Embryology Quiz: Kidney Development

Development of the Urinary System

• The urinary and genital systems develop from a common mesodermal ridge, specifically the intermediate mesoderm.
• The pronephros is the first excretory system, rudimentary and non-functional.
• The mesonephros, the second system, may function briefly in early fetal development.
• The metanephros, the third system, develops into the permanent kidney.

Kidney Systems

• Pronephros: Rudimentary, non-functional, early in development.
• Mesonephros: May function for a short time during fetal development.
• Metanephros: Forms the permanent kidney.

Pronephros

• Early in the fourth week, the pronephros consists of 7 to 10 solid cell groups in the cervical region.
• These groups form rudimentary excretory units (nephrotomes).
• The pronephric system disappears by the end of the fourth week.

Mesonephros

• Forms in the thorax and lumbar regions; large excretory units (nephrons).
• Forms its own collecting duct, the mesonephric (or Wolffian) duct.
• In males, few caudal tubules persist and participate in genital system development; in females, these structures disappear.

Metanephros (Definitive Kidney)

• The third urinary organ, the metanephros appears in the fifth week.
• Its excretory units come from metanephric mesoderm.

Collecting System

• Collecting tubules develop from the ureteric bud, an outgrowth of the mesonephric duct, inside the metanephric tissue.
• The bud penetrates the tissue, resulting in dilation of that portion to form the primitive renal pelvis.
• The collecting system further develops to include major and minor calyces.
• Tubules proliferate until the end of the fifth month.
• Third-, second-, and first-order tubules enlarge and absorb those of lower orders, to form the minor calyces of the renal pelvis.
• The collecting tubules of later generations elongate considerably, converge on the minor calyx, and form renal pyramids.

Nephrons

• Glomeruli form in the pockets at one end of the S-shaped tubules.
• Renal vesicles form small S-shaped tubules, where the metanephric tissue cap forms these.
• The proximal end of each nephron develops Bowman's capsule, indented by a glomerulus.
• The distal end of the tubules establishes a connection with a collecting tubule.
• The kidney develops from two sources:
  • Metanephric mesoderm (excretory units)
  • Ureteric bud (collecting system)
• Nephrons are formed until birth, resulting in approximately 1 million nephrons per kidney.

Urine Production

• Urine production begins early in gestation, after glomerular capillaries start forming around the 10th week.

Kidney Appearance at Birth

• At birth, kidneys have a lobulate appearance.
• This lobulation disappears during infancy due to further nephron growth, although the number of nephrons does not increase.

Renal Dysplasia and Agenesis

• Renal dysplasia and agenesis represent a spectrum of severe malformations in the kidneys.
• These malformations often require dialysis and transplantation in early years of life.
• Multicystic dysplastic kidney is a common example, where many ducts are surrounded by undifferentiated cells, preventing normal nephron development.
• Renal agenesis may be unilateral or bilateral, resulting in renal failure. Other severe defects (vagina, uterus, vas deferens, seminal vesicles) are often seen accompanying this condition.

Congenital Polycystic Kidney Disease

• Numerous cysts develop within the kidneys, inherited as autosomal recessive or dominant diseases.
• Autosomal recessive polycystic kidney disease occurs in 1/5000 births. This condition leads to progressive disease with renal failure in infancy or childhood.
• Autosomal dominant polycystic kidney disease is more prevalent (1/500-1/1000 births) but less progressive; renal failure is typically not seen until adulthood due to slower, lower levels of cyst formation, compared to recessive disease.

Duplication of the Ureter

• Results from early splitting of the ureteric bud.
• This splitting may be partial or complete, resulting in metanephric tissues separated into two or more parts.
• In rare cases, one ureter connects to the bladder and the other is ectopic, entering the vagina, urethra or vestibule

Position of the Kidney

• Kidneys initially located in the pelvis, later shifting to the abdominal region.
• Movement (ascent) involves crania shifting and lateral moving/meeting up with adrenal glands. A rotation also occurs that places the renal pelvis facing the midline.
• Factors motivating the rise of kidneys involve body curvature and growth in lumbar and sacral regions.
• The metanephros receives arterial supply from a pelvic branch of the aorta during its ascent.

Clinical Correlates: Abnormal Location of the Kidneys

• Occasionally, during kidney ascent, one kidney fails to pass through umbilical arteries and remains in the pelvis close to the common iliac artery as a pelvic kidney.
• Sometimes, kidneys are pushed together during ascent, causing fusion of the lower poles of the kidneys to form a horseshoe kidney.
• Usually located at the level of the lower lumbar vertebrae; blocked from moving upward by inferior mesenteric artery.
• Approximately 1/600 people have a horseshoe kidney. Renal arteries arise commonly from the aorta to supply kidneys.

Function of the Kidney

• The metanephros-formed definitive kidney becomes functional near the 12th week of gestation.
• Urine is passed into the amniotic cavity and mixes with amniotic fluid.
• The fluid is swallowed by the fetus, and recycled through the kidneys.
• In fetal development, the placenta handles waste product excretion, not the kidneys.

Bladder and Urethra

• The cloaca divides to form the urogenital sinus (anterior) and anal canal (posterior) during development weeks 4-7.
• The urorectal septum is a mesodermal layer separating the primitive anal canal from the urogenital sinus. It forms the perineal body.
• The urogenital sinus is divided into three portions:
  • The upper/largest part is the urinary bladder.
  • The second is the pelvic part of urogenital sinus that forms the prostatic and membranous portions of the urethra in males.
  • The third part forms the phallic portion of the genital urethra. This part's development differs significantly between sexes.

Ureters

• The caudal portions of the mesonephric ducts are incorporated into the bladder wall, forming the ureters.
• During kidney ascent, the openings of the ureters move upward.
• The mesonephric ducts' openings approach each other and become the ejaculatory ducts in males.
• The mesodermal lining of the bladder trigone is gradually replaced by endodermal epithelium. This leads to the bladder's complete endodermal lining.

Urethra

• Both sexes' urethral epithelium originates in endoderm.
• The connective and smooth muscle tissue comes from visceral mesoderm.
• In males, prostatic urethra epithelium proliferates to form the prostate gland by the third month.
• In females, the cranial portion of the urethra gives rise to urethral and paraurethral glands.

Clinical Correlates: Bladder Defects

• Urachal fistula: Occurs when the intraembryonic portion of the allantois persists, causing urine to drain from the umbilicus.
• Urachal cyst: A local area of the allantois persists, leading to cystic dilation due to secretory activity.
• Urachal sinus: Upper portion of the allantois persists, resulting in a sinus usually connected to the urinary bladder.

Exstrophy of the Bladder

• A ventral body wall defect exposing the bladder mucosa.
• Likely due to failure of the lateral body wall folds to close in the midline in the pelvic region.
• A rare anomaly, occurring in approximately 2/100,000 live births.