An introduction to the determination of the gonads

This is because testosterone is converted to the more potent DHT by 5-alpha reductase. In females, without excess androgens, these become the clitorisurethra and vaginaand labia Differentiation between the sexes of the sex organs occurs throughout embryological, fetal and later life.

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Fgf9, like many of the founding signals in the gonads, is initially expressed in gonads of both sexes, but becomes male-specific after Sry is expressed. XO humans have a number of abnormalities Turner syndrome ; see Figureincluding short stature, mental retardation, and mere traces of gonads, but they are clearly female in morphology.

Paul Sternberg has written about a single hermaphrodite-specific organ, the vulva, which forms during larval development and provides an opening between the uterus and the external environment.

Unlike the situation in Drosophila discussed belowthe mammalian Y chromosome is a crucial factor for determining sex in mammals.

Introduction to the Reproductive System

Early in embryogenesis, germ cells are specified as distinct from somatic cells. A Polymerase chain reaction followed by electrophoresis shows the presence of the Sry gene in normal XY males and in a transgenic XX Sry mouse. The ventral portions of the gonadal rudiments are composed of the genital ridge epithelium.

Figure Development of the mammalian urogenital system.

Environmental sex determination

The mouse homologue of this gene, Sox9, is expressed only in male XY but not in female XX genital ridges. After Haqq et al. Examination of FOXL2 in Development of the reproductive system By 7 weeks, a fetus has a genital tubercleurogenital groove and sinus, and labioscrotal folds.

The embryonic cell lineages in the two sexes are essentially identical, although a few cells in each sex are programmed to die sex-specifically during late embryogenesis; for example, males get rid of two cells that in hermaphrodites would become neurons required for egg-laying.

The adrenal cortex is the outer layer of the adrenal gland, which rests upon each kidney. These studies indicate that Sox9 plays a central role in sex determination. These secondary sex characteristics are usually determined by hormones secreted from the gonads.

Lovell-Badge and Goodfellow used a transgene to provide spectacular evidence in support of this identity Figure These epithelial layers form the sex cords.

Koopman and colleagues took the kilobase region of DNA that includes the Sry gene and presumably its regulatory elements and microinjected this sequence into the pronuclei of newly fertilized mouse zygotes.

Specialized ribonucleoprotein particles, called P granules, are maternally provided and segregated to the P blastomeres. These genetic and molecular regulation insights come from multi-species studies involving the analyses of genetic mutations, gene expression and in vitro molecular studies.

Thus, the androgen— receptor complex binds to androgen-responsive enhancer elements, leading to the activation of malespecific gene expression. XY sex-determination system Most mammalsincluding humans, have an XY sex-determination system: The existence of these two independent systems of masculinization is demonstrated by people having androgen insensitivity syndrome.

B After 3 days in culture with more In both XY and XX gonads, the sex cords remain connected to the surface epithelium. The role of chromatin remodeling in the germ line has been further established by characterization of maternal-effect sterile mes genes and modifications of specific histone residues see Specification of the germ line ; Germline chromatin.

Development of the gonads is a complex process, which starts with a period of undifferentiated, bipotential gonads. During this period the expression of sex-determining genes is initiated.

Sex determination is a process triggering differentiation of the gonads into the testis or ovary. Sex determination in mammals: coordinated control by the endocrine system An analysis of sex- chromosome aneuploids demonstrates that mammalian sex determination and differentiation are quite different from those of Drosophila (see Table ).

The murine gonad forms at days post-coitum (dpc) and is initially indistinguishable between XY and XX embryos. At this stage, the cells of the gonad are bipotential and can follow either the male or female pathway. Many lines of evidence suggest that sex determination in mammals centers on a.

the two somatic gonads are clear variants of each other.

Sexual differentiation in humans

1. Introduction to sex determination 2 In his chapter, Michael Herman focuses on cell fate specifications that occur only in hermaphrodites. Both All WormBook content, except where otherwise noted, is licensed under a Creative. Sex-determining mechanisms can be divided mainly into two categories: genetic sex determination (GSD) and environmental sex determination (ESD).

In GSD, genetic elements specify the sex of the individual independent of the environment. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts, breasts, body hair, and plays a role in gender identification.

An introduction to the determination of the gonads
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