Development of the Urinary System

Development of the Urinary System

• The urinary and genital systems develop from a common mesodermal ridge (intermediate mesoderm).
• The pronephros, the first excretory organ, is rudimentary and nonfunctional.
• The mesonephros, the second excretory organ, may function briefly during early fetal development. It is primarily involved in the male.
• The metanephros forms the permanent kidneys and appears in the fifth week of development.

Kidney Systems

• The metanephros contains excretory units
• These units develop from metanephric mesoderm.
• The ureteric bud forms the collecting system.

Kidney Development

• At the beginning of the fourth week, the pronephros forms groups of cells (7-10) in the cervical region.
• Vestigial excretory units (nephrotomes) form.
• Internal and external glomeruli and nephric tubules develop and open into the intraembryonic cavity.
• By the end of the fourth week, the pronephric system is gone.

The Mesonephros

• Forms in the thoracic and lumbar regions.
• Characterized by excretory units and its own collecting duct (mesonephric or Wolffian duct)
• In males, some caudal tubules from the mesonephric duct participate in the formation of the genital system .
• In females, the mesonephric duct disappears.

The Metanephros (The Definitive Kidney):

• Forms the definitive kidney.
• Appears in the fifth week.
• Excretory units develop from metanephric mesoderm.

Collecting System

• Develops from the ureteric bud.
• The bud penetrates the metanephric tissue,
• The bud dilates and forms the primitive renal pelvis.
• Splits into cranial and caudal portions (future major calyces).
• The first two new buds are produced by the calyx while penetrating the metanephric tissue
• Tubules subsequently form until the fifth month.
• Tubules enlarge, absorb other tubules, forming the minor calyces of the renal pelvis.

Collecting Tubules

• Fifth and successive generations of collecting tubules elongate and converge on the minor calyx.
• These tubules form the renal pyramid.
• These tubules are part of the ureter, renal pelvis, major and minor calyces, and around 1 to 3 million collecting tubules.

Excretory System (Nephrons)

• Collecting tubule has a metanephric tissue cap.
• The cells in the cap form small vesicles (renal vesicles).
• The renal vesicles develop into small S-shaped tubules
• Capillaries grow into a pocket at one end of the tubules; these become glomeruli.
• The tubules plus their glomeruli form nephrons.
• The proximal end of each nephron forms Bowman's capsule, indented by the glomerulus.

Kidney Formation Process

• The distal end of the tubules connects to a collecting tubule
• Formation of proximal convoluted tubule, Henle's loop, and distal convoluted tubule.
• The kidney develops from metanephric mesoderm and the ureteric bud.

Nephrons Formation and Function

• Nephrons are formed until birth (around 1 million in each kidney).
• Urine production begins early in gestation.
• Glomerular capillaries differentiate by the 10th week.
• At birth, the kidneys have a lobulate appearance, that disappears during infancy.

Renal Dysplasias and Agenesis

• A spectrum of severe malformations.
• Can require dialysis and transplantation early in life.
• Examples include multicystic dysplastic kidney.

Multicystic Dysplastic Kidney

• An example of renal anomalies where multiple cysts develop and surround undifferentiated cells.
• Nephrons fail to develop
• The ureteric bud fails to branch
• Collecting ducts never form.
• Can lead to kidney involution and agenesis.

Renal Agenesis

• Can be unilateral (1 per 1000 births) or bilateral (1 in 10,000 births).
• May be asymptomatic if unilateral, but bilateral results in renal failure.
• Characteristic of Potter Sequence: anuria, oligohydramnios, hypoplastic lungs.
• Other defects like abnormal vagina/uterus, vas deferens, or seminal vesicles may occur with renal agenesis in 85% of cases.

Congenital Polycystic Kidney Disease

• Cysts develop from collecting ducts.
• Autosomal recessive (1/5000 births) results in renal failure early in life.
• Autosomal dominant (1/500-1/1000 births) is less severe and failure may occur in adulthood

Ureter Duplication

• Occurs from early splitting of the ureteric bud.
• Ureter may be partial or complete.
• Can result in one ureter opening into the bladder, and the other ectopic (opening into the vagina, urethra, or vestibule).

Kidney Position

• Kidneys initially in pelvic region, migrate cranially, laterally, and rotate.
• Position influenced by body curvature development in lumbar/sacral regions.
• Renal vessels develop at different levels, with lower vessels often degenerating.

Abnormal Kidney Location (Clinical Correlates)

• Pelvic kidneys: one or both kidneys don't ascend properly, rest near iliac artery
• Horseshoe kidney (1/600): fusion of kidneys, often blocked by inferior mesenteric artery

Kidney Function

• Functional metanephros forms near 12th week.
• Urine flows into amniotic cavity and recycles through the fetus.
• Kidneys aren't responsible for excretion of waste during fetal life; placenta does it.

Bladder and Urethra

• Cloaca divides into urogenital sinus (anterior) and anal canal (posterior) during weeks 4-7
• Urorectal septum a layer of mesoderm dividing the anus and urogenital structures.
• Urogenital sinus forms the urinary bladder and urethra.
• 3 parts to urogenital sinus (1) urinary bladder, (2) pelvic part of urethra, (3) phallic part of urethra.

Ureters

• Caudal portions of mesonephric ducts absorbed into the bladder.
• Ureters emerge separately from the bladder.
• Ureteric openings move cranially as kidneys ascend.
• The mesodermal lining of the trigone changes to endodermal epithelium of the bladder.

Urethra

• Urethra epithelium of both sexes originates in endoderm.
• Surrounding connective tissue and smooth muscle come from visceral mesoderm.
• Male prostatic urethra proliferates to create prostate gland.
• Female urethra has urogenital and paraurethral glands.

Clinical Correlates: Bladder Defects:

• Urachal fistula: allantois remains open, urine drains from umbilicus.
• Urachal cyst: allantois partially persists, forming a cyst.
• Urachal sinus: upper part of allantois persists, forming a sinus connecting to bladder.

Exstrophy of the Bladder

• Ventral body wall defect.
• Bladder mucosa exposed.
• Related to failure of lateral body wall folds to fuse in the midline.
• Rare anomaly (2/100,000 births).