238 Development of the Reproductive System

Embryological and Fetal Events

Reproductive organs develop from the Wolffian ducts in males and the Mullerian ducts in females.

The development of the reproductive system is the part of prenatal development that concerns the sex organs. Because it largely overlaps the urinary system, the development of these organs is often described together.

The reproductive organs develop from the intermediate mesoderm. The permanent organs of the adult are preceded by a set of structures that are purely embryonic; with the exception of the ducts, they disappear almost entirely before birth. These embryonic structures are the Wolffian and Müllerian ducts, also known as mesonephric and paramesonephric ducts, respectively. The Wolffian duct remains in males and the Müllerian duct persists in females.

The Role of the Wolffian Duct

In males, the Wolffian duct persists and forms the tube of the epididymis, the ductus deferens, and the ejaculatory duct, while the seminal vesicle arises during the third month of fetal development as a lateral diverticulum from its hinder end. A large part of the head of the mesonephros atrophies and disappears; of the remainder, the anterior tubules form the efferent ducts of the testis while the posterior tubules are represented by the ductuli aberrantes and paradidymis, which is sometimes found in front of the spermatic cord above the head of the epididymis. In females, the Wolffian bodies and ducts atrophy.

The Role of the Mullerian Duct

Shortly after the formation of the Wolffian ducts, the Müllerian ducts develop. In the female, the Müllerian ducts persist and undergo further development. The portions that lie in the genital cord fuse to form the uterus and vagina. This fusion begins in the third month, and the septum formed by their fused medial walls disappears from below upward. The parts outside this cord remain separate, each forming the corresponding Fallopian tube.

About the fifth month, a ring-like constriction marks the position of the cervix, and after the sixth month the walls of the uterus begin to thicken. For a time, the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornix. At about the fifth or sixth month, the lumen of the vagina is produced by the breaking down of the central cells of the epithelium. The hymen represents the remains of the Müllerian eminence.
In males, the Mullerian ducts atrophy.

Development of Gonads and External Genitalia

The gonads are the precursors of the testes in males and ovaries in females. They initially develop from the mesothelial layer of the peritoneum. The ovary is differentiated into a central part, the medulla, covered by a surface layer, the germinal epithelium. The immature ova originate from cells from the dorsal endoderm of the yolk sac.

The periphery of the testes is converted into the tunica albuginea. Cords of the central mass run together and form a network that becomes the rete testis, and another network that develops the seminiferous tubules. The descent of the testes consists of a connection from the testis to its final location at the anterior abdominal wall, followed by the development of the gubernaculum, which subsequently translocates the testis down into the developing scrotum. Ultimately, the passageway closes behind the testis. A failure in this process causes indirect inguinal hernia.

Until about the ninth week of gestational age, the external genitalia of males and females look the same and follow common development. This includes the development of a genital tubercle and dorsal membrane covering the developing urogenital opening, as well as the development of labioscrotal folds. Even after differentiation can be seen between the sexes, some stages are common, like the disappearing of the membrane. Sex-dependent development includes further protrusion of the genital tubercle in the male to form the penis. The labioscrotal folds evolve into the scrotum in males and into labia in females.

Development of male and female external genitalia from a common developmental beginning: A: common development. C,E: male development. B,D, F: female development.

Puberty

Puberty is the process through which a child matures and becomes capable of sexual reproduction.

Puberty is the process of physical changes by which a child’s body matures into an adult body capable of sexual reproduction.

Gender Differences in Puberty

Puberty is initiated by hormonal signals from the brain to the gonads ( ovaries in females, testes in males). In response to these signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sexual organs.

Although there is a wide range of normal ages, girls typically begin the process of puberty at age 10 or 11, boys at age 12 or 13. Girls usually complete puberty by ages 15–17, while boys usually complete puberty by ages 16–18. Any increase in height beyond the post-pubertal age is uncommon. Girls attain reproductive maturity about four years after the first physical changes of puberty appear. In contrast, boys accelerate more slowly, but continue to grow for about six years after the first visible pubertal changes.

The major landmark of puberty for males is the first ejaculation, which occurs on average at age 13. For females, it is menarche, the onset of menstruation, which occurs on average between ages 12 and 13. Puberty that starts earlier than average is known as precocious puberty. Puberty that starts later than usual is known as delayed puberty.

The development of secondary sex characteristics is notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body. The male “growth spurt” also begins later, accelerates more slowly, and lasts longer before the epiphyses fuse. Although boys are on average 2 cm shorter than girls before puberty begins, adult men are on average about 13 cm (5.2 inches) taller than women. Most of this sex difference in adult heights is attributable to a later onset of the growth spurt and a slower progression to completion, a direct result of the later rise and lower adult male levels of estradiol.

Hormonal Roles in Puberty

For boys, an androgen called testosterone is the principal sex hormone. An estrogen called estradiol drives female development during puberty. Estradiol is a substantial product of testosterone metabolism. The conversion of testosterone to estradiol depends on the amount of body fat and estradiol levels in boys, which are typically much lower than in girls. While estradiol promotes growth of breasts and uterus, it is also the principal hormone driving the pubertal growth spurt and epiphyseal maturation and closure. Estradiol levels rise earlier and reach higher levels in women than in men.

Normal puberty is initiated in the hypothalamus. GnRH signals from the hypothalamus induce pulsed secretion of LH (and to a lesser degree, FSH) at roughly one to two-hour intervals. Early stages of male hypothalamic maturation are very similar to the early stages of female puberty, though occurring about one to two years later. LH stimulates the Leydig cells of the testes to make testosterone and blood levels begin to rise. For females, as the amplitude of LH pulses increases, the theca cells of the ovaries begin to produce testosterone and smaller amounts of progesterone. Much of the testosterone moves into nearby cells called granulosa cells. Smaller increases of FSH induce an increase in the aromatase activity of these granulosa cells, which converts most of the testosterone to estradiol for secretion into the circulation. The remaining testosterone, together with adrenal androgens, is responsible for the typical androgenic changes of female puberty: pubic hair, other androgenic hair as outlined above, body odor, and acne.

Under the influence of androgens, the voice box, or larynx, grows in both sexes. This growth is far more prominent in boys, causing the male voice to drop and deepen about an octave (sometimes abruptly).

By the end of puberty, adult men have heavier bones and nearly twice as much skeletal muscle. Some of the bone growth (e.g. shoulder width and jaw) is disproportionately greater, resulting in noticeably different male and female skeletal shapes. The average adult male has about 150% of the lean body mass of an average female and about 50% of the body fat. This muscle develops mainly during the later stages of puberty, and muscle growth can continue even after boys are biologically adult.

Hormone feedbacks in males and female stimulating puberty: Puberty, and sexual maturity thereafter, is regulated by a series of hormones and feedbacks in the body, illustrated here. Hormones include: 1 Follicle-stimulating hormone – FSH. 2 Luteinizing hormone – LH. 3 Progesterone. 4 Estrogen. 5 Hypothalamus. 6 Pituitary gland. 7 Ovary. 8 Pregnancy – hCG (Human chorionic gonadotropin). 9 Testosterone. 10 Testicle. 11 Incentives. 12 Prolactin – PRL.