Chapter 9: Reproductive Physiology

The Reproductive System

Overview

The male and female gonads and the genitalia develop from common precursors in the first weeks of embryonic life, but the final stages of sexual differentiation are not completed until puberty. Human reproduction and sexuality are unique in that they are defined by recreation and pleasure in addition to procreation; physiological and psychological satisfaction are both of central importance. In sexually mature males, millions of gametes (spermatozoa) are continually produced by the testes, whereas in females, fertility is cyclical and yields a single gamete (oocyte) approximately once per month. If intercourse occurs at the appropriate time, ejaculation of sperm into the female reproductive tract enables the fertilization of the oocyte by a single sperm. Implantation of the developing embryo occurs in the uterus, and is rapidly followed by development of the placenta. The normal gestation period of 40 weeks ends with labor and parturition (childbirth). The final differentiation of the female breast occurs during pregnancy to allow lactation and breast-feeding. The complex events in male and female reproductive physiology are orchestrated by the pituitary, the gonadal, and (during pregnancy) the placental hormones.

Sexual Differentiation

Sexual differentiation refers to the transformation of the undifferentiated gonads of the early embryo into functional male and female reproductive systems. Anatomic differentiation occurs in utero, but the final maturation that produces fully functional reproductive organs is not completed until puberty.

Reproductive Embryology

The process that determines whether male or female reproductive organs develop depends on the complement of sex chromosomes present; female gametes (oocytes) all have the same 22X chromosomal makeup, whereas male gametes (spermatozoa) are either 22X or 22Y. The chromosomal sex of the fetus is determined at fertilization when the male and female gametes combine; XX is female and XY is male.

The default phenotypic sex is female if the fetus does not have a Y chromosome. The presence of a Y chromosome directs the undifferentiated gonad to become a testis rather than an ovary. A single gene (SRY), located in the sex-determining region of the Y chromosome, is required for male sexual differentiation.

Before sexual differentiation begins, the fetus has developed two parallel duct systems located near the undifferentiated gonads: the mesonephric (wolffian) duct and the paramesonephric (müllerian) duct (Figure 9-1). By week 10 of gestation, the fetal gonads can be distinguished as either testes or ovaries.