DEVELOPMENT OF THE UROGENITAL SYSTEMS I. The Urinary System and Adrenal Gland The early development of the urinary (excretory) and genital (reproductive) systems are closely associated. Both systems develop primarily from a common mesodermal ridge along the posterior wall of the abdominal cavity which is derived from intermediate mesoderm. The close association of these two systems exists in the adult anatomy as well. For example, in the male the urethra is a common pathway for sperm and urine. In the female, the urethra and vagina open into a common vestibule. Although the development of these systems is closely related, this CAI will separate urinary and genital development. 1. The nephrogenic cords originate from intermediate mesoderm will contribute to the development of the urinary and genital systems form from endoderm A and B only A, B and C Correct! No, the answer is D. The nephrogenic cords are derived from intermediate mesoderm and make a major contribution to the development of both the urinary and genital systems. 2. During the fetal development of the urinary tract which of the following kidneys normally develop(s) pronephros mesonephros metanephros B and C only A, B and C Correct! No, the answer is E. The development of the kidneys in the human embryo and fetus represents an example of ontogeny recapitulating phylogeny.Three sets of kidneys are formed 1) the pronephros (forekidney), 2) the mesonephros (midkidney), and 3) metanephros (hindkidney). The pronephros persists in adult cyclostomes and some teleosts, the mesonephros is the adult kidney of most fish and amphibia (anamniota) while the metanephros is the adult permanent kidney of mammals including man (amniota). There is overlap between the three developmental stages of kidney ontogeny. 3. The pronephros is a rudimentary structure which only contributes its duct to the continuation of kidney development is the permanent kidney of mammals consists of 35-40 renal tubules which form from the nephrogenic cords A and C only A, B and C Correct! No, the answer is A. The pronephroi (plural of pronephros) are bilateral structures consisting of several tubules which form from t the nephrogenic cord in the cervical region of the embryo. Tubules form in a cranio-caudal sequence, but the pronephroi only contain a few tubules since the cranial tubules regress as new tubules form at a more caudal level. The pronephroi form early in the 4th week and disappear by the end of the 4th week. The pronephroi are believed to be non-functional(rudimentary) structures. The pronephric ducts which run caudally from the cervical region to open into the cloaca are used by the next developing set of kidneys, the mesonephroi. 4. The mesonephros forms in the cervical region consists of as many as 40 tubules with both glomerular capsules and mesonephric (renal) tubules present is the permanent kidney of humans A and B only A, B and C Right! No, the answer is B. The mesonephric kidneys form in the thoracic and lumbar regions of the developing embryo during the 4th week. Mesonephric tubules form from the nephrogenic cord and make contact with the pronephric duct which is now called the mesonephric (Wolffian) duct. The medial end of each of the mesonephric tubules is invaginated by a cluster of capillaries which form a glomerulus. The medial indented end of the mesonephric tubule forms the Bowman's (glomerular) capsule. The glomerular capsule and the glomerulus constitute the mesonephric (renal) corpuscle. As in the pronephros, tubule development occurs in a cranio- caudal sequence. Each mesonephric kidney contains no more than 40 tubules at any one time. The mesonephric kidneys usually degenerate completely by the 16th week. However,the mesonephric duct and a few mesonephric tubules are important embryological structures which give rise to the reproductive duct system of the male and vestigial remnants in the female. 5. The mesonephroi are believed to be functional kidneys in the human embryo True False Right! No, the answer is A. The development of complete mesonephric kidney tubules with glomeruli and extensive convolutions of the tubular structure and direct evidence from other mammalian species suggest that the human mesonephric kidneys are probably functional as interim kidneys until the metanephroi assume the excretory function. 