Urinary System Embryology Quiz

Urinary System Embryology

• The urinary system develops from intermediate mesoderm, also called the nephrogenic cord.
• Kidney development occurs in a cranio-caudal gradient
• Three distinct kidney structures appear sequentially: pronephros, mesonephros, and metanephros.
• Pronephros: forms in the cervical region, is non-functional, and regresses by week 5.
• Mesonephros: forms in the thoracolumbar region, is transiently functional, appearing in the 4th week and disappearing around the end of the 2nd month (except for the mesonephric duct in males). It is considered a "transitory kidney"
• Metanephros: forms in the sacral or pelvic region in the 5th week, and is the "definitive kidney".

Germ Tissues

• The chordamesoderm and paraxial mesoderm create the axial skeleton, while the intermediate mesoderm gives rise to the kidneys and gonads.
• Lateral plate mesoderm forms the circulatory system, body wall, and limbs.

Embryonic Renal Origins

• The kidney develops from intermediate mesoderm.
• The nephron, glomerulus, renal corpuscle, urorectal septum, pronephros, mesonephros, and metanephros are all components of the urinary system during development.
• The ureteric bud and metanephric mesoderm contribute to kidney development.
• Mesenchymal-epithelial transitions are crucial in kidney development.
• Defects like renal agenesis, cystic kidney, double ureter, horseshoe kidney, and bladder exstrophy have embryonic origins.

Pronephros

• Appears during week 4.
• Disappears by week 5.
• Located in the neck region.
• A non-functional structure.
• Made of nephrogenic cord, nephrotomes, pronephric tubules, and glomeruli
• Develops from nephrotomes after the 13-14th somite.

Mesonephros

• Appears during week 4.
• Forms the mesonephric tubules and duct.
• Has its own nephrons and tubules.
• Mesonephric tubules are S-shaped loops.
• The mesonephric (Wolffian) duct is involved in the development of the male reproductive system (collecting duct) and can contribute also to excretion..
• The mesonephric ducts are involved in part of the male reproductive structures, but degenerate in the female.

Paramesonephric (Müllerian) Duct

• Not part of the mesonephros, but derived from intermediate mesoderm.
• No role in the kidneys, instead plays a role in the reproductive system.

Mesonephros - Mostly Disappears

• Disappears after week 8-10.
• Except:
  • Mesonephric ducts - Give rise to male reproductive structures.
  • Degenerate in females.
• Analogous to Paramesonephric Ducts (also known as Müllerian ducts).

Metanephros

• Appears in week 5 and becomes functional by week 10.
• Metanephric blastema(i.e. tissue) is differentiated by the ureteric bud(metanephrogenic diverticulum)(outgrowth of mesonephric ducts)
• Formation of nephrons begins
• Becomes the functional kidneys, the "definitive kidney"

Collecting System

• The ureteric bud (which is a derivative of the mesonephros) subdivides into 1-3 million collecting tubules.
• Major and minor calyces act as convergence points for collecting tubules.
• Pelvis is the convergence point for major calyces.
• The ureter is the main tube that carries urine to the bladder.

Filtration System

• It comprises of the primordial metanephric mesoderm.
• This tissue initially exists as undifferentiated tissue caps.
• Ureteric bud influences the formation of renal vesicles and subsequently nephrons.
• Final development culminates into nephrons, basic structures of the kidney

Nephron (Excretory Unit)

• Renal corpuscle (glomerulus and Bowman's capsule).
• Proximal convoluted tubule.
• Loop of Henle.
• Distal convoluted tubule.
• Connect to collecting ducts.
• Formed until birth, ~1e6 nephrons
• Urine production begins by week 10

Filtering System Derivation

• Nephrons develop from metanephros.
• Collecting duct system (ureter, pelvis, calyx) is formed from the ureteric bud, a mesonephric derivative.

Molecular Signals

• WT1 gene is expressed in metanephric mesenchyme, maintaining nephron development competence.
• Mesenchymal signals affect the ureteric bud, influencing branching and division.
• Ureteric bud signals to metanephric mesoderm, inducing proliferation, blocking cell death, and inducing the formation of the renal epithelium.
• Several proteins and signaling pathways are involved in these developmental processes.

Renal Agenesis

• A developmental defect where one or both kidneys fail to develop, Often associated with other abnormalities.
• Possible causes include mutations in Genes like WT1 or GDNF or other developmental defects or errors.

Congenital Cystic Kidney

• Autosomal recessive polycystic kidney disease (ARPKD) involves cyst formation in collecting tubules and hypertrophy of surrounding nephrons, which can cause early kidney failure.
• Autosomal dominant polycystic kidney disease (ADPKD) sees cyst formation impacting all parts of the nephron, causing renal failure in adulthood.

Ciliopathies

• Defects in primary cilia, involved in fluid transport signal transduction, are associated with congenital cystic diseases affecting the kidneys.
• Primary cilia defects interfere with renal function.

Ureter Duplication

• A common renal abnormality.
• It can present either as a partial or complete splitting of the ureteric bud.
• Ectopic ureter termination into the vagina, urethra, or vestibule may occur.

Kidney Ascent

• Kidneys move from their pelvic to their abdominal position (T12-L3).
• Factors contributing to their movement include changes in body curvature and growth.

Kidney Positioning Defects

• Pelvic kidney: one kidney is retained in the pelvic region.
• Horseshoe kidney: fusion of kidney at upper or lower poles (90 percent). This is often caused by the isthmus of the kidneys getting trapped beneath the inferior mesenteric artery.

Cloaca

• The common embryonic cavity initially recieves urine and feces.
• Eventually divides and forms the urogenital sinus (anterior part) and anorectal canal (posterior part).

Urogenital Sinus (endoderm)

• Develops into the urinary bladder, and parts of the urethra.

Urachal Defects

• Urachal defects involve abnormalities in the development of the urachus, which is located between the urinary bladder and the umbilicus.
• These defects can result in the urachus remaining patent, forming a fistula, developing into a cyst, or a serous discharge from the umbilicus..

Bladder Formation

• The urinary bladder develops from the allantois-derived endoderm. and from the urogenital sinus.
• The urogenital sinus will develop into the pelvic, spongy, and phallic regions of the urethra in males and the urethra in females.

Trigone Formation

• The trigone is the posterior part of the urinary bladder.
• The mesonephric ducts and ureter play a role in its development as they incorporate into the posterior part of the bladder, forming the trigone region during embryonic development.
• The trigone's lining develops from the endoderm, replacing the mesodermal origin.

Epispadias

• A condition where the urethra opens dorsally on the penis or clitoris.

Exstrophy

• A condition where the abdominal wall fails to close around the developing bladder.
• Often associated with other structural anomalies