The development of the reproductive systems begins soon after fertilization 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 Fetus

Egg producing and conducting (EPC) organs are considered the “fundamental” anatomy —that is, without much chemical prompting, all fertilized eggs would develop into these organs. To develop sperm producing and conducting (SPC) organs, an individual must be 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). Embryos that continue to develop EPC organs do not have a Y chromosome, and so they do not have the SRY gene.

The same group of cells has the potential to develop into either EPC or SPC organs; 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 the development of EPC organs. 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 to become SPC organs. For example, with exposure to testosterone, cells that could become either the glans penis or the glans clitoris form the glans penis. Without testosterone, these same cells differentiate into the clitoris.

Not all tissues in the reproductive tract are bipotential. The internal reproductive structures (for example egg conducting organs like the uterus, uterine tubes, and part of the vagina; and sperm conducting organs like the epididymis, ductus deferens, and seminal vesicles) 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 those with SPC organs, secretions from sustentacular cells trigger a degradation of the EPC organs (called the Müllerian duct). At the same time, testosterone secretion stimulates growth of the SPC organs (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 develop EPC organs.

Further Sexual Development Occurs at Puberty

Puberty is the stage of development at which individuals become sexually mature. As shown in the image below, a concerted release of hormones from the hypothalamus (GnRH), the anterior pituitary (LH and FSH), and the gonads (either testosterone or estrogen) is responsible for the maturation of the reproductive systems and the development of secondary sex characteristics, which are physical changes that serve auxiliary roles in reproduction.

The first changes begin around the age of eight or nine when the production of LH becomes detectable. The release of LH occurs primarily at night during sleep and precedes the physical changes of puberty by several years. In pre-pubertal children, the sensitivity of the negative feedback system in the hypothalamus and pituitary is very high. This means that very low concentrations of androgens or estrogens will negatively feed back onto the hypothalamus and pituitary, keeping the production of GnRH, LH, and FSH low.

As an individual approaches puberty, two changes in sensitivity occur. The first is a decrease of sensitivity in the hypothalamus and pituitary to negative feedback, meaning that it takes increasingly larger concentrations of sex steroid hormones to stop the production of LH and FSH. The second change in sensitivity is an increase in sensitivity of the gonads to the FSH and LH signals, meaning the gonads of adults are more responsive to gonadotropins than are the gonads of children. As a result of these two changes, the levels of LH and FSH slowly increase and lead to the enlargement and maturation of the gonads, which in turn leads to secretion of higher levels of sex hormones and the initiation of spermatogenesis and folliculogenesis.

In addition to age, multiple factors can affect the age of onset of puberty, including genetics, environment, and psychological stress. One of the more important influences may be nutrition; historical data demonstrate the effect of better and more consistent nutrition on the age of menarche (the beginning of menstruation or a period) in the United States, which decreased from an average age of approximately 17 years of age in 1860 to the current age of approximately 12.75 years in 1960, as it remains today. Some studies indicate a link between puberty onset and the amount of stored fat in an individual. This effect is more pronounced in those with EPC organs, but has been documented in those with SPC organs as well. Body fat, corresponding with secretion of the hormone leptin by adipose cells, appears to have a strong role in determining menarche. This may reflect to some extent the high metabolic costs of gestation and lactation. In those with EPC organs who are lean and highly active, such as gymnasts, there is often a delay in the onset of puberty.

Figure 1. Click to view a larger image. During puberty, the release of LH and FSH from the anterior pituitary stimulates the gonads to produce sex hormones.

Signs of Puberty

Different sex steroid hormone concentrations also contribute to the development and function of secondary sexual characteristics. Examples of secondary sexual characteristics are listed in Table 1.

As an individual with EPC organs reaches puberty, typically the first change that is visible is the development of the breast tissue. This is followed by the growth of axillary and pubic hair. A growth spurt normally starts at approximately age 9 to 11, and may last two years or more. During this time, height can increase 3 inches a year. The next step in puberty is menarche, the start of menstruation.

In individuals with SPC organs, the growth of the testes is typically the first physical sign of the beginning of puberty, which is followed by growth and pigmentation of the scrotum and growth of the penis. The next step is the growth of hair, including armpit, pubic, chest, and facial hair. Testosterone stimulates the growth of the larynx and thickening and lengthening of the vocal folds, which causes the voice to drop in pitch. The first fertile ejaculations typically appear at approximately 15 years of age, but this age can vary widely across individuals. Unlike the early growth spurt observed in those with EPC organs, the growth spurt in those with SPC organs occurs toward the end of puberty, at approximately age 11 to 13, and height can increase as much as 4 inches a year. In some with SPC organs, pubertal development can continue through the early 20s.

Chapter Review

The reproductive systems of those with EPC and SPC organs begin to develop soon after conception. A gene on the Y chromosome called SRY is critical in stimulating a cascade of events that simultaneously stimulate testis development and repress the development of EPC organs. Testosterone produced by Interstitial cells in the embryonic testis stimulates the development of SPC organs. If testosterone is not present, EPC organs will develop.

Whereas the gonads and some other reproductive tissues are considered bipotential, the tissue that forms the internal reproductive structures stems from ducts that will develop into only SPC (Wolffian) or EPC (Müllerian) structures. The expression of hormones will cause one of these systems to continue to develop and the other to degrade.

Further development of the reproductive systems occurs at puberty. The initiation of the changes that occur in puberty is the result of a decrease in sensitivity to negative feedback in the hypothalamus and pituitary gland, and an increase in sensitivity of the gonads to FSH and LH stimulation. These changes lead to increases in either estrogen or testosterone, in adolescents with EPC and SPC organs, respectively. The increase in sex steroid hormones leads to maturation of the gonads and other reproductive organs. The initiation of spermatogenesis begins in those with SPC organs, and those with EPC organs begin ovulating and menstruating. Increases in sex steroid hormones also lead to the development of secondary sex characteristics such as breast development those with EPC organs and facial hair and larynx growth in those with SPC organs.

Self Check

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Glossary

Müllerian duct: duct system present in the embryo that will eventually form the EPC structures

puberty: life stage during which the EPC or SPC organs becomes anatomically and physiologically mature

secondary sex characteristics: physical characteristics that are influenced by sex steroid hormones and have supporting roles in reproductive function

Wolffian duct: duct system present in the embryo that will eventually form the internal SPC structures