6. The metanephros is essential for the prenatal existence of the fetus forms only from nephrogenic cord forms on the posterior abdominal wall A and C only none of the above Very good! No, the answer is E. The metanephroi are the permanent kidneys of man and other mammals. They form from two sources 1) the metanephric (ureteric) bud which is an outgrowth of the mesonephric duct and 2) the metanephrogenic cell mass (blastema) which is derived from the nephrogenic cord caudal to the level of the mesonephros. The metanephroi form as pelvic organs and later ascend by a passive process to their adult location on the posterior abdominal wall. The metanephroi are normally functional in the human fetus, but are not essential as excretory organs since the mother's placenta subserves this excretory function. However, the absence of urine production will result in a reduced volume of amniotic fluid (oligohydramnios), which may lead to other developmenal problems. The kidneys only become essential at birth when the placenta is no longer available for urinary excretion. 7. Which of the following is (are) true of the metanephros the ureteric bud gives rise to the nephron portion of the permanent kidney the metanephrogenic cell mass is the source of the collecting portions of the adult kidney the surface of the kidney is smooth and the hilum faces medially when it is first formed the kidneys undergo a passive ascent from the pelvic region to the posterior abdominal wall which is caused by the rapid growth of the body in the lumbar and sacral regions C and D only Very good! No, the answer is D. In the development of the metanephroi the ureteric bud or metanephric diverticulum forms the collecting portions of the kidney the ureter, renal pelvis the calyces and collecting tubules. The urine producing portion of the kidney (nephrons) consists of the glomerular (Bowman's) capsules, the proximal and distal convoluted tubules and the loops of Henle. The nephron portion of the kidney is derived from the metanephrogenic cell mass. When the metanephroi first form they are lobulated and the hila faces ventrally. The lobulation disappears soon after birth due to extensive cortical growth of nephrons. The kidneys ascend from their original location in the pelvis to their final position on the posterior abdominal wall. This ascent is a passive process due to differential growth of the fetal body. Rapid growth of the body caudal to the developing kidneys in the lumbar and sacral region is primarily responsible for the ascent of the kidney. During the ascent the kidneys rotate 90 degrees so that the hilum is positioned anteromedially. 8. The cloaca is the caudal part of the hindgut receives the allantois and the mesonephric duct is divided into a dorsal part (the primitive rectum) and a ventral part (the primitive urogenital sinus) by the downgrowth of the urorectal septum A and B only A, B and C Very good! No, the answer is E. The caudal dilated portion of the hindgut which receives the allantois and the mesonephric duct is known as the cloaca. The urorectal septum forms in the angle between the allantois and hindgut and grows down toward the cloacal membrane. This downgrowth separates the cloaca into a dorsal rectum and a ventral urogenital sinus. 9. The urogenital sinus includes the cranial vesico-urethral canal which is continuous with the allantois a middle pelvic portion a caudal phallic portion enclosed externally by the urogenital membrane A, B and C none of the above Correct! No, the answer is D. The 3 parts of the urogenital sinus are 1) the vesico-urethral canal, 2) a middle pelvic portion, and 3) a caudal phallic portion enclosed externally by the urogenital membrane. 10. Which of the following is (are) true of bladder development the epithelium of the adult bladder is derived from the mesoderm of the mesonephric ducts the lamina propria,muscle layers,and the serosa develop from the endoderm of the urogenital sinus the urachus develops from the metanephric diverticulum in the adult, the urachus is called the median umbilical ligament Correct! No. Only the fourth foil is correct. The trigone of the bladder is originally formed by the epithelium of the mesonephric ducts and therefore, is formed from mesoderm. This mesoderm is overgrown by the epithelium of the vesico-urethral portion of the urogenital sinus. Therefore, the epithelium of the adult bladder is formed from endoderm not mesoderm. The lamina propria (connective tissue), muscle develop from the splanchnopleuric mesenchyme. The allantois becomes a thick fibrous cord called the urachus which is attached to the apex of the bladder and the umbilicus. In the adult, the urachus is called the median umbilical ligament. 