15.3 Development of the Male and Female Reproductive Systems
15.3 Development of the Male and Female Reproductive Systems
Learning Objectives
By the end of this section, you will be able to:
- Explain how bipotential tissues are directed to develop into male or female sex organs
- Name the rudimentary duct systems in the embryo that are precursors to male or female internal sex organs
- Describe the hormonal changes that bring about puberty and the secondary sex characteristics of men and women
The development of the reproductive systems begins soon after fertilisation of the egg, with primordial gonads beginning to develop approximately one month after conception. Reproductive development continues in utero, but there is little change in the reproductive system between infancy and puberty.
Development of the Sexual Organs in the Embryo and Foetus
Females are considered the “fundamental” sex—that is, without much chemical prompting, all fertilised eggs would develop into females. This is called the SRY (Sex-determining Region of the Y chromosome). Because females do not have a Y chromosome, they do not have the SRY gene. Without a functional SRY gene, an individual will be female.
In both male and female embryos, the same group of cells has the potential to develop into either the male or female gonads; this tissue is considered bipotential. The SRY gene actively recruits other genes that begin to develop the testes and suppresses genes that are important in female development. As part of this SRY-prompted cascade, germ cells in the bipotential gonads differentiate into spermatogonia. Without SRY, different genes are expressed, oogonia form, and primordial follicles develop in the primitive ovary.
Soon after the formation of the testis, the Leydig cells begin to secrete testosterone. Testosterone can influence tissues that are bipotential to become male reproductive structures. Without testosterone, these same cells differentiate into the clitoris.
Not all tissues in the reproductive tract are bipotential. The internal reproductive structures (for example the uterus, uterine tubes, and part of the vagina in females; and the epididymis, ductus deferens, and seminal vesicles in males) form from one of two rudimentary duct systems in the embryo. For proper reproductive function in the adult, one set of these ducts must develop properly, and the other must degrade. In males, secretions from sustentacular cells trigger a degradation of the female duct, called the Müllerian duct. At the same time, testosterone secretion stimulates growth of the male tract, the Wolffian duct. Without such sustentacular cell secretion, the Müllerian duct will develop; without testosterone, the Wolffian duct will degrade. Thus, the developing offspring will be female. For more information and a figure of differentiation of the gonads, seek additional content on foetal development.
Further Sexual Development Occurs at Puberty
Puberty is the stage of development at which individuals become sexually mature. Though the outcomes of pu