Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Internal Organs
Reproductive System
Development of the Reproductive System

The Reproductive System is initiated as early as the end of the first month of intrauterine development. This primordium differentiates into Gonads, genital ducts, and external genitalia. However, the primordium is morphologically indifferent—the structures forming it cannot be identified as male or female. At this stage of development, parallel to the mesonephric ducts (see 4.4.5), the paramesonephric ducts are formed, which open with their cranial ends into the coelom and their caudal ends into the cloaca (Fig. 4.56). The gonads develop in close contact with the Urinary System. Even while the mesonephros is functioning, a ridge-like thickening—the gonadal primordium—appears on its ventromedial surface. Each primordium, formed by mesenchyme and covered by epithelium, begins to be populated by primordial Germ Cells migrating into it from the yolk sac endoderm.

The sex of the embryo is determined only at the beginning of the third month. The direction of gonadal differentiation is determined by several factors. In the presence of the Y chromosome in the Cells of the gonadal primordium, it develops into a Testis and begins to produce Male Sex Hormones (testosterone and others). Testosterone induces the transformation of the mesonephric ducts into the vas deferens, which connect the testis to the Urethra. Under the Influence of other hormonal factors produced by the testis, the paramesonephric ducts degenerate. In the absence of testosterone, the mesonephric ducts degenerate, while the paramesonephric ducts transform into ovarian ducts. These ducts give rise to the oviducts, Uterus, and part of the Vagina.

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Fig. 4.56. Diagram of The Development of the reproductive system Organs (after Patten):

A — indifferent primordium: 1 — diaphragmatic ligament of the mesonephros; 2 — degenerating tubules of the mesonephros; 3 — gonad; 4 — mesonephric tubule; 5 — mesonephric duct; 6 — Urinary Bladder; 7 — ureteric orifices; 8 — rectum; 9 — genital tubercle; 10 — urorectal septum; 11 — anal and 12 — urogenital PARTS OF THE cloaca; 13 — inguinal ligament of the mesonephros; 14 — metanephric duct (Ureter); 15 — Kidney; 16 — Müllerian duct; B — male embryo: 1 — testis (before descent); 2 — Epididymis; 3 — prostatic sinus; 4 — Prostate Gland; 5 — Bulbourethral gland; 6 — urethra; 7 — Scrotum; 8 — testis (after descent); 9 — opening of the ejaculatory duct; 10 — inguinal ligament; 11 — Ductus deferens; 12 — Müllerian duct; 13 — kidney; C — female embryo: 1 — opening of the Uterine tube; 2 — Ovary; 3 — urethra; 4 — labium minus; 5 — vestibule; 6 — labium majus; 7 — vagina; 8 — round ligament of the uterus; 9 — round ligament of the ovary; 10 — ovary; 11 — uterine tube after descent; 12 — mesonephric duct; 13 — ureter; 14 — uterine tube (before descent); 15 — Kidneys

During the development of the male genital ducts, the mesonephric tubules closely located to the Testes persist as efferent ductules. Together with a portion of the mesonephric duct, they form the epididymis. Distal to the epididymis, the mesonephric duct becomes surrounded by smooth Muscle cells and transforms into the ductus deferens. Near its entry into the urogenital sinus, outpocketings appear, which are the primordia of the Seminal Vesicles.

Under METABOLISM/18.html">The Influence of testosterone, the prostate gland develops from the epithelium of the urethra.

Rapid growth of the mesonephros causes it to project into the coelomic cavity. The surrounding Peritoneum folds into ridges, which later transform into ligaments—the diaphragmatic and inguinal ligaments of the mesonephros (Fig. 4.56, A). Subsequently, the mesonephros gradually regresses, while the testes increase in size and become enclosed in the inguinal ligament, which becomes the ligament of the testis. As they grow, the testes and their associated structures shift caudally and eventually end up outside the Abdominal cavity, under the Skin in the scrotum. The canal connecting the abdominal cavity to the scrotum (the Inguinal Canal) then closes.

