Reproductive System Development
Development of the reproductive system is a complex process relying on numerous systems and conditions to ensure appropriate structures are formed and the rate of development is maintained within the embryo. There are several important criteria that determine appropriate reproductive system development; genetic sex which determines the sex of the embryo, gonadal sex and phenotypic sex which both determine sexual differentiation. Genetic sex determines gonadal sex which in turn determines phenotypic sex. Sex determination relates to the genetic or environmental process which determines the sex of an individual whilst sexual differentiation represents the development of internal and external genitalia.
Sex Determination in Mammals
In mammals the male is the heterogametic species (XY chromosomes) and females are the homogametic species (XX chromosomes) and this is the first stage in the development of the reproductive system. This stage occurs in the zygote and genes in the Y chromosome regulate the development of gonads towards the male phenotype. Therefore the Y chromosome must be present for the development of the male gender, regardless of the number of X chromosomes (see Developmental Abnormalities section below). Each Y chromosome contains a Sex Determining Region (SRY) and testes must be present for the formation of male characteristics. Sex hormones produced by the developing gonads affect the further development of the reproductive organs and the nervous system and are therefore crucial. For further information in sexual differentation please see here.
Fetal development in mammals occurs in a highly hormonal environment with relatively high concentrations of oestrogens and progesterone and therefore during the early stages of embryonic development the gonads must produce testosterone and other developmental signalling molecules in order for the testes to develop. If no testosterone is produced, the reproductive organs will take on female characteristics as a default. Abnormalities can occur if the gonads in a genetic male do not produce sufficient levels of testosterone.
Genital development under testosterone therefore induces the penis and scrotum to develop. Testosterone is converted to dihydrotestosterone which is the signalling molecule responsible for the external appearence of the penis and scrotum. An absence of testosterone results in the development of the clitoris, labia and vaginal opening.
Sex Determination in Birds
Birds have sex chromosomes labelled Z and W. The chromosomal requirements for a male bird are the opposite of that in mammals as the female is the heterogametic species rather than the male. Therefore ZZ results in a male and ZW results in a female.
Sex Determination in Reptiles
Crocodilians, marine turtles, some terrestrial turtles and lizards undergo environmental sex determination. The important factor for sex determination in these species is the incubation temperature of the eggs as this determines the development of the gonads towards testes or ovaries. In crocodiles a higher temperature is male determining whilst in turtles a higher temperature is female determining.
Sexual Differentiation of the Brain
In the male testosterone is secreted from the fetal testes into the blood and is able to cross the blood brain barrier as it does not bind to alpha-fetoprotein. Once across this barrier the testosterone is converted into estradiol and dihydrotestosterone via the enzyme aromatase. Although it is counter-intuitive, esradiol actually causes masculinisation of the male brain. This brain masculinisation process results in the prevention of the development of a surge centre.
In the female alpha-fetoprotein binds to estradiol which has been secreted by the fetal ovary. This binding results in the complex being unable to cross the blood brain barrier. This mechanism protects the female brain from the masculinising effects of estradiol and allows the development of a 'feminising' surge centre at the point of the rostral part of the mesencephalon.
Normal Sexual Development
The development of the reproductive system occurs in close proximity and concurrently to the renal system. Throughout normal sexual development there are series of defined phases;
The pregonadal phase represents the stage where germ cells begin to migrate from the yolk sac of the developing embryo. Primitive primordial germ cells are formed in the yolk sac (the yolk sac is part of the digestive system in the embryo). These primitive germ cells migrate from the yolk sac through the mesentry to the genital ridges which are small elongated pieces of tissue beneath the peritoneum situated at the dorsal aspect of the abdominal cavity. These germ cells then begin to divide by mitosis and the gonads are developed from these genital ridges.
Migration & Proliferation of Germ Cells
Germs cells migrate via an amoeboid movement. These germs cells travel from the primitive streak in the yolk sac to the allantois, then to the hind gut before reaching the urogenital ridge and gonads. In the male the XY germ cells undergo mitosis from leaving the primitive streak until reaching the urogenital ridge and/or the gonads. At this stage the XY germ cells arrest at the G0 phase of the cell cycle due to a meiosis inhibitor factor.
In females the XX germ cells undergo the same migration but on reaching the urogenital ridge and gonads no such meiosis inhibitor factor is present. Therefore the XX germ cells undergo mitosis between the primitive streak until the urogenital ridge where they switch to meiosis. This process continues in the female until birth.
