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Sexual differentiation is the process of development of the differences between
males and females from an undifferentiated
zygote (fertilization
ovum). 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 are developments which are wholly characteristic of one sex only. Examples of sex-dichotomous differences include aspects of the sex-specific genital organs such as ovary, a
uterus or a phallic urethra). In contrast,
sexual dimorphism differences are matters of degree (e.g., size of Phallus#In physical anatomy). Some of these (e.g.,
height, 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 sex-determination system 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 entirely physical (e.g., presence of a uterus) and some differences are just as obviously purely a matter of 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 versus 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
society.
The various ways that genes, hormones, and upbringing affect different human behaviors and mental traits are difficult to test experimentally and charged with political conflict.
Chromosomal sex differences
Humans have forty-six chromosomes, including two sex chromosomes,
XY sex-determination system in females and XY sex-determination system in males. It is obvious that the
Y chromosome must carry at least one essential gene which determines testis 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 male syndromes),
SRY alone is sufficient to induce male differentiation.
Investigation of other cases of human sex reversal (
XX male syndromes, Swyer syndromes) 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 Gonadal 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
testis, which become microscopically identifiable and begin to produce hormones by week 8. Germ cells become Spermatogonium.
Without
SRY, ovary form during months 2-6. Failure of ovarian development in 45,X girls (
Turner syndrome) implies that two functional copies of several
X chromosome and X chromosome genes are needed. Germ cells become
ovarian follicles. Supporting and steroidogenic cells become
theca cells and granulosa cells, respectively.
Hormonal differentiation
In a male fetus, testes produce
steroid hormone 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.
Genital differentiation
A differentiation of the
sex organ can be seen. However, this is only the external genital differentiation. There is also an internal genital differentiation.
Internal genital differentiation
Gonads are histologically distinguishable by 6-8 weeks of gestation. A fetus of that age has both mesonephric (
wolffian ducts) and paramesonephric (mullerian ducts) ducts. Subsequent development of one set and degeneration of the other depends on the presence or absence of two testicular hormones: testosterone and antimullerian hormone. Disruption of typical development may result in the development of both, or neither, duct system, which may produce morphologically
intersexual 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
virilization. The most potent is dihydrotestosterone (DHT), generated from testosterone in skin and genital tissue by the action of 5α-reductase. A male fetus may be incompletely masculinized if this enzyme is 5-alpha-reductase deficiency. 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.
Hair differentiation
The amount and distribution of body
hair differs between the sexes. Males have more
terminal hair, especially on the
facial hair,
chest hair, abdominal hair and back, and females have more vellus hair, which is less visible. This may also be linked to
neoteny in humans, as
vellus hair is a
adolescent characteristic.
Other body differentiation
The differentiation of other parts of the body than the sex organ creates the secondary sex characteristics.
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 epiphyses.
In females, in addition to breast differentiation, 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
menstrual cycle.
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.This seems to be the case in humans as well; sex hormone levels in male and female fetuses and infants differ, and both androgen and estrogen
receptor (biochemistry)s 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 hypothalamus nuclei, and the
gonadotropin feedback response to
estradiol.
Psychological and behavioral differentiation
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 are demonstrable across cultures and may have both biological and learned determinants. For example, girls are, on average, more verbally fluent than boys, but males, on average, are better at spatial calculation. Because we cannot explore hormonal influences on human 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
sexual orientation) remain unsettled and controversial.
Current theories of mechanisms of sexual differentiation of brain and behaviors in humans are based primarily on three sources of evidence: animal research involving manipulation of hormones in early life, observation of outcomes of small numbers of individuals with disorders of sexual development (intersex conditions or cases of early
sex reassignment), and statistical distribution of traits in populations (e.g., rates of homosexuality in twins). Many of these cases suggest some genetic or hormonal effect on sex differentiation of behavior and mental traitsSteven Pinker.
The Blank Slate. New York: Penguin. 2002. pages 346-350; others do not.
In addition to affecting development, changing hormone levels affect certain behaviors or traits that are gender dimorphic, such as superior verbal fluency among women.Steven Pinker.
The Blank Slate. New York: Penguin. 2002. pages 347-348.
In most
mammals species, and in other hominid species, females are more oriented toward child rearing and males toward competition with other males.
Biology of gender
Biology of gender is the scientific analysis of the physical basis for behavioural differences between men and women. It deals with gender identity, gender roles and
sexual 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. 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 case studies suggested hormonal, genetic, or other physical influence Reiner, W. G. (for example, see David Reimer).
