Renal System Development: Pronephros & Mesonephros

• The renal system develops from the same origin as the reproductive system, but slightly ahead in development.
• It originates from the urogenital ridge of the intermediate mesoderm.
• Three sequential systems contribute to renal development: pronephros, mesonephros, and metanephros.

Pronephros

• It emerges during the 4th week of development.
• Development begins in the cervical region of the embryo.
• Nephrotomes develop in the intermediate mesoderm, functioning as primitive kidneys.
• The pronephros consists of 6-10 pairs.
• They join and drain into the pronephric duct.
• The pronephros is non-functional and regresses completely by the end of week 4.
• Crucial for the developmental cascade that leads to the formation of permanent kidneys.

Mesonephros

• Develops below the pronephros.
• Pronephric duct induces nearby intermediate mesoderm in thoracolumbar region to form mesonephric tubules.
• Receives capillaries from dorsal aorta (initial filtration of blood) which drain into mesonephric duct (continuation of pronephric duct).
• It serves as a primitive excretory system in the embryo.
• Most mesonephric tubules atrophy and disappear by 10-12 weeks, but the mesonephric duct persists.
• Mesonephric duct produces the ureteric bud caudally.
• Ureteric bud contributes to the development of the definitive kidney.

Mesonephric Duct

• It first appears in the 4th week.
• Grows by proliferation and migration of cells at the caudal end.
• Establishes contact with the cloaca on day 26.
• Produces the ureteric bud from its caudal end on day 28.
• The ureteric bud grows into the metanephric blastema on day 32.
• Metanephric blastema develops from the caudal region of the intermediate mesoderm of metanephros.

Metanephros

• Produces the definitive kidney.
• Development begins in the 5th week of gestation and ends by the 12th week.
• The ureteric bud from mesonephric duct merges within the caudal region of intermediate mesoderm, this is known as metanephric blastema which then produces the definitive kidney (day 32).

The Definitive Kidney

• The definitive kidney is formed from the ureteric bud and metanephric blastema
• The ureteric bud branches repeatedly, and each branch is covered by a cap of metanephric blastema to form a renal lobe.
• Further branching of the ureteric bud gives rise to collecting tubules.
• The tip of the collecting tubule induces the formation of renal vesicles.

Collecting System

• Derived from the ureteric bud, which develops from the mesonephros.
• The ureteric bud dilates and develops to form the ureter, renal pelvis, major and minor calyces, collecting tubules and terminates at the distal convoluted tubule (DCT).
• If the ureteric bud splits occurs too early, two ureters can be formed, or two renal pelvices connecting to one ureter may occur (duplex kidney).

Excretory System

• Derived from the metanephric blastema
• The metanephric tissue cap covers each collecting tubule, giving rise to excretory tubules.
• Excretory tubules and the developing glomeruli form the nephron (functional unit of the kidney).
• The proximal end of the excretory tubule forms Bowman's capsule around the glomerulus.
• The distal end of the excretory tubule elongates to form the proximal convoluted tubule PCT, Loop of Henle LOH, and the distal convoluted tubule DCT.
• Ureteric bud and metanephric blastema exert effects on each other, contributing to the development of the collecting ducts, ureter, major and minor calyces, nephrons, Bowman's capsule , PCT, LOH, and DCT.

Kidney Ascent

• The kidneys initially develop in the pelvic region before ascending into the retroperitoneal space in the abdomen.
• The kidney receives blood supply from the pelvic branch of the abdominal aorta while in the pelvis.
• As the kidney ascends, it receives new vessels from the abdominal aorta.
• Pelvic vessels usually regress but can persist as accessory renal arteries.

Clinical Relevance

• Horseshoe kidney, where two developing kidneys fuse into a single, horseshoe-shaped structure.
• This occurs if the kidneys come in close proximity during their ascent.
• The kidneys usually fuse at the inferior pole.
• Horseshoe kidneys are typically drained by two ureters and are often asymptomatic.
• Individuals with horseshoe kidney may be prone to infections.

Development of the Bladder and Urethra

• The bladder and urethra are derived from the cloaca, a hindgut structure that serves as a common chamber for gastrointestinal and urinary waste.
• Between the 4th and 7th weeks of development, the cloaca is divided into two parts via the uro-rectal septum
• The dorsal (inferior) portion subsequently develops into the rectum and anal canal.
• The ventral portion develops into the bladder and urogenital sinus.
• The urinary bladder is initially drained by the allantois.
• As fetal development progresses , the allantois becomes obliterated and becomes a fibrous cord known as the urachus.
• In adults, a remnant oft he urachus is the median umbilical ligament.
• It connects the apex of the bladder to the umbilicus.
• The ureters form as outgrowths of the mesonephric duct and enter the bladder at the base of the trigone.

Urogenital Sinus

• It is divided into three parts: upper, pelvic, and lower.
• The upper third forms the bladder.
• The pelvic third forms the entire urethra.
  • In females, the pelvic third contributes to the reproductive tract.
  • In males, the pelvic third forms the prostatic and membranous urethra.
• The lower third forms part of the reproductive tract in females and the spongy urethra in males.

Variations Between Male and Female Fetus:

• Male:
• During ascent, ureteric openings move cranially and the mesonephric duct (Wolffian ducts) move caudally, closing together and entering the prostatic urethra to become ejaculatory ducts.
• The pre-prostatic, prostatic, and membranous urethra form from the pelvic part of the urogenital sinus, while the spongy urethra forms from the phallic part of the urogenital sinus. -Female:
• During ascent, the ureteric openings move cranially, and the mesonephric ducts degenerate due to the lack of testicular androgens.
• The urethra forms from the pelvic part of the urogenital sinus.

An overview of the initial stages of renal system development. This includes the pronephros, which appears first but regresses quickly, and the mesonephros, which develops below the pronephros and contributes to the formation of mesonephric tubules.