objections to Blanchard's theory and is consistent with a variety of clinical observations concerning autogynephilic MtF transsexualism.     

On the evocability of a positive oestrogen feedback action on LH secretion in female and male rats.

Following a single injection of oestradiol benzoate (15 mug/100 g body weight) postpubertally castrated and oestrogen-primed female rats showed a distinct surge of LH secretion, while castrated and androgen-primed females displayed a diminished and delayed surge of LH secretion. On the other hand, postpubertally castrated and oestrogen-primed male rats exhibited only a slight, but significant surge of LH secretion, whereas castrated and androgen-primed males did not display any surge of LH secretion following oestrogen injection. In view of these findings the evocability of a positive oestrogen feedback action on LH secretion is dependent on the sex hormone level during the critical hypothalamic differentiation phase and the functional (priming) phase as well.     

Female-specific target sites for both oestrogen and androgen in the teleost brain

To dissect the molecular and cellular basis of sexual differentiation of the teleost brain, which maintains marked sexual plasticity throughout life, we examined sex differences in neural expression of all subtypes of nuclear oestrogen and androgen receptors (ER and AR) in medaka. All receptors were differentially expressed between the sexes in specific nuclei in the forebrain. The most pronounced sex differences were found in several nuclei in the ventral telencephalic and preoptic areas, where ER and AR expression were prominent in females but almost completely absent in males, indicating that these nuclei represent female-specific target sites for both oestrogen and androgen in the brain. Subsequent analyses revealed that the female-specific expression of ER and AR is not under the direct control of sex-linked genes but is instead regulated positively by oestrogen and negatively by androgen in a transient and reversible manner. Taken together, the present study demonstrates that sex-specific target sites for both oestrogen and androgen occur in the brain as a result of the activational effects of gonadal steroids. The consequent sex-specific but reversible steroid sensitivity of the adult brain probably contributes substantially to the process of sexual differentiation and the persistent sexual plasticity of the teleost brain.     

Estrogen treatment effects on cognition, memory and mood in male-to-female transsexuals

Gonadal hormones, particularly estrogens, have been suggested to influence memory and cognitive tasks that show sex differences. Previously, we reported that male-to-female (M-F) transsexuals undergoing estrogen treatment for sex re-assignment scored higher on verbal Paired Associate Learning (PAL) than a transsexual control group awaiting estrogen treatment. The present study used a more robust design to examine further associations between estrogen and cognition. We assessed additional aspects of memory, including visual, spatial, object and location memory, other cognitive abilities that show reliable sex differences, including verbal and visual-spatial abilities, and mood variables that could mediate associations between estrogen and cognition. In addition to comparing groups of individuals on and off estrogen, we used two repeated measures designs (AB and BA). The AB group was tested prior to hormone treatment and then again after treatment had begun; the BA group was tested while on estrogen treatment and then again when hormones had been withdrawn prior to surgery. Few changes in memory or cognition were observed, and changes that were observed were not consistent across study designs. The lack of significant effects did not relate to mood changes or to the sexual orientation of participants. These findings suggest that estrogen treatment associated with sex change for M-F transsexuals has little or no influence on sex-typed aspects of cognition or memory.     

The ratio of 2nd to 4th digit length and male homosexuality

Sexual orientation may be influenced by prenatal levels of testosterone and oestrogen. There is evidence that the ratio of the length of 2nd and 4th digits (2D:4D) is negatively related to prenatal testosterone and positively to oestrogen. We report that (a) 2D:4D was lower in a sample of 88 homosexual men than in 88 sex- and age-matched controls recruited without regard to sexual orientation, (b) within the homosexual sample, there was a significant positive relationship between mean 2D:4D ratio and exclusive homosexuality, (c) overall, there was a decrease in 2D:4D from controls to homosexual men to bisexual men and (d) fraternal birth order, a positive predictor of male homosexuality, was not associated with 2D:4D in a sample of 240 Caucasian men recruited without regard to sexual orientation and 45 homosexual men.Further work is needed to confirm the relationships between 2D:4D and sexual orientation. However, these and other recent data tend to support an association between male homosexuality and high fetal testosterone. Very high testosterone levels may be associated with a sexual preference for both men and women.     

