Human Anatomy - Kotsan I. Y. 2009

Urogenital system
Genitalia - External female genitalia
Development of genital organs

The Reproductive System in higher animals consists of Gonads and genital tracts.

In vertebrates, the gonads are paired structures that develop on the posterior body wall, medial to the primary Kidneys. The efferent ducts for the Germ Cells are formed from the mesonephric ducts (in males) and paramesonephric ducts (in females).

In mammals, the gonads migrate from their initial embryonic site into the pelvic region; the Ovaries remain within the lesser pelvis, whereas the Testes (testicles) in most mammals (except for certain insectivores, cetaceans, etc.) descend from the Abdominal cavity into a cutaneous fold known as the Scrotum. The urogenital sinus separates from the anterior part of the cloaca in mammals, and the genital ducts open into it. In viviparous mammals, the urogenital sinus develops into the vaginal vestibule in females, while in males it elongates and forms the Urethra. The Uterus in placental mammals develops through the fusion of paired oviducts—which persist in most rodents, elephants, and some other animals—into a duplex uterus divided by a longitudinal septum (found in certain rodents, carnivores, and pigs). In ungulates, cetaceans, and insectivores, the oviducts fuse to form a bicornuate uterus, and only in some primates and humans does the uterus consist of a single cavity opening into the Vagina.

In the human embryo, indifferent internal and external genitalia develop first, and only subsequently do the definitive male or female internal and external Organs take shape.

The primordia of the indifferent gonads in the human embryo appear during the fourth week of embryonic development from the germinal epithelium located on the medial side of the right and left primary kidneys, spanning from the IV cervical to the V lumbar body segments. By the fifth week, along the lateral margin of the primary Kidney and the mesonephric duct, cells lining the body cavity give rise to the paramesonephric duct, which opens into the urogenital sinus. Flanking the dorsal mesentery, the mesonephric and paramesonephric ducts form ridge-like elevations known as urogenital folds. Subsequently, each of these folds is divided by a longitudinal sulcus into a medial part—the genital fold, where the gonad will develop—and a lateral part, which contains the primary kidney as well as the mesonephric and paramesonephric ducts. By the seventh week of embryonic development, the indifferent gonads begin to differentiate into either testes or ovaries. During testicular development, the mesonephric ducts transform into the efferent ducts of the male gonads, whereas the paramesonephric ducts almost completely regress. Conversely, during ovarian development, the mesonephric ducts degenerate into rudimentary structures, while the paramesonephric ducts serve as the precursors for the uterine tubes, uterus, and part of the vagina.

By the seventh week of embryonic development, the external genitalia also begin to form as an indifferent genital tubercle, genital folds, and genital swellings. These precursors subsequently develop into either the male or female external genitalia.

Development of Internal Male Genital organs. By the seventh month of intrauterine life, the Connective Tissue surrounding the developing male gonad forms the tunica albuginea. By this time, the gonad assumes a more rounded shape, and cords develop within it that differentiate into seminiferous tubules.

During The Development of the male gonad, the tubules of the primary kidney give rise to the efferent ductules of the Testis, while the cranial part of the mesonephric duct forms the duct of the Epididymis. Several cranially positioned tubules of the primary kidney transform into the Appendix of the epididymis, whereas caudally positioned tubules become the paradidymis. The terminal portion of the mesonephric duct, surrounded by a muscular coat, forms the Ductus deferens (vas deferens). The distal segment of the ductus deferens dilates to form the ampulla of the ductus deferens, and a Seminal Vesicle sprouts as a lateral outpocketing of this duct. The terminal, narrowed segment of the mesonephric duct forms the ejaculatory duct, which opens into the male urethra.

The cranial end of the paramesonephric duct gives rise to the appendix of the testis, whereas the fused caudal ends of these ducts form the prostatic utricle (masculine uterus). The remaining PARTS OF THE paramesonephric ducts regress in male embryos.

In humans, the testis and its epididymis do not remain in their initial embryonic Location; during ontogeny, they undergo a caudal displacement known as testicular descent, in which the gubernaculum testis plays a crucial role. By the third month of intrauterine life, the testis lies in the iliac fossa. By the sixth month, it reaches the deep inguinal ring. During the seventh and eighth months, the testis passes through the Inguinal Canal alongside the ductus deferens, vessels, and nerves, which together constitute the Spermatic Cord formed during the descent.

The Prostate Gland develops from the epithelium of the developing urethra in the form of cellular cords, which subsequently branch to form the lobules of the gland.

The Bulbourethral Glands develop from epithelial outgrowths of the spongy urethra.

Development of Internal Female Genital organs. In the Ovary of a female embryo, the connective tissue zone beneath the germinal epithelium layer is less pronounced than in the male gonad. The cellular cords are less distinct, and the germ cells are scattered throughout the mesenchymal stroma of the organ. Some of these cells proliferate more actively and enlarge, becoming surrounded by smaller cells to form primordial ovarian follicles. Subsequently, the cortex and medulla of the ovary are established, with Blood Vessels and nerves growing into the latter. As development proceeds, the primordial germ cells gradually shift into the cortical substance.

During development, the ovaries and uterine tubes migrate from their original embryonic site into the pelvic region. The descent of the ovaries is accompanied by a topographical shift in the uterine tubes, which transition from a vertical to a horizontal orientation.

During ovarian development, the remaining tubules of the primary kidney give rise to rudimentary structures—the epoophoron and paroophoron. The longitudinal duct of the epoophoron develops from remnants of the mesonephric duct.

The paramesonephric ducts give rise to the uterine tubes, uterus, and proximal part of the vagina. The distal part of the vagina and its vestibule are formed from the urogenital sinus.

Development of external genitalia. In the third month of human intrauterine life, anterior to the cloacal membrane, the mesenchyme gives rise to the genital tubercle. At its base, extending toward the anus, lies the urogenital groove, flanked on both sides by the genital folds. Flanking the genital tubercle and folds, crescent-shaped elevations of Skin and subcutaneous tissue—the genital swellings—form. All these structures constitute the indifferent external genitalia, which later differentiate into the external male or female organs.

In male development, the genital tubercle grows rapidly and elongates, transforming into the corpora cavernosa of the Penis. On the ventral (caudal) surface of the penis, the genital folds become more prominent. As these folds enlarge, the urogenital groove between them progressively deepens into a sulcus, and subsequent fusion of its edges forms the spongy urethra and the corpus spongiosum of the penis. As the penis grows, the urogenital orifice shifts from its initial position at the ROOT of the organ to the distal tip. The line of closure of the urethral groove remains visible as a scar known as the penile raphe. Simultaneously with The formation of the urethra, the prepuce (foreskin) develops over the distal end of the penis through the proliferation of an epithelial fold around the glans.

The genital swellings also enlarge, become more convex—particularly in their caudal regions—and approach each other, fusing along the midline to form the scrotum. The line of fusion creates the scrotal raphe, which extends across the entire Perineum from the root of the penis to the anus.

In female development, the genital tubercle exhibits limited growth and differentiates into the Clitoris. The genital folds enlarge to become the Labia minora. The distal portion of the urogenital groove widens to form the vaginal vestibule, into which the female urethra and vagina open. The genital swellings enlarge and transform into the Labia Majora, accumulating a substantial amount of adipose tissue that ultimately conceals the labia minora.