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 ambiguous genitalia or
intersex conditions.
Sex and intelligence
There is uncertainty whether there is any difference in absolute intelligence between men and women. Males have a larger brain, but women have more connections between the neurons. Males have a larger tendency to end up in extreme values, both on the lower and the upper end of the intelligence scale, while women generally are closer to the average. Females often outperform males on various measures of verbal ability, while males tend to outperform females on measures of mathematical and spatial ability.
Defeminization and masculinization
Defeminization and masculinization are the processes that a fetus goes through to become a male in sexual differentiation. In this perspective, the female is the default path for a developing human being, was it not for intervening factors that alter the path to the male one.
Biologically, this perspective is supported by that there is neither corresponding female genes nor female hormones to the ones that are active in males only. Estrogen, for instance, is present in both male and female fetuses.
See also
References
- Baum MJ. Mammalian animal models of psychosexual differentiation: When is
‘translation’ to the human situation possible? (2007)Hormones and Behavior 50:579–88.
- Crouch RA. Betwixt and between: the past and future of intersexuality. J Clin Ethics 9:372-384.
Hughes IA, Houk C, Ahmed SF, Lee PA, LWPES/ESPE Consensus Group. (2006) Consensus statement on management of intersex disorders. Arch Dis Childhood.
- Martin, J. T. and Nguyen, D. H. (2004). Anthropometric analysis of homosexuals and heterosexuals: implications for early hormone exposure. Hormones and Behavior 45. 31-39.
- Phoenix, C.H., Goy, R.W., Gerall, A.A. and Young, W.C. (1978). Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65, 369-382.
- Wallen, K. (2005) Hormonal influences on sexually differentiated behavior in nonhuman primates. Frontiers in Neuroendocrinology 26, 7-26.
- Wilson BE, Reiner WE. (1998) Management of intersex: a changing paradigm. J Clin Ethics 9:360-9.
External links
- Human Sexual Differentiation by P. C. Sizonenko
- The Ciba Collection of Medical Illustrations: Vol.2, Reproductive System by Frank H. Netter, M.D. comparing female and male reproductive systems development and anatomy
- Development of the Female Sexual & Reproductive Organs – illustrations comparing female and male genitalia during the early development
Sexual differentiation is the process of development of the differences between males and females from an undifferentiated
zygote (
fertilization ovum). 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 are developments which are wholly characteristic of one sex only. Examples of sex-dichotomous differences include aspects of the sex-specific genital organs such as ovary, a
uterus or a phallic
urethra). In contrast, sexual dimorphism differences are matters of degree (e.g., size of
Phallus#In physical anatomy). Some of these (e.g., height, 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 sex-determination system
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 entirely physical (e.g., presence of a uterus) and some differences are just as obviously purely a matter of 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 versus 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
society.
The various ways that genes, hormones, and upbringing affect different human behaviors and mental traits are difficult to test experimentally and charged with political conflict.
Chromosomal sex differences
Humans have forty-six chromosomes, including two sex chromosomes, XY sex-determination system in females and XY sex-determination system in males. It is obvious that the Y chromosome must carry at least one essential gene which determines testis 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 male syndromes),
SRY alone is sufficient to induce male differentiation.
Investigation of other cases of human sex reversal (
XX male syndromes,
Swyer syndromes) 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 Gonadal 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 testis, which become microscopically identifiable and begin to produce hormones by week 8. Germ cells become
Spermatogonium.
Without
SRY, ovary form during months 2-6. Failure of ovarian development in 45,X girls (Turner syndrome) implies that two functional copies of several X chromosome and X chromosome genes are needed. Germ cells become ovarian follicles. Supporting and steroidogenic cells become
theca cells and granulosa cells, respectively.
Hormonal differentiation
In a male fetus, testes produce steroid hormone 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.
Genital differentiation
A differentiation of the
sex organ can be seen. However, this is only the external genital differentiation. There is also an internal genital differentiation.
Internal genital differentiation
Gonads are histologically distinguishable by 6-8 weeks of gestation. A fetus of that age has both mesonephric (wolffian ducts) and paramesonephric (
mullerian ducts) ducts. Subsequent development of one set and degeneration of the other depends on the presence or absence of two testicular hormones:
testosterone and antimullerian hormone. Disruption of typical development may result in the development of both, or neither, duct system, which may produce morphologically
intersexual 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 virilization. The most potent is dihydrotestosterone (DHT), generated from testosterone in skin and genital tissue by the action of 5α-reductase. A male fetus may be incompletely masculinized if this enzyme is 5-alpha-reductase deficiency. 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.
