Sexual differentiation

Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote (fertilized egg). As male and female individuals develop from zygotes into fetuses, into infants, children, adolescents, and eventually into adults, sex and gender differences at many levels develop: genes, chromosomes, gonads, hormones, anatomy, psyche, and social behaviors.

Sex differences range from nearly absolute to simply statistical. Sex-dichotomous differences refer to aspects of development which are wholly characteristic of one sex only. Examples of sex-dichotomous differences include aspects of the sex-specific genital organs such as ovaries, a uterus or a phallic urethra). In contrast, sex-dimorphic differences are matters of degree (e.g., size of phallus). Some of these (e.g., stature, behaviors) are mainly statistical, with much overlap between male and female populations.

Nevertheless, even the sex-dichotomous differences are not absolute in the human population, and there are individuals who are exceptions (e.g., males with a uterus, or females with an XY karyotype), or who exhibit biological and/or behavioral characteristics of both sexes.

Sex differences may be induced by specific genes, by hormones, by anatomy, or by social learning. Some of the differences are determined entirely by biological processes (e.g., presence of a uterus) and some differences are just as obviously purely a matter or social learning and custom (e.g., relative hair length). Many differences, though, such as gender identity, appear to be influenced by both biological and social factors ("nature" and "nurture").

The early stages of human differentiation appear to be quite similar to the same biological processes in other mammals and the interaction of genes, hormones and body structures is fairly well understood. In the first weeks of life, a fetus has no anatomic or hormonal sex, and only a karyotype distinguishes male from female. Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading to psychological and behavioral differences, some of which are innate and some induced by the social environment.

The relationship of biological factors to more advanced and more specifically human aspects of differentiation, and especially the ways and degrees by which social and environmental influences contribute to psychological and behavioral differentiation, is less well understood and a common subject of political controversy.

Chromosomal sex differences
Humans have 46 chromosomes, including 2 sex chromosomes, XX in females and XY in males. It is obvious that the Y chromosome must carry at least one essential gene which determines testicular formation (originally termed TDF). A gene in the sex-determining region of the short arm of the Y, now referred to as SRY, has been found to direct production of a protein which binds to DNA, inducing differentiation of cells derived from the genital ridges into testes. In transgenic XX mice (and some human XX males), SRY alone is sufficient to induce male differentiation.

Investigation of other cases of human sex reversal (XX males, XY females) has led to discovery of other genes crucial to testicular differentiation on autosomes (e.g., WT-1, SOX9, SF-1), and the short arm of X (DSS).

Gonadal differentiation
Early in fetal life, germ cells migrate to the genital ridge. By week 6, undifferentiated gonads consist of germ cells, supporting cells, and steroidogenic cells.

In a male, SRY and other genes induce differentiation of supporting cells into Sertoli cells and (indirectly) steroidogenic cells into Leydig cells to form testes, which become microscopically identifiable and begin to produce hormones by week 8. Germ cells become spermatogonia.

Without SRY, ovaries form during months 2-6. Failure of ovarian development in 45,X girls (Turner syndrome) implies that two functional copies of several Xp and Xq genes are needed. Germ cells become ovarian follicles. Supporting and steroidogenic cells become granulosa cells and theca cells, respectively.

Hormonal differentiation
In a male fetus, testes produce steroid and protein hormones essential for internal and external anatomic differentiation. Leydig cells begin to make testosterone by the end of month 2 of gestation. From then on, male fetuses have higher levels of androgens in their systemic blood than females. The difference is even greater in pelvic and genital tissues. Antimullerian hormone (AMH) is a protein hormone produced by Sertoli cells from the 8th week on. AMH suppresses development of müllerian ducts in males, preventing development of a uterus.

Fetal ovaries produce estradiol, which supports follicular maturation but plays little part in other aspects of prenatal sexual differentiation, as maternal estrogen floods fetuses of both sexes.

Internal genital differentiation
Gonads are histologically distinguishable by 6-8 weeks of gestation. A fetus of that age has both mesonephric (wolffian) and paramesonephric (mullerian) ducts. Subsequent development of one set and degeneration of the other depends on the presence or absence of two testicular hormones: testosterone and AMH. Disruption of typical development may result in the development of both, or neither, duct system, which may produce morpologically bisexual individuals.