Passing through the abdominal wall, the testis pushes all of its layers ahead of itself, which subsequently form its tunics.

In a newborn, the mass of the testis with the epididymis is only 0.3 g. Its rapid growth begins with the onset of Puberty—by age 20, the mass of the testis reaches 20 g. In adulthood, the size and mass of the testis increase insignificantly, and after age 60, they decrease slightly. The same pattern is observed in the growth of the epididymis. In newborns, the lumens of the seminiferous tubules are absent—they appear by age 15–16; in adolescence, the tubule diameter doubles, and in adult men, it increases threefold compared to newborns. By the time of birth, the testes should descend into the scrotum; however, if descent is delayed, they may remain in the inguinal canal (retroperitoneally) and descend later, with the right testis typically situated higher than the left.

The presence of testes in the scrotum is one of the signs of maturity and full-term status in a newborn boy.

In a newborn, the ductus deferens is very thin, and the muscular layer in its wall is absent (it appears by age 5). In a 15-year-old adolescent, the thickness of the Spermatic Cord is approximately 6 mm, and that of the ductus deferens is 1.6 mm.

The seminal vesicles of a newborn are small, coiled tubules. Their growth begins during puberty. They reach their peak development by age 40. Thereafter, involutional changes occur, particularly in the mucosa: it thins, leading to a decline in secretory function.

In children, the glandular tissue of the prostate is undeveloped; its formation begins during puberty, when the gland increases tenfold. It reaches its peak functional activity between 30 and 45 years of age, followed by a gradual decline in function. During this process, the glandular tissue progressively atrophies. In old age, the gland can significantly increase in size due to the proliferation of fibrous structures, which causes narrowing of the urethra.

In female embryos, the paramesonephric (Müllerian) ducts approach each other at the caudal end of the embryo and empty into the urogenital sinus (Fig. 4.56). These ducts fuse over a considerable distance to form the uterus. In the dorsal wall of the urogenital sinus, near The entry of the ducts, a thickening arises from which the vagina later develops. The portion of the Müllerian duct located between the uterus and the ovary becomes the uterine tube (oviduct).

In female embryos, the primordium of the gonads (Ovaries) and their projection along with the mesonephros into the coelom follow the same pattern. However, in this case, the mesonephros degenerates to a greater extent than in male embryos. The peritoneal folds thin out and resemble mesenteries. They support the Müllerian ducts and ovaries and subsequently become the upper part of the broad ligament of the uterus. In the region where the uterus is formed by the fusion of the ducts, the peritoneal folds also fuse along the midline, forming the rest of the broad ligament of the uterus. During development, the ovaries also descend slightly. The inguinal ligament, which is well-developed in male embryos, is incorporated into the broad ligament, which later transforms into the round ligaments of the ovary and uterus. The caudal end of the ligament becomes incorporated into the Connective Tissue of the Labia Majora.

Fig. 4.56. Formation of the uterus and vagina in a female embryo (after Patten):

1 — mesonephros; 2 — Müllerian duct; 3 — fusing Müllerian ducts; 4 — mesonephric duct; 5 — urogenital sinus; 6 — vaginal plate; 7 — uterine tubes; 8 — body of the uterus; 9 — degenerating mesonephric ducts; 10 — cervix of the uterus; 11 — vagina; 12 — hymen; A–D — successive Selection/3.html">Stages of development

In a newborn girl, the ovaries lie above the pelvic inlet. They occupy their permanent position only by age five. After age 35, the ovaries begin to shrink; this process is particularly noticeable after age 45, when ovulation typically ceases. In elderly women, the ovaries undergo severe atrophy and are almost completely replaced by Fibrous connective tissue.

The uterine tubes of newborns are relatively longer, form several curves, and do not contact the ovaries. During puberty, the tubes begin to grow and approach the ovaries. Each tube straightens, with only a single curve remaining. In elderly women, the curves of the tube disappear, its wall thins, and the fimbriae at the edges of the infundibulum atrophy.

