Development of the urinary system embryology

Development of the urinary system

1. ANA 223 EMBRYOLOGY OF THE URINARY SYSTEM, excretory units, kidney, Ureters, bladder and urethra

2. Overview The development of the urinary system involves the transient formation and remodeling of the intermediate mesoderm, which is the middle germ layer of the developing embryo. During development, three successive pairs of kidneys appear: the pronephros , the mesonephros , and the metanephros . The  pronephros  appears and subsequently degenerates, the  mesonephros  becomes temporarily functional before degenerating, and the permanent  metanephros  ascends from the  pelvis  to the lumbar region. Concurrently , the urogenital sinus gives rise to the  bladder , its neck, the penile urethra, and the vestibule of the vagina.

3. Pronephros By week 4, the intermediate mesoderm condenses and reorganizes into a series of epithelial buds. At the cranial level, these buds form the first pair of kidneys, the  pronephros  (plural, pronephroi ). In humans, the pronephros degenerates as rapidly as it forms . T he formation of the pronephros lays the foundation for induction of the mesonephros , which in turn lays the foundation for induction of the metanephros . Hence, the pronephros is crucial to the developmental cascade that leads to the formation of the permanent kidneys. In vertebrates with free-swimming larvae, such as teleost fishes and certain amphibians, the pronephros is the functional kidney of their early larval life and is crucial for proper systemic osmoregulation.

4. Pronephros While the pronephros forms and degenerates, a small duct appears lateral to the pronephric region. This small duct is a primary excretory duct known as the  mesonephric duct (or Wolffian duct). It differentiates in a cranial-to-caudal sequence to eventually connect and open into the walls of the cloaca. This region of fusion is what eventually forms the posterior wall of the future bladder.

5. Diagram showing the gross anatomy of the excretory system

6. Mesonephros During the degeneration of the pronephros , the epithelial buds at the thoracic and lumbar levels form the second pair of kidneys, the  mesonephros  (plural, mesonephroi ). The mesonephros degenerates at the thoracic level, but persists at the lumbar level. Simultaneously and also at the lumbar level, 20-30 pairs of  mesonephric tubules lengthen from the mesonephric buds. Their lateral ends fuse with the mesonephric duct, while their medial ends differentiate into a cup-shaped sac known as the Bowman’s capsule. T he Bowman’s capsule wraps around a knot of capillaries, known as the  glomerulus . Together, this forms a rudimentary  renal corpuscle , representing an abbreviated version that mirrors the adult nephron.

7. Mesonephros With the fusion of the lateral tips of mesonephric tubules to the mesonephric duct, a passage from the excretory units to the cloaca opens. The mesonephros thus functions as a temporary excretory system, producing small amounts of urine during week 6-10. After week 10, the mesonephric system ceases to function and completely regresses in females. Whereas in males, the mesonephric ducts and ductules persist to eventually form important elements of the male genital system, such as the  epididymis , the  ductus deferens , the  seminal vesicles , and the ejaculatory duct.

8. Metanephros While the mesonephros functions a temporary excretory system, the definitive kidneys, the  metanephros  (plural, metanephroi ) forms at the sacral level. The formation of the metanephros consists of two structures that are initially separated: the metanephric blastema and the ureteric bud. The  metanephric blastema  forms on each side of the body axis at the sacral level as a result of a  mesenchymal condensation induced by the intermediate mesoderm . Simultaneously, a ureteric bud  evaginates from the caudal end of each mesonephric duct, lengthens, and penetrates its corresponding metanephric blastema .

9. Metanephros By week 5, the ureteric bud undergoes a sequence of bifurcations and branching to eventually form the  renal pelvis , the major and minor calyces, and the collecting ducts. the metanephric blastema is what forms the Bowman’s capsule, the  proximal  convoluted tubules, the loops of Henle , and the  distal  convoluted tubules. T he ureteric bud and the metanephric blastema respectively form the excretory portions and collecting portions of the permanent kidneys, a process that takes place over a period of ten weeks.

10. RELOCATION OF THE KIDNEYS During week 6 to week 9, the metanephric kidneys relocate themselves by following a path on both sides of the dorsal  aorta . This relocation process is characterized by three mechanisms that occur concurrently: ascension, medial rotation, and revascularization. First , the kidneys ascend from the sacral to the lumbar region. Second, the hilum of each kidney –  initially facing ventrally – rotates medially to face the dorsal aorta. Third, the ascending and medially rotating kidney is progressively  revascularized  by a series of arterial sprouts from the dorsal aorta: a permanent  renal artery  is formed in the lumbar region for each kidney, whereas the original renal arteries in the sacral region and the subsequent ones degenerate.

11. Take note Right kidney is usually positioned lower than the left kidney, Right kidney does not ascend as high as the left kidney due to the presence of the  liver  on the right side. T he process of relocation, the formation of the excretory and collecting portions of the metanephric kidneys, and the temporary functioning of the mesonephric system all occur at the same time. Also, realize that because of the ascension of the kidney, the  ureter  is now much longer than it once was as a ureteric bud in the sacral region.