Hair differentiation
The amount and distribution of body hair differs between the sexes. Males have more terminal hair, especially on the facial hair, chest hair, abdominal hair and back, and females have more vellus hair, which is less visible. This may also be linked to neoteny in humans, as
vellus hair is a
adolescent characteristic.
Other body differentiation
The differentiation of other parts of the body than the
sex organ creates the secondary sex characteristics.
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
epiphyses.
In females, in addition to breast differentiation, 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 menstrual cycle.
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.This seems to be the case in humans as well; sex hormone levels in male and female fetuses and infants differ, and both androgen and estrogen receptor (biochemistry)s 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 hypothalamus nuclei, and the gonadotropin feedback response to
estradiol.
Psychological and behavioral differentiation
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 are demonstrable across cultures and may have both biological and learned determinants. For example, girls are, on average, more verbally fluent than boys, but males, on average, are better at spatial calculation. Because we cannot explore hormonal influences on human 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 sexual orientation) remain unsettled and controversial.
Current theories of mechanisms of sexual differentiation of brain and behaviors in humans are based primarily on three sources of evidence: animal research involving manipulation of hormones in early life, observation of outcomes of small numbers of individuals with disorders of sexual development (intersex conditions or cases of early
sex reassignment), and statistical distribution of traits in populations (e.g., rates of homosexuality in twins). Many of these cases suggest some genetic or hormonal effect on sex differentiation of behavior and mental traits
Steven Pinker.
The Blank Slate. New York: Penguin. 2002. pages 346-350; others do not.
In addition to affecting development, changing hormone levels affect certain behaviors or traits that are gender dimorphic, such as superior verbal fluency among women.
Steven Pinker. The Blank Slate. New York: Penguin. 2002. pages 347-348.
In most
mammals species, and in other
hominid species, females are more oriented toward child rearing and males toward competition with other males.
Biology of gender
Biology of gender is the scientific analysis of the physical basis for behavioural differences between men and women. It deals with
gender identity, gender roles and sexual 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. 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 case studies suggested hormonal, genetic, or other physical influence Reiner, W. G. (for example, see David Reimer).
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
ambiguous genitalia or intersex conditions.
Sex and intelligence
There is uncertainty whether there is any difference in absolute intelligence between men and women. Males have a larger brain, but women have more connections between the neurons. Males have a larger tendency to end up in extreme values, both on the lower and the upper end of the intelligence scale, while women generally are closer to the average. Females often outperform males on various measures of verbal ability, while males tend to outperform females on measures of mathematical and spatial ability.
Defeminization and masculinization
Defeminization and masculinization are the processes that a fetus goes through to become a male in sexual differentiation. In this perspective, the female is the default path for a developing human being, was it not for intervening factors that alter the path to the male one.
Biologically, this perspective is supported by that there is neither corresponding female genes nor female hormones to the ones that are active in males only. Estrogen, for instance, is present in both male and female fetuses.
See also
References
- Baum MJ. Mammalian animal models of psychosexual differentiation: When is
‘translation’ to the human situation possible? (2007)Hormones and Behavior 50:579–88.
- Crouch RA. Betwixt and between: the past and future of intersexuality. J Clin Ethics 9:372-384.
Hughes IA, Houk C, Ahmed SF, Lee PA, LWPES/ESPE Consensus Group. (2006) Consensus statement on management of intersex disorders. Arch Dis Childhood.
- Martin, J. T. and Nguyen, D. H. (2004). Anthropometric analysis of homosexuals and heterosexuals: implications for early hormone exposure. Hormones and Behavior 45. 31-39.
- Phoenix, C.H., Goy, R.W., Gerall, A.A. and Young, W.C. (1978). Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65, 369-382.
- Wallen, K. (2005) Hormonal influences on sexually differentiated behavior in nonhuman primates. Frontiers in Neuroendocrinology 26, 7-26.
- Wilson BE, Reiner WE. (1998) Management of intersex: a changing paradigm. J Clin Ethics 9:360-9.
External links
- Human Sexual Differentiation by P. C. Sizonenko
- The Ciba Collection of Medical Illustrations: Vol.2, Reproductive System by Frank H. Netter, M.D. comparing female and male reproductive systems development and anatomy
- Development of the Female Sexual & Reproductive Organs – illustrations comparing female and male genitalia during the early development