Age-related Changes in the uterus are highly significant. In a newborn girl, the uterus is located in the greater pelvis and even partially in the abdominal cavity. It has a cylindrical shape, a length of 25–35 mm, and a mass of 2 g. By the end of the first month, a flexion forms that persists into adulthood. Until age 10, the cervix of the uterus is longer than its body. During puberty (ages 13–14), the uterus acquires adult proportions. During menopause, with the cessation of menstruation, it gradually decreases in size, and in elderly women, it becomes half its former size. The most pronounced changes occur in the cervix.

In a newborn girl, the vagina is short (up to 35 mm), arched, and has a narrow lumen. Rapid growth begins in adolescence, when mucosal folds develop. By ages 45–50, the vaginal epithelium becomes keratinized.

The development of the external genitalia begins with The formation of the genital Swelling in embryos (indifferent stage) (Fig. 4.57). It soon transforms into the genital tubercle, from which a pair of urethral folds extends toward the anus. Between the folds lies the urogenital opening, and lateral to them are the labioscrotal swellings. In male embryos, the genital tubercle elongates to form the Penis, and the labioscrotal swellings transform into the scrotum. A longitudinal groove appears on its caudal surface, extending to the opening of the urogenital sinus. Later, the urethral folds proliferate along the sides of the groove and, by fusing, form the urethra. The penis begins to grow rapidly during puberty. In elderly individuals, increased keratinization of the glans epithelium and skin atrophy are observed.

In female embryos, the genital tubercle transforms into the Clitoris, the urethral folds into the Labia minora, and the labioscrotal swellings into the labia majora. The opening of the urogenital sinus does not shift. In a newborn girl, the clitoris and labia minora protrude from the pudendal cleft. By age 10, the pudendal cleft opens only when the thighs are abducted. After ages 45–50, Atrophy of the labia and mucous glands occurs, and the epithelium of the pudendal cleft mucosa thins and undergoes keratinization.

Fig. 4.57. Development of the external genitalia: A — male; B — female; 1 — genital tubercle; 2 — urethral folds; 3 — labioscrotal swellings; 4 — urogenital sinus; 5 — anus; 6 — tail; 7 — glans penis; 8 — urethral groove; 9 — scrotum; 10 — scrotal raphe; 11 — penile raphe; 12 — clitoris; 13 — epithelial tag; 14 — glans clitoridis; 15 — labia majora; 16 — urethral orifice; 17 — labia minora; 18 — hymen; 19 — vaginal orifice

The Mammary Glands develop from the 6th week of intrauterine life. Two cords of ectodermal cells, known as "milk lines," appear on the lateral aspects of the body. In the thoracic region, some of these cells grow into the underlying mesenchyme, forming the primordia of glandular tissue. In animals, multiple pairs of such buds develop instead of just one, subsequently giving rise to two rows of mammary glands. By the time of birth, the lactiferous ducts are formed. Clear differences

between boys and girls in The Structure of the glands are not observed during this period. With the onset of puberty in girls, under the influence of sex hormones, the glands gradually increase in size, and the nipples become more prominent. This increase in breast volume is primarily due to the accumulation of adipose tissue within the connective tissue between the lobes of the gland and the development of the ductal system. In boys, the mammary glands do not undergo significant changes during puberty, remaining flat.

Following menopause, the glandular and Connective Tissues of the mammary glands begin to undergo atrophy.

Review Questions

1. Describe the characteristics of male and female external and internal reproductive organs.

2. Structure and Tunics of the testis.

3. What is the STRUCTURE OF THE ductus deferens, ejaculatory ducts, and accessory Glands of the Male Reproductive System?

4. Describe the topography and Structural Features of the ovaries and uterine (fallopian) tubes.

5. What is The Relationship of the ovaries and uterine tubes to the peritoneum?

6. The uterus: topography, regions, Functions, and structural Features of the wall. Ligaments of the uterus.

7. How do the Organs of the reproductive system develop during ontogeny?

8. What is the structure of the mammary glands?