Learning Objectives
By the end of this section, you will be able to:
- Explain how bipotential tissues are directed to develop into sex organs
- Name the rudimentary duct systems in the embryo that are precursors to internal sex organs
- Describe the hormonal changes that bring about puberty, and the secondary sex characteristics
Introduction
The development of the sexual systems begins soon after fertilization of the egg, with primordial gonads beginning to develop approximately one month after conception. Sexual system development continues in utero, but there is little change in the system between infancy and puberty.
Development of the Sexual Organs in the Embryo and Fetus
Without chemical prompting, all fertilized eggs would develop a clitoris and vagina. This would be different if an individual was exposed to the cascade of factors initiated by a single gene on the Y chromosome. This is called the SRY (Sex-determining Region of the Y chromosome). Individuals without a Y chromosome also do not have the SRY gene. Without a functional SRY gene, an individual will typically develop a uterus and ovaries.
In all embryos, the same group of cells has the potential to develop into either testes and ovaries; this tissue is considered bipotential. The SRY gene actively recruits other genes that begin to develop the testes, and suppresses other genes that would lead to development of ovaries. 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 interstitial (Leydig) cells begin to secrete testosterone. Testosterone can influence tissues that are bipotential. 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; and the epididymis, ductus deferens, and seminal vesicles) form from one of two rudimentary duct systems in the embryo.
Development of the internal sexual organs requires one set of ducts to develop and the other set to degrade. A hormone secreted from sustentacular (Sertoli) cells trigger a degradation of the paramesonephric (Müllerian) duct, and therefore a uterus is unlikely to develop. At the same time, testosterone secretion stimulates growth of the mesonephric (Wolffian) duct, leading to development of the epididymis and vas deferens. Without such sustentacular cell hormone secretion, the paramesonephric duct will now develop; and without testosterone, the mesonephric duct will degrade. Thus, the offspring in this circumstance will likely develop a uterus, and not an epididymis or vas deferens. For more information and a figure of differentiation of the gonads, seek additional content on fetal development.
There are many reasons why sexual anatomy would