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Genitourinary apparatus in ontogeny
Last reviewed: 07.07.2025
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The urinary and reproductive systems are interconnected in their development. In humans, the reproductive organs are later formed from the embryonic structures of some organs that are formed as urinary organs.
In lower vertebrates, three generations of excretory organs are distinguished, replacing each other: the pronephron, the primary kidney, and the definitive kidney.
The pronephros is the simplest form of excretory organs, it is laid in the embryos of all vertebrate fish species. It is a paired organ consisting of several excretory canals, or protonephridia. At one end of the canal there is a funnel that opens into the body cavity, and the other end of the canal connects to the excretory duct running along the body. The right and left ducts in the caudal part of the body open outward or flow into the final section of the digestive tube. Near the funnels of the protonephridia are vascular glomeruli, where the filtration of fluid occurs, which is first excreted into the body cavity and then enters the lumen of the canal.
In embryos of higher vertebrates, the anlage of the pronephros is reduced very early. It is replaced by the second generation of excretory organs - the primary kidney.
The primary kidney (mesonephros), or Wolffian body, is a paired organ that is laid down caudally to the pronephros and consists of segmental convoluted tubules, or mesonephridia. In higher vertebrates, these tubules begin blindly at one end and have the appearance of a double-walled cup (capsule). The vascular glomerulus (glomerulus) is invaginated into the capsule. Together, they form the renal corpuscle (corpusculus renis). The other end of the tubules connects with the remaining duct of the pronephros, which becomes the excretory duct of the primary kidney - the Wolffian duct (ductus mesonephricus). The primary kidney is laid down and functions as an excretory organ in all vertebrates in the embryonic period, and in adults - only in cyclostomes and some fish. In higher vertebrates, the primary kidney and its mesonephric duct function for a certain time and then are partially reduced. The surviving parts of the primary kidney serve as rudiments for the development of some sexual organs.
Lateral to the duct of the primary kidney, in parallel with it, in the embryo of vertebrates, from the cells lining the body cavity, a paired paramesonephric duct (ductus paramesonephricus), or Müllerian duct, develops. The cranial ends of these ducts open into the body cavity, and the caudal ends join with each other to form a common duct, which opens into the urogenital sinus (sinus urogenitalis). From the paramesonephric ducts in the female, the fallopian tubes and vagina are laid and develop, and in the male, the appendages of the genital organs. The primary kidney is replaced by the third generation of excretory organs - the definitive kidney.
The definitive kidney (metanephros) is a paired excretory organ that is laid down in the embryo later than the pronephron and primary kidney. The definitive kidney is formed from nephrogenic tissue and the proximal portion of the ureteric outgrowth of the duct of the primary kidney. The definitive kidney functions as a urine-forming organ in higher vertebrates.
The reproductive system in higher animals consists of the sex glands and the reproductive tract. The sex glands in vertebrates are paired and develop on the posterior wall of the body, medial to the primary kidney. The excretory ducts for the reproductive cells are formed from the duct of the primary kidney (in males) or the paramesonephric duct (in females).
In mammals, the sex glands move from their place of origin to the pelvic region. The ovaries remain in the pelvic cavity, the testicles (testes) in most mammals (except for some insectivores, cetaceans, etc.) exit the abdominal cavity into a skin fold - the scrotum. From the anterior part of the cloaca in mammals, the urogenital sinus is isolated, into which the genital ducts open. In viviparous mammals, the urogenital sinus is transformed into the vestibule of the vagina. In males, the urogenital sinus extends into the urethra.
In males, the urogenital ducts also open into the seminal vesicles, prostate, and bulbourethral glands; in females, large vestibular glands (vagina) are formed.
The formation of the uterus in placental mammals occurs by the fusion of the paired oviducts (double uterus) present in most rodents, elephants and some other animals into a bipartite uterus divided by a longitudinal partition (in some rodents, carnivores, pigs). In ungulates, cetaceans and insectivores, the fusion of the oviducts is such that a bicornuate uterus is formed, and only in some bats and humans does the uterus have one cavity (simple uterus), which opens into the vagina.
In humans, the pronephros is formed during embryogenesis and then completely disappears. The primary kidney at a certain stage of development ceases to perform its function and is partially reduced. The remaining parts of the primary kidney serve as a source of development of some sexual organs and their appendages. In the process of their development, the organs of the urinary and reproductive systems enter into close anatomical relationships.
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