11. Which of the following is (are) true statements the mesonephric duct originally represents a common pathway of the urinary and reproductive systems into the urogenital sinus of the male absorption of the caudal portions of the mesonephric ducts into the wall of the urinary bladder results in the ureters entering the bladder separately from the mesonephric ducts due to the ascent of the kidneys and the downward growth of the mesonephric ducts the orifices of the ureters move to a more cranial position on the dorsal surface of the bladder the ureters in the adult male open laterally and cranially to the ejaculatory ducts Very good! No. All foils are correct. During division of the cloaca into rectum and urogenital sinus, the caudal portions of the mesonephric ducts are gradually absorbed into the wall of the urinary bladder. This developmental sequence results in the ureters, initially outgrowths of the mesonephric ducts, entering the bladder separately . As a result of the traction exerted by the kidneys during their cranial ascent and because of the continued downward growth of the mesonephric ducts, the ureters will eventually open laterally and cranially to the ejaculatory ducts, which are the adult derivatives of the mesonephric ducts in the male. In the female, the mesonephric ducts degenerate except for several vestigial structures epoophoron, paroophoron, and the duct of Gartner. 12. In the male urethra the epithelium of the prostatic urethra proximal to the orifices of the ejaculatory ducts is derived from mesoderm of the mesonephric ducts the connective tissue and smooth muscle develop from the endoderm of the urogenital sinus the epithelium of the membranous and spongy urethra is derived from mesonephric duct mesoderm the glandular epithelium (epithelium of the navicular fossa) forms from ectoderm Right! No. Only the fourth foil is correct. The epithelium of the prostatic part of the male urethra proximal to the ejaculatory duct orifices is derived from the vesical part of the urogenital sinus. The epithelium of the remainder of the prostatic portion of the urethra (distal to the orifices of the ejaculatory ducts) and the membranous urethra is derived from the endoderm of the middle portion of the urogenital sinus. The phallic (spongy) portion of the male urethra forms from the endoderm of the phallic part of the urogenital sinus. The epithelium of the navicular fossa (glandular part of the penis) forms from the glandular plate which is of ectodermal origin. The connective tissue and smooth muscle of all portions of the male urethra form from splanchnic mesenchyme. 13. In the female urethra the epithelium of the proximal part of the urethra is formed from the mesonephric duct epithelium and is therefore derived from mesoderm the epithelium of the entire female urethra is formed from the endoderm of the urogenital sinus the connective tissue and the smooth muscle form from the endoderm of the urogenital sinus opens into the vestibule of the vagina in adults Correct! No. The second and fourth foils are correct. In the female urethra the entire epithelium is formed from the endoderm of the vesical portion of the urogenital sinus while the connective tissue and muscle layers develop from the splanchnic mesenchyme. In the adult the female urethra and the vagina open into a common area between the labia minora known as the vestibule of the vagina. 14. The adrenal (suprarenal) gland consists of a cortex and medulla in the adult is derived only from mesoderm of the fetus is much larger than the adult gland is completely developed at birth Correct! No. The first and third foils are correct. The adrenal or suprarenal gland of the adult consists of 2 parts cortex and medulla. The cortex forms from mesoderm associated with the coelomic epithelium lining the posterior abdominal wall while the adrenal medulla forms from neural crest. The fetal cortex forms first from the coelomic epithelium and encapsulates the neural crest derived chromaffin cells which form the adrenal medulla. Subsequently, a second group of mesenchymal cells arise from the coelomic epithelium and surround the fetal cortex. This layer is known as the primordium of the permanent adrenal cortex. The zona glomerulosa and zona fasciculata form from the primordium of the permanent cortex before birth. The last layer of the adult adrenal cortex, the zona reticularis, forms during the first year after birth as the fetal adrenal cortex disappears. The adrenal gland of the human fetus is up to 20 times larger than the adult gland, relative to body weight. The large fetal adrenal gland results primarily from the presence of a relatively thick fetal cortex which is responsible for the production of the androgen, dehydroepiandrosterone. The fetal adrenal cortex participates in the materno-fetoplacental unit which involves the liver and adrenal glands of the mother and fetus and the placenta. Steroids formed in the adrenal and modified in the liver are the precursors for the synthesis of various estrogens. 