12. Bladder and urethra T he  cloaca is divided by the urorectal septum to form the ventral urogenital sinus and the dorsal anorectal canal. The urogenital sinus itself further develops into three distinctive parts: the bladder, the neck, and the phallic segment. The   bladder  continues to take the shape that we normally see in adults. The neck contributes to the development of the membranous urethra and the prostatic urethra in males and the membranous urethra in females. The phallic segment contributes to the development of the penile urethra in males and the vestibule of the  vagina  in females. Concurrent with the partitioning of the cloaca, the allantois regresses and forms a ligamentous band known as the urachus or the  median umbilical ligament , which runs through the subperitoneal fat from the apex of the bladder to the  umbilicus .

13. Bladder and urethra The caudal ends of the  mesonephric ducts are attached to cloaca. While the septation of the cloaca occurs, these caudal ends expand, flatten, and become incorporated into the posterior wall of the presumptive bladder. These caudal ends fuse and migrate caudally until they open into the pelvic urethra, right below the neck of the bladder. Concurrently, the ureters that were once tiny ureteric buds, dissociate themselves from the caudal ends of the mesonephric ducts, migrate cranio -laterally and also become incorporated into the posterior wall. As a result, lying between the openings of the ureters (laterally and superiorly) and the opening of the pelvic urethra (inferiorly) forms a triangular area known as the  trigone .

14. Development of bladder and urethra

15. Development of the bladder and urethra The bladder and urethra of the urinary system are ultimately derived from the  cloaca  – a hindgut structure that is a common chamber for gastrointestinal and urinary waste . In the 4th-7th weeks of development, the cloaca is divided into two parts by the  uro -rectal septum : Urogenital sinus  (anterior) – divided into three parts: The upper part of the urogenital sinus forms the bladder. The pelvic part forms the entire urethra and some of the reproductive tract in females, and the prostatic and membranous urethra in males. The phallic/caudal part forms part of the female reproductive tract, and the spongy urethra in males. Anal canal  (posterior )

16. Development of bladder and urethra The urinary bladder is initially drained by the  allantois , whic obliterates during fetal development and becomes a fibrous cord – the urachus . A remnant of the urachus can be found in adults; the median umbilical ligament, which connects the apex of the bladder to the umbilicus. As the bladder develops from the urogenital sinus, it absorbs the caudal parts of the  mesonephric ducts  (also known as the  Wolffian ducts), becoming the trigone of the bladder. The ureters, which have formed as outgrowths of the mesonephric ducts, enter the bladder at the base of the trigone . The final structure varies between sexes:  

17. Difference between male and female development Male female Bladder As the kidneys ascend into the abdomen, the ureteric openings move cranially. The mesonephric ducts (Wolffian ducts) move caudally and closer together, entering the prostatic urethra to become the ejaculatory ducts. As the kidneys ascend into the abdomen, the ureteric openings move cranially. The mesonephric ducts degenerate due to a lack of testicular androgens.

18. Difference between male and female in development Male female Urethra The pre-prostatic, prostatic and membranous urethra is formed from the pelvic part of the urogenital sinus. The spongy urethra is formed from the phallic part of the urogenital sinus. Urethra is formed from the pelvic part of the urogenital sinus

19. Diagram picturing the bladder

20. Clinicals Relocation anomalies On rare occasions, one of the kidneys fails to ascend, it remains close to the  common iliac artery  and forms a pelvic kidney. When both kidneys do not ascend, they may fuse at their cranial poles and form a pelvic rosette kidney (or discoid kidney). When kidneys are pushed close together during their ascension, their caudal poles fuse and form a horseshoe kidney. A horseshoe kidney’s relocation is often incomplete because it becomes hooked right under the  inferior mesenteric artery . These conditions rarely cause any symptoms and are typically an incidental finding. Although the insertion of the ureters is not completely distorted, they can be prone to reflux.

21. Clinical aspects Urachal anomalies Recall that the allantois normally regresses and forms the urachus (or the median umbilical ligament) at the apex of the bladder. In rare occasions, the allantois persists and instead forms either a  urachal fistula, an umbilical urachal sinus, a  vesicourachal diverticulum, or a  urachal cyst. Symptoms may include leakage of urine from the umbilicus, urinary tract infections, and peritonitis. In the event of an infection linked to these conditions, the initial symptoms can easily be confused with those of an  appendicitis

22. Clinicals Defective partitioning of cloaca If the urorectal septum fails to properly separate the cloaca into the urogenital sinus and the anorectal canal, this can result in a variety of fistulas between the urethra and the anorectal canal. In males, a  rectourethral fistula can form, connecting the prostatic urethra to the  rectum  or the  anal canal . This can cause the penile urethra to be frequently stenotic , causing urine to exit out the anorectal canal via the rectourethral fistula. In females, a  rectovaginal fistula can form, connecting the vagina to the rectum or the anal canal. Although the urethra is not affected in this case, fecal matter may empty into the vaginal canal.

23. Please take note 1. embryology without gestational age is meaningless 2. study to show your selves approved 3. notes are guides, your textbooks are also important 4. compliments of the season. Jesus CHRIST is the reason for the season; HE cares. See you all in the new year.