Learning aims

At the end of this module students should be able to:

• list the genetic and hormonal factors that lead to sexual differentiation
• describe the steps that occur in the differentiation of the testicles and ovaries
• explain the formation of the internal and external genitals of both sexes
• name the abnormalities that indicated disorders in the most important mechanisms of genital development

What you should already know

• gametogenesis
• Embryonic disk
• Urinary system

Delving deeper

• Which genetic and epigenetic factors cause a common embryonic anlage such as the genital system to differentiate itself into the differing genitals of the two sexes?
• How can one explain the differences and commonalities between the female and male genitals, given that they possess the same origin?
• The primordial germ cells have a large capacity for differentiation. How can these cells avoid being influenced by factors that induce differentiation during their migration from the extra-embryonic regions into the genital ridges?

Introduction

The genital system consists of ther:

• gonads
• internal genitals
• external genitals

Although the genetic gender (karyotype) of the embryo (46, XX or 46, XY) is already determined at the time of fertilization, undifferentiated genitals - ones that do not differ - arise in both sexes only in the 6th week.
A bit later (stages 18 - 20), though, a differentiation occurs - step by step - that is determined by hormonal and genetic factors (inhibiting and/or inducing).

Given the genetic gender, the differentiation of the gonads (ovary or testis) takes place first and is followed by that of the phenotypes (morphologically visible alterations of the internal and external genitals, hair growth, size, etc.). Only then does the differentiation of the psyche (feminine or masculine differentiation of the brain, primarily in the median, preoptic region of the hypothalamus) occur.