15. Renal agenesis occurs unilaterally as a relatively infrequent teratologic malformation occurring bilaterally is incompatible with postnatal life is usually diagnosed during gestation is probably due to failure of development of the metanephric diverticulum (ureteric bud) Correct! No. The second and fourth foils are correct. Unilateral renal agenesis is a relatively common malformation occurring about 1/1,000 births. It is often not discovered until many years after birth as long as the remaining kidney is completely functional. Bilateral renal agenesis is a rare malformation. Although oligohydramnios (reduced volume of amniotic fluid) results from bilateral kidney agenesis, the fetus exists normally without kidneys because the mother's placenta subserves the urinary excretory function. After birth the kidneys are essential for life. 16. An ectopic kidney is most frequently found in the pelvis and may be caused by the failure of the kidneys to ascend from their original location may frequently occur as a "horseshoe kidney" in which the ascent of the kidney during development is blocked by the inferior mesenteric artery may occur as part of a "pancake" kidney or a unilateral double kidney may produce no symptoms unless a ureter is obstructed resulting in urinary stasis Very good! No. All foils are correct. Ectopic kidneys are most frequently found in the pelvis and probably result from failure of the kidneys to ascend to their normal adult position. "Horse shoe kidneys" occur in about 1/600 births and result from the fusion of the most medial portions of the ureteric buds. The kidneys begin their ascent from the pelvis but are trapped by the inferior mesenteric artery. The "pancake" or rosette kidney results from early fusion of the ureteric buds. In a unilateral double kidney the kidney on one side is double with separate ureters and blood supply . Obstruction or kinking of the ureters may occur resulting in urinary stasis and eventual hydronephrosis from the back pressures on the kidney cortex. 17. Which of the following urinary tract malformations may lead to serious postnatal complicatons multiple renal vessels duplications of the upper urinary tract congenital bilateral polycystic kidney disease unilateral renal agenesis Correct! No. The first three foils are correct. Multiple renal vessels develop as the kidneys undergo their ascent from the pelvis to the posterior abdominal wall. Normally, as the kidneys move out of the pelvis they are supplied by vessels at successively higher levels, and the lower vessels degenerate. Duplications of the ureters and renal pelves are common and result from premature division of the ureteric bud. Vascular variations and ureteric duplications may obstruct the ureters. Congenital bilateral polycystic disease of the kidney is relatively common and results from abnormal development of the collecting tubules. Severe renal insufficiency and uremia result in the death of these neonates at or soon after birth. Unilateral renal agenesis is rarely fatal and may not be diagnosed for many years as long as the remaining kidney is functional. 18. Exstrophy of the bladder is characterized by exposure and protrusion of the posterior bladder wall through the anterior abdominal wall is caused by a mesodermal defect in the inferior part of the anterior abdominal wall is more common in males than females and is normally associated with epispadias is a common congenital malformation Right! No. The first three foils are correct. Exstrophy of the bladder is a severe congenital malformation which occurs rarely (1/50,000 births), but is 3 times more common in males than in females. It results from the failure of mesoderm to invade the embryonic disc caudal to the cloacal membrane. The anterior abdominal wall and the anterior bladder wall are affected so that the posterior bladder wall protrudes through the defect in the anterior abdominal wall. Exstrophy of the bladder is associated with disruption of general growth processes in the area resulting in separation of the pubes and abnormal development of the penis or clitoris. In males exstrophy of the bladder is often associated with epispadias (opening of the urethra onto the dorsal surface of the penis). 19. Which of the following is (are) true of the development of the urachus the adult derivative of the urachus is the median umbilical ligament remnants of the urachal lumen may form urachal cysts rarely, the entire urachus remains open as a fistula that allows urine to escape at the umbilical orifice the urachus lies between the umbilical arteries and connects the urinary bladder with the umbilicus Very good! No. All foils are correct. The median umbilical ligament is the adult derivative of the urachus. The urachus remains patent at the bladder in about 50% of cases. Urachal fistulae and cysts are rare, but do occur. The presence of a urachal fistula allows urine to escape at the umbilicus. Internal and external urachal sinuses may also occur in a similar fashion to internal and external branchial sinuses.