Bipotential Gonadal Phase
This phase represents the development of gonadal precursor tissue near the kidneys from the genital ridges. The initial development of the gonads is the same between males and females at this stage. The initial structures present at this stage are the early developmental stages of the rectum, the metanephros which are the early kidneys, the mesonephros which are tissues associated with the early gonads, the paramesonephric duct which runs between the mesonephros and the rectum/urogenital sinus and the mesonephric duct which connects the mesonephros with the metanephros.
The paramesonephric ducts then decreases in length towards the urogenital tract and the early developmental gonads begin to enlarge. By the end of this early developmental stage the mesonephros becomes a remnant and eventually becomes the future efferent ducts. In the female the paramesonephric duct ultimately becomes the female urinogenital tract, but for the next developmental stage it becomes the Mullerian duct. The mesonephric ducts in the male eventually become the future epididymis but for the next developmental stage becomes the Wolfian duct.
There are a number of genes involved in this stage of bipotential gonadal determination; WT-1, SF-1, Lim-1 and EMx-2. The development of either the Wolfian ducts is caused by the genes SRY, SOX-9, DMRT and 1/2. The Mullerian ducts develop as a result of the genes DAX-1 and Wnt-4.
Primary Sexual Differentiation Phase
This phase represents the initial phase of differentiation of reproductive tissues. The male Y chromosome in mammals contains genetic code that results in the production of testes-determining factor (TDF) which is responsible for causing the gonads to develop into testes. In the female fetus, TDF is not produced as there is no Y chromosome and therefore the gonads develop into ovaries. As the testes begin to develop under the influence of TDF, the Sertoli cells within the testes begin to produce Anti-Mullerian hormone (AMH) and this has two main effects. It causes the Leydig cells within the testes to develop which are able to produce testosterone and this testosterone begins the development of the male duct system. Under the influence of AMH these Leydig cells also begin to produce dihydrotestosterone which, per the above explanation, is also a key signalling molecule in reproductive system development resulting in the development of the penis, scrotum and accessory sex glands. The second key effect of AMH is the regression of the paramesonephric duct system. In females, the lack of TDF and AMH allows the paramesonephric ducts to become the oviducts, uterus, cervix and internal parts of the vagina.
At this stage of development the gonads do not have their own excretory ducts. In males the excretory ducts from the developing kidney, the Wolfian Ducts facilitate the excretion of spermatozoa away from the testes. These Wolfian Ducts develop into male accessory reproductive structures that include the epididymis and spermatic ducts aswell as connecting the developing testes where they connect a system of tubules, the rete testis. These tubular connections collect spermatozoa from the seminiferous tubules resulting in a continuous tube system that facilitates the transport of spermatozoa to the external environment. In females the Wolfian ducts undergo regression at this stage of development.
In both sexes a pair of Mullerian ducts are formed which are epithelialised ducts which extend from the lumbar region to the urogenital cleft. In females, the fallopian tubes or oviducts, uterus and internal elements of the vagina all develop from these ducts. Although the Mullerian ducts are formed in males, they eventually regress.
In the fetal ovary, active development occurs much later than in the testes and there is little hormone production in the first two trimesters. Follicle maturation occurs as gestation progresses.
Secondary Sexual Differentiation Phase
This phase corresponds to the development of secondary sexual characteristics.
Testicular descent usually occurs between the middle and the end of detal development. In horses this can be in very late pregnancy or even after birth. In canines it can be up to 8 days post partum. In other species including whales and elephants, the testes will remain in their original positions and do not descend. For further information regarding testicular descent, please see here.
Abnormal Reproductive Development
Freemartinism occurs where a heifer is born twinned with a bull and there is a common blood supply between the male and female fetuses. The female fetus is therefore exposed to AMH and testosterone from the male. The result is incompletely developed paramesonephric ducts and ovaries that do not have a full complement of germ cells. Together with this, the gonads may produce testosterone and androstenedione and the nervous systemk may have differentiated such that a female will display male characteristics and behaviours. For further information on freemartinism, please see here.
Cryptorchism represents a failure of the testes to descend into the scrotum. Cryptorchidism has been linked to environmental factors. For further information regarding this abnormal development please see here.
Inguinal herniation occurs when a portion of the intestine passes through the inguinal cavity into the vaginal cavity. For further information on this abnormal development please see here.
This abnormal development occurs where is a urethral opening at the base of the penis. This abnormality has been linked to environmental factors.
Puberty is the final stage in the development of the reproductive system and is defined as the period in life where sexual activity commences and the animal becomes physically able to reproduce. There is cyclic activity in the ovaries in the female and the production of spermatozoa in the male. For further information on the physiology of puberty please see here. For information regarding the endocrinology of puberty please see here. For information regarding the time of onset of puberty, please see here and for onset influencing factors please see here.
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