Sex-related differences in hematological values. A study on the erythrocyte and granulocyte count, plasma iron and iron-binding proteins in human transsexuals on contrasexual hormone therapy

Hematological data known or supposed to be influenced by individual sex hormones were evaluated in 18 untreated transsexuals (TS) and in 20 castrated or non-castrated TS on androgen and estrogen treatment, respectively. Profiting from a situation of clinically controlled hormonal sex-transformation it was tested, whether the circulating erythrocyte and granulocyte mass and iron metabolism are linked to a male and female sex-hormone constellation. The erythrocyte and granulocyte counts were significantly higher in untreated males and treated female-to-male TS than in untreated females and treated male-to-female TS. The unexpected finding of sex hormone-dependent granulocyte fluctuations was corroborated by parallel concentration changes of lactoferrin, a granulocyte-derived plasma protein. Iron metabolism as judged from plasma iron, total iron-binding capacity and serum ferritin was unaffected by sexual transformation. Plasma iron and the total iron-binding capacity did not differ significantly in untreated and treated TS of either type. The serum ferritin concentration, however, was significantly lower in untreated as well as in virilized females than in untreated and in feminized males, but was not significantly changed by long-term androgen or estrogen treatment. The present study demonstrates the potential of human transsexualism as a model for the study of sex-related biological processes.     

Neurological effects of aromatase deficiency in the mouse

In the brain, the conversion from androgen into estrogen is an important process for the differentiation of the brain function in male rodents. The aromatase is expressed in some nucleus of the brain. To assess the functional significance of the aromatase gene in development and activation of sex-specific behavior, we analyzed behavioral phenotypes of the aromatase knockout (ArKO) male mice. ArKO males obviously decreased their fertility and showed deficits in male sexual behavior including mount, intromission and ejaculation. Noncontact penile erection was not significantly affected by defect of the aromatase gene. A reduction of aggressive behavior against male intruders was also observed in ArKO males, while they tend to exhibit aggression toward estrous females during male copulatory tests. Moreover, the infanticide toward the pups was observed in the ArKO males, whereas characteristic parental behavior, but not infanticide was observed in wild-type males. These results indicate that aromatase gene expression is a critical step not only for motivational and consummatory aspects of male sexual behavior, but also for aggressive and parental behaviors in male mice.     

Estrogen-Independent Ovary Formation in the Medaka Fish, Oryzias latipes

To investigate whether a female sex steroid, estrogen, acts as a natural inducer of female gonadal sex determination (or ovary formation) in the medaka fish, Oryzias latipes, the effects of an aromatase inhibitor and anti-estrogens on sexual differentiation of gonads were examined. We found that both drugs did not show any discernible effects on the genetically determined sex differentiation in both sexes. However, the aromatase inhibitor impaired the paradoxical effects of androgen (a male sex steroid), and the anti-estrogens inhibited the male-to-female sex reversal caused by estrogen. Treatments of the fertilized eggs with androgen disturbed the gonadal sex developments in both sexes, suggesting that sex steroid synthesis is detrimental to the gonadal sex developments in the medaka embryos. These results are consistent with the previous observation that sex steroids are not synthesized before the onset of gonadal sex differentiation, and suggest that ovary formation in the genetic females of the medaka fish is not dependent on estrogen.     

Sparing a testis during vaginoplasty in male-to-female transsexuals: does it benefit our patients?

Testosterone and its metabolite dihydrotestosterone are the libido hormones for the male, vital to sex drive and sexual function. Fear of loss of libido and orgasm is the main reason to retain at least one testis in male-to-female transsexuals during vaginoplasty. We report on four South American transsexual patients in whom we resected a remaining testis to illustrate the superfluity of retaining it. Because there are multiple reasons for castration, we advise that bilateral orchidectomy be performed in the course of sex reassignment surgery for male-to-female transsexuals.     

Sex Steroids and the Differentiation of Avian Reproductive Behavior

Experiments in which avian embryos are treated with sex steroidsor steroid antagonists suggest that sexual differentiation ofreproductive behavior (and thus differentiation of the brainmechanisms for such behavior) is controlled by steroids producedby the embryonic gonads. In chickens and Japanese quail, maleshatched from eggs treated with estradiol or testosterone duringincubation are feminized (demasculinized); they fail to exhibitmasculine sexual behavior as adults, and no longer are behaviorallydistinguishable from females. Some evidence suggests that testosteronemay mimic the feminizing action of estradiol by being convertedto an estrogen in the embryonic brain. Genetic female quailexposed to an antiestrogen during embryonic development aremasculinized; they exhibit an increased ability to display themasculine copulatory pattern. Thus the behavior of these speciesis feminized by embryonic exposure to sex steroids, the anhormonal(neutral) sex for behavioral differentiation appears to be themale, and females appear to result from estrogen produced bythe embryonic ovaries. In contrast, sex steroid treatment ofmammals early in development masculinizes behavior, the femaleis the neutral sex, and males result from fetal androgen secretion.These opposite patterns of psychosexual differentiation in birdsand mammals are correlated with a major difference between theavian and mammalian sex-determining mechanism. Implicationsfor other vertebrates are discussed.     

