Intermediate mesoderm gives rise to the kidneys, ureters, gonads, and genital ducts. Along with the coelomic mesothelial cells, it forms the urogenital ridge. The urogenital ridge differentiates into:

• A lateral nephrogenic cord
• A medial genital ridge

The urinary system develops craniocaudally in three evolutionary distinct stages:

• Pronephros
• Mesonephros
• Metanephros

The adult kidney develops from the metanephros. The adult kidney initially develops in the pelvic region and ascends to its normal position in the abdomen by the end of the 9th week of development. This ascent is not an active process; it results from rapid longitudinal growth of the lumbosacral vertebrae and growth of the abdominal and thoracic viscera.

At the same time, the renal hilum changes position from ventral to medial by undergoing a 90-degree rotation.

The metanephros has two distinct components:

• The ureteric bud, derived from the mesonephric (Wolffian) duct
• The metanephric blastema, derived from intermediate mesoderm

The ureteric bud develops into:

• Ureter
• Pelvicalyceal system
• Collecting ducts

The metanephric blastema develops into the nephrons, including:

• Podocytes
• Epithelium of Bowman’s capsule
• PCT
• DCT
• Loop of Henle

These two systems induce each other’s development through a mechanism involving retinoic acid. Signaling via the NOTCH pathway is crucial for development of proximal renal structures such as the glomeruli and PCT. The glomerular capillaries are derived from mesoderm.

The allantois extends from the yolk sac and becomes part of the umbilical cord. It gives rise to the urachus. The urachus connects to the cranial end of the urogenital sinus (endoderm), which gives rise to the urinary bladder and urethra.

The urinary bladder and urethra develop from endoderm, except for:

• The trigone of the bladder, which is mesodermal (from the mesonephric ducts)
• Smooth muscle, which originates from splanchnic mesoderm

Adrenal cortex is derived from the intermediate mesoderm of the mesonephric tubules. Adrenal medulla develops from ectoderm of the neural crest.

Polycystic kidneys

Aberrant differentiation, induction, and proliferation of the developing renal tubular system leads to formation of cysts. Multiple cysts are seen in the kidneys in polycystic disease. Although cysts may become symptomatic later in life, they begin forming during the intrauterine period.

Mutations in the gene coding for a transmembrane protein called fibrocystin (polyductin) have been implicated as the root cause. Fibrocystin/polyductin, or polycystin PC1 and PC2, is thought to mediate terminal differentiation of the renal collecting ducts and intrahepatic biliary ducts.

Fibrocystin is normally located at the basolateral plasma membrane, primary cilia, and the centrosome in renal epithelial cells. ADPKD kidneys and ADPKD polycystic livers show an approximately 2-fold increase in PC1 mRNA compared to normal kidneys and livers.

Renal agenesis or hypoplasia

Absent or underdeveloped kidneys present as oligohydramnios and Potter