Fetal genital development
Encyclopedia
Fetal genital development refers to the sequence of events which leads to the development of the genitals and their adnexa.
behind the protuberance.
In typical fetal
development, the presence of the SRY gene
causes the fetal gonad
s to become testes; the absence of it allows the gonads to continue to develop into ovaries. Thereafter, the development of the internal reproductive organs
and the external genitalia is determined by hormones produced by fetal gonads (ovaries or testes) and the cells' response to them.
If the fetus has testes, and if the testes produce testosterone
, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum
; the protuberance grows larger and straighter to form the penis
; the inner urogenital swellings swell, wrap around the penis, and fuse in the midline to form the penile urethra.
If testosterone is not present, normal female development continues, with the development of a perineal urethra
and the formation of a uterus
, clitoris
and vagina
.
The Müllerian ducts, which are paired ducts of the embryo which empty into the cloaca
, and which develop into the upper vagina
, cervix
, uterus
and oviducts; in the male they disappear except for the vestigial vagina masculina and the appendix testis
.
Although female development is usually considered the "default" configuration in the absence of induction, recent evidence has indicated that Wnt4
plays a role in the development of female anatomy.
; for example, the clitoris and penis are homologous with one another, as are Skene's glands in females and the prostate gland in males.
See congenital anomalies of the genitalia
for a list of congenital disorder
s caused by problems with fetal genital development.
Sequence
A few weeks after fertilization, the initial appearance of the human fetal genitalia is basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethraUrethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
behind the protuberance.
In typical fetal
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
development, the presence of the SRY gene
SRY
SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the SOX gene family of DNA-binding proteins...
causes the fetal gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s to become testes; the absence of it allows the gonads to continue to develop into ovaries. Thereafter, the development of the internal reproductive organs
Sex organ
A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; flowers are the reproductive organs of flowering plants, cones are the reproductive...
and the external genitalia is determined by hormones produced by fetal gonads (ovaries or testes) and the cells' response to them.
If the fetus has testes, and if the testes produce testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum
Scrotum
In some male mammals the scrotum is a dual-chambered protuberance of skin and muscle containing the testicles and divided by a septum. It is an extension of the perineum, and is located between the penis and anus. In humans and some other mammals, the base of the scrotum becomes covered with curly...
; the protuberance grows larger and straighter to form the penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
; the inner urogenital swellings swell, wrap around the penis, and fuse in the midline to form the penile urethra.
If testosterone is not present, normal female development continues, with the development of a perineal urethra
Perineal urethra
A perineal urethra is a human urethral opening that is located on the perineum rather than at the tip of the phallus. Urethral position is an aspect of human sexual dimorphism. A phallic urethra is typical of male anatomy and a perineal urethra is typical of female anatomy...
and the formation of a uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
, clitoris
Clitoris
The clitoris is a sexual organ that is present only in female mammals. In humans, the visible button-like portion is located near the anterior junction of the labia minora, above the opening of the urethra and vagina. Unlike the penis, which is homologous to the clitoris, the clitoris does not...
and vagina
Vagina
The vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
.
The Müllerian ducts, which are paired ducts of the embryo which empty into the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
, and which develop into the upper vagina
Vagina
The vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
, cervix
Cervix
The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall...
, uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
and oviducts; in the male they disappear except for the vestigial vagina masculina and the appendix testis
Appendix testis
The appendix testis is a vestigial remnant of the Müllerian duct, present on the upper pole of the testis and attached to the tunica vaginalis. It is present about 90% of the time.-Clinical significance:...
.
Although female development is usually considered the "default" configuration in the absence of induction, recent evidence has indicated that Wnt4
WNT4
Protein Wnt-4 is a protein that in humans is encoded by the WNT4 gene.WNT4 is a secreted protein involved in female fetal genital development.- Function :...
plays a role in the development of female anatomy.
Homologies
Because of their common origin in fetal anatomy, a number of male and female anatomical features are said to be homologousHomology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
; for example, the clitoris and penis are homologous with one another, as are Skene's glands in females and the prostate gland in males.
Intersexuality
Most of the time, the result of fetal genital development follows the stereotypical male or stereotypical female development path. However, in a small but significant minority of cases, the path of development follows an intermediate or other pathway, leading to what are called "ambiguous genitalia", one condition of those known as intersexuality.See congenital anomalies of the genitalia
Congenital anomalies of the genitalia
Congenital anomaly of the genitalia is a medical term referring to any physical abnormality of the male or female internal or external genitalia present at birth...
for a list of congenital disorder
Congenital disorder
A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life , regardless of causation...
s caused by problems with fetal genital development.