Face perception is modulated by sexual preference

Face perception is mediated by a distributed neural system in the human brain . The response to faces is modulated by cognitive factors such as attention, visual imagery, and emotion ; however, the effects of gender and sexual orientation are currently unknown. We used fMRI to test whether subjects would respond more to their sexually preferred faces and predicted such modulation in the reward circuitry. Forty heterosexual and homosexual men and women viewed photographs of male and female faces and assessed facial attractiveness. Regardless of their gender and sexual orientation, all subjects similarly rated the attractiveness of both male and female faces. Within multiple, bilateral face-selective regions in the visual cortex, limbic system, and prefrontal cortex, similar patterns of activation were found in all subjects in response to both male and female faces. Consistent with our hypothesis, we found a significant interaction between stimulus gender and the sexual preference of the subject in the thalamus and medial orbitofrontal cortex, where heterosexual men and homosexual women responded more to female faces and heterosexual women and homosexual men responded more to male faces. Our findings suggest that sexual preference modulates face-evoked activation in the reward circuitry.     

Estrogen and Androgen Receptor Proteins in Embryonic and Neonatal Brain: Hypotheses for Roles in Sexual Differentiation and Behavior

We have demonstrated and partially characterized putative estrogenand androgen receptors from mouse hypothalamus for a range ofperinatal ages. For the first time, estrogen and androgen receptorsfrom embryonic mouse and rat hypothalamus are described andcharacterized; they display similar parameters as the receptorproteins of adult mice and rats. The ontogeny of these proteinsis discussed in the context of models for the control of the"critical period" of sexual differentiation of the brain. The androgen-binding proteins, presumed to be receptors, arecompared for hypothalamus and kidney and for the androgen-resistantmutant mouse, testicular feminization (Tfm). The putative receptorforms that are observed help to define the possible functionof brain androgen receptors during sexual differentiation. Development and modification of DNA—cellulose chromatographyfor the affinity separation of steroid receptors of brain isdescribed. The methods allow complete separations of receptorproteins from non—receptor, steroid-binding proteins andsubsequent analysis of the resultant receptors.     

Steroid 5α-reductase in adult rat brain after neonatal testosterone administration

Testosterone (T) plays an important role in developing brain, dictating sex-specific behavior and physiology. 3α,5α-Reduced neurosteroids also regulate reproductive behavior. The key enzyme in the biosynthesis of these neurosteroids is 5α-reductase (5α-R), expressed as two isozymes, 5α-R1 and 5α-R2. In this study, T and sesame oil (vehicle) were administered during postnatal sexual differentiation of the central nervous system (CNS) and mRNA levels of 5α-R isozymes, were measured using quantitative RT-PCR in prefrontal cortex of male and female rats with different androgenic status at adulthood. Our results indicate that T concentrations during postnatal sexual differentiation of the rat CNS, among other sex-dependent factors, influence brain levels of 5α-R isozymes in adulthood and the pattern of their regulation by androgen hormones.     

Treatment of pregnant rhesus macaques with testosterone propionate: observations on its fate in the fetus

Castrated male primates, unlike castrated male rodents, respond to exogenous estrogen by releasing gonadotropin. Although this disparity is unexplained, it may occur because the amount of testicular androgen secreted during a critical period for sexual differentiation is not sufficient to completely androgenize the anlagen of the central nervous system (CNS) in male primates. Therefore, male primates might be incompletely masculinized, and if fetal males were exposed to additional androgen during sexual development, they would no longer display the positive feedback to estrogen that usually characterizes females. Besides the development of mechanisms mediating the release of gonadotropins, questions about relationships between adult male sexual behaviors and the intensity of the androgen stimulus upon developing neural structures are of interest. We tested some of these possibilities by injecting androgen into 8 pregnant rhesus macaques from Days 40 through 50 of gestation, and we compared serum levels of testosterone (T), dihydrotestosterone (DHT), and androstenedione (delta 4) in the fetal circulation with that of 5 untreated control males. Fetal sera (both male and female) from treated pregnancies did not contain significantly greater quantities of T and DHT than sera of intact control males from untreated mothers. The maternal sera, however, contained large amounts of T (125.05 +/- 27.40 [SEM] ng/ml, n = 8) and significant elevations of DHT and delta 4 after treatment. The concentrations of delta 4 in the fetal circulation were significantly elevated (p less than 0.01) in treated fetuses compared to intact control males, probably due to the actions of the 17 beta-hydroxysteroid dehydrogenases in the placenta, the fetus, or both.(ABSTRACT TRUNCATED AT 250 WORDS)