Human Anatomy - Kotsan I. Ya. 2009

Urogenital Apparatus
Genitalia - External Female Genitalia
Development of the Urogenital Apparatus in Phylogeny and Ontogeny

During the phylo- and ontogeny of vertebrates, three successive stages of Kidney development replace one another: the pronephros, the mesonephros, and the metanephros (definitive kidney).

The pronephros is a paired organ present during the Cytology/cytology/16.html">Early stages of embryonic development in all vertebrates. In adult organisms, the pronephros Functions throughout life only in some lower fish species, whereas in humans and higher vertebrates, it rapidly disappears during the early stages of Embryogenesis.

In the human embryo, the pronephros appears in the third week of intrauterine development from the nephrotomes of the lower cervical and upper thoracic segments (mesoderm in the region where somites transition into lateral plates). Each pronephros consists of 5–8 excretory tubules, or protonephridia. These tubules open into the body cavity with their expanded funnel-shaped ends, while their other ends open into a common excretory (protonephric) duct. Glomeruli are located near the funnels of the excretory tubules; fluid is filtered through them, initially entering the body cavity and subsequently flowing into the lumen of the tubule. In the caudal region of the body, the right and left excretory (protonephric) ducts open externally or empty into the terminal section of the hindgut. Pronephric tubules (protonephridia) are transient structures (functioning for 40–50 hours) and subsequently undergo complete regression. The excretory (protonephric) duct persists and serves as the duct for the next Generation of the kidney—the mesonephros.

The mesonephros (Wolffian body) is a paired organ that develops caudal to the pronephros and consists of convoluted tubules (mesonephridia). In higher vertebrates, one end of these tubules ends blindly and has the shape of a double-walled cup (capsule) into which a vascular glomerulus invaginates. Together, the capsule and the glomerulus form a renal corpuscle. The other end of the tubule empties into the common excretory duct, which remains from the pronephros and transforms into the excretory duct of the mesonephros—the mesonephric (Wolffian) duct. The mesonephros develops and functions as an excretory organ in all vertebrates during the Embryonic period. In adults, it functions throughout life only in cyclostomes and certain fish. In higher vertebrates, the mesonephros functions for a certain period before partially regressing, with its remaining parts serving as primordia for The Development of the reproductive Organs and the definitive (final) kidney.

In the human embryo, the mesonephros forms at the end of the third week of intrauterine development from the nephrotomes of the thoracic and lumbar segments and consists of 25–30 segmental convoluted tubules. As an excretory organ, it functions during the 1st and 2nd months of embryonic development. At the end of the second month, partial regression of the mesonephric tubules begins, and the mesonephros ceases its function. In males, the remaining mesonephric tubules and duct form the deferent ducts and the Epididymis, whereas in females, they form the epoophoron and paroophoron.

Lateral to the mesonephric duct and parallel to it, the paramesonephric (Müllerian) duct develops from the Cells lining the embryonic body cavity. The upper ends of the paramesonephric ducts open into the body cavity, while their lower ends fuse and open via a common orifice into the urogenital sinus. In females, the uterine tubes, Uterus, and Vagina develop from the paramesonephric ducts, whereas in males, they give rise to the Appendix of the Testis and the prostatic utricle.

The metanephros (definitive kidney) is a paired organ that develops later than the two preceding ones. In the human embryo, it begins to develop during the 4th–5th weeks of embryonic development caudal to the mesonephros (in the pelvic region) from two sources: the metanephrogenic tissue (a region of the mesoderm) and the proximal end of the ureteric bud arising from the mesonephric duct. The metanephrogenic tissue gives rise to the renal parenchyma, whereas the proximal segment of the mesonephric duct forms the renal calyces, renal pelvis, and Ureter. Beginning in the third month of embryonic development, the definitive kidney starts to function and completely replaces the mesonephros. The Development of the definitive kidney is completed only after birth. During development, the definitive kidney appears to ascend from the pelvic cavity into the future lumbar region, which is associated with the uneven growth of various body segments.

The development of the Urinary Bladder and Urethra is associated with the transformation of the cloaca, allantois, and the caudal sections of the mesonephric ducts.

The cloaca is a common cavity into which the urinary tracts, reproductive tracts, and hindgut open during the early stages of Human Development and in certain animals. In the human embryo, the cloaca is divided by a frontal septum into two compartments: an anterior compartment—the urogenital sinus, part of which contributes to The formation of the walls of the urinary bladder and urethra—and a posterior compartment, which becomes the future rectum. The allantois (urinary sac), as well as the mesonephric and paramesonephric ducts, are connected to the urogenital sinus. In the second month of embryonic development, the base and trigone of the urinary bladder are formed from the lower part of the allantois and the caudal sections of the mesonephric ducts. The apex and body of the urinary bladder develop from the middle part of the allantois and the urogenital sinus of the cloaca, whereas the upper part of the allantois transforms into the urachus, which later becomes a fibrous cord known as the median umbilical ligament.