Reproductive System Development
Introduction
Development of the reproductive system is a complex process relying on numerous systems and conditions to ensure appropriate structures and rate of development are 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.
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 in birds is 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.
EMBRYOLOGICAL DEVELOPMENT OF THE GENITAL TRACT
Early embryo
- Small indifferent gonads.
- Primitive structures which will become gonads and genital tract are identical at this stage.
- Paired mesonephric ducts (Wolffian) or paramesonephric (Mullerian) ducts fuse caudally with urogenital sinus.
(Metanephros - becomes the kidney).
Gonadal differentiation
- Determined by the presence of the SRY gene of the Y chromosome (sex determining region of the Y chromosome) which encodes “testicular determining factor” protein (SRY protein).
- Lack of SRY gene (i.e. XX females) leads to ovarian differentiation.
Development of phenotypic sex
- The sex of the embryonic gonad normally determines which reproductive duct system develops.
- Testes induce development of mesonephric (Wolffian) duct system in to epididymis and vas deferens and cause regression of paramesonephric (Mullerian) duct by ”mullerian inhibitory substance”, (MIS).
- Absence of testes in presence of ovaries allows paramesonephric ducts to develop under influence of oestrogen to oviduct, uterus, cervix and cranial vagina.
Summary of determinants of sexual development
Chromosomal or genetic sex = sex chromosomes
Gonadal sex = ovary or testis
Phenotypic sex = usually determined by gonad
Information by permission of Professor RW Else
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