Local testosterone causes each wolffian duct to develop into epididymis, vas deferens, and seminal vesicles. Without male testosterone levels, wolffian ducts degenerate and disappear. Müllerian ducts develop into a uterus, fallopian tubes, and upper vagina unless AMH induces degeneration. The presence of a uterus is stronger evidence of absence of testes than the state of the external genitalia.

External genital differentiation
For illustrations, see the External links section.

By 7 weeks, a fetus has a genital tubercle, urogenital groove and sinus, and labioscrotal folds. In females, without excess androgens, these become the clitoris, urethra and vagina, and labia.

Males become externally distinct between 8 and 12 weeks, as androgens enlarge the phallus and cause the urogenital groove and sinus to fuse in the midline, producing an unambiguous penis with a phallic urethra, and a thinned, rugated scrotum.

A sufficient amount of any androgen can cause external masculinization. The most potent is dihydrotestosterone (DHT), generated from testosterone in skin and genital tissue by the action of 5&alpha;-reductase. A male fetus may be incompletely masculinized if this enzyme is deficient. In some diseases and circumstances, other androgens may be present in high enough concentrations to cause partial or (rarely) complete masculinization of the external genitalia of a genetically female fetus.

Further sex differentiation of the external genitalia occurs at puberty, when androgen levels again become disparate. Male levels of testosterone directly induce growth of the penis, and indirectly (via DHT) the prostate.

Breast differentiation
Visible differentiation occurs at puberty, when estradiol and other hormones cause breasts to develop in girls. However, fetal or neonatal androgens may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen.

Other body differentiation
General habitus and shape of body and face, as well as sex hormone levels, are similar in prepubertal boys and girls. As puberty progresses and sex hormone levels rise, obvious differences appear.

In males, testosterone directly increases size and mass of muscles, vocal cords, and bones, enhancing strength, deepening the voice, and changing the shape of the face and skeleton. Converted into DHT in the skin, it accelerates growth of androgen-responsive facial and body hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion.

In females, breasts are the most obvious manifestation of higher levels of estrogen, but estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, and menses.

The difference in adult masculine and feminine faces is largely a result of heavier jaw and jaw muscle development induced by testosterone in late adolescence. Masculine features on average are slightly thicker and coarser. Androgen-induced recession of the male hairline accentuates these differences by middle adult life.

Sexual dimorphism of skeletal structure develops during childhood, and becomes more pronounced at adolescence. Sexual orientation has been demonstrated to correlate with skeletal characters that become dimorphic during early childhood (such as arm length to stature ratio) but not with characters that become dimorphic during puberty (such as shoulder width) (Martin & Nguyen, 2004).

Brain differentiation
In most animals, differences of exposure of a fetal or infant brain to sex hormones produce significant and irreversible differences of brain structure and function which correlate with adult reproductive behavior. In humans, sex hormone levels in male and female fetuses and infants differ, and both androgen and estrogen receptors have been identified in brains. Several sex-specific genes not dependent on sex steroids are expressed differently in male and female human brains. Structural sex differences begin to be recognizable by 2 years of age, and in adult men and women include size and shape of corpus callosum and certain hypothalamic nuclei, and the gonadotropin feedback response to estradiol.

Psychological and behavioral differentiation
Sex steroid differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some mammals, adult sex-dimorphic reproductive behavior (e.g., mounting or receptive lordosis) can be shifted to that of the other sex by supplementation or deprivation of androgens in fetal life or early infancy, even if adult levels are normal.

Psychological and behavioral differentiation in humans
Human adults and children show many psychological and behavioral sex differences, both dichotomous and dimorphic. Some (e.g., dress) are learned and obviously cultural. Others (e.g., early verbal fluency, spatial reasoning) are demonstrable across cultures and may have both biological and learned determinants. Because we cannot explore hormonal influences on human reproductive behavior experimentally, and because potential political implications are so unwelcome to many factions of society, the relative contributions of biological factors and learning to human psychological and behavioral sex differences (especially gender identity, role, and orientation) remain unsettled and controversial.

Gender identity, role, and orientation
Gender identity is the subjective sense of being male or female-- it cannot be externally measured, only asserted by a person or sometimes inferred from the gender role, which consists of all behaviors which are sex-dimorphic in that person's culture. For an anatomically normal person, a sense that one's true gender identity differs from the sex of anatomy and rearing is termed gender identity disorder, also known as transsexualism or transgender.

In the 20th century it was widely assumed and taught by academics that gender identity and gender role are purely learned, with minimal biological determination. However, many individual cases are suggestive of hormonal and other physical influence.

Sexual orientation, the sex to which one is erotically most attracted is the most politically contentious aspect of psychosexual differentiation. Although the idea of a biological "cause" of homosexuality was mostly rejected in academic quarters in the 1970s and early 80s, recent reports of structural brain differences and mendelian inheritance patterns make a persuasive case for reconsidering a role for biologic factors in male homosexuality.

Although people are often simply either "male" or "female" in many of their relations with the institutions of their society, the degree to which various aspects of gender identity, gender role and sexual orientation are sex-dimorphic, rather than dichotomous, varies widely among cultures. Some argue that social gender roles should be even less dimorphic, or that more than two sexes/genders should be recognised.

These issues complicate management of infants with anatomic ambiguity or intersex conditions.

Defeminization and masculinization (female as the "default" path)
Sexual differentiation in mammals is biased towards developing as a female, so that it is often said that female is the default developmental pathway. Two processes: defeminization, and masculinization, are involved in producing male typical morphology and behavior. Disruption of either of these processes in males produces female-typical development. The opposite is not true, disruption of normal sexual development in females does not lead to male-typical endpoints.

Defeminization involves the suppression of the development of female typical morphology (development of the Mullerian ducts into the fallopian tubes, uterus and vagina) and behavioural predispositions. Masculinization involves the production of male typical morphology (development of the Wolffian ducts into male reproductive structures) and behavioural predispositions. Both defeminization and masculinization are required for a mammalian zygote to become a fully reproductively functional male.

A brief version of the female default paradigm can be stated as follows:


 * 1) A set of specific genetic instructions must be present and a series of differentiating events mediated by hormones must occur in order for a mammalian zygote to become a fully reproductively functional male.
 * 2) The SRY, SOX9, and SF1 genes must be present and functional.
 * 3) Functional Leydig cells must form in the gonads.
 * 4) The Leydig cells must be able to produce testosterone.
 * 5) The target cells must have the hormone receptors to respond to the testosterone. The target cells of the external genitalia must have functional 5-alpha-reductase enzyme to convert some of the testosterone to more active dihydrotestosterone.
 * 6) There is some evidence that the brain must be exposed and respond to androgens either prenatally or early in life to produce characteristic mating behavior. This is well demonstrated in many animal species but remains mostly speculative with respect to humans.
 * 7) To a large extent, each step builds on the previous. If anything goes wrong at any of the first four steps, the subsequent pathway of development results in female anatomy and behavior.
 * 8) No ovarian organizing gene homologous to SRY has been discovered. Both sexes are exposed to maternal estrogen prenatally. No hormones have yet been discovered that are necessary early in life to produce female sexual development. Estrogen seems not to be necessary until puberty for purposes of differentiation.

This paradigm dates back to the 1950s. Even stronger versions were commonly stated in the 1960s and 1970s. One version, perhaps most associated with John Money, (who termed it the Adam principle), held that additional steps in the cascade to male identity were the recognition by parents and doctor that the external genitalia were male, which resulted in a male sex assignment, which in turn resulted in a male sex of rearing by parents and society, which in turn (coupled with the reinforcing appearance of male genitalia) resulted in a male gender identity. At least by implication,  female gender identity simply required a female sex of rearing and lack of an obvious penis.

In Germany in the 1970s, Günter Dõrner extrapolated the cascade to include direct testosterone effect on the brain as necessary for a male gender identity and sexual orientation, proposing that transsexualism and or homosexuality in biological males could result from deficiency of prenatal or early postnatal testosterone effect on the brain.

In the 1970s, Money's version was widely accepted in liberal academic circles and Dõrner's was fiercely denounced, especially in the gay and transsexual communities. The last decade has seen somewhat of a reversal of appeal in similar circles. This turnabout perhaps illustrates the difficulties of investigating these relationships in humans rather than rodents, the complexities of attempting to explain core aspects of the human psyche, and perhaps the influence of intellectual and political fashion in the application of scientific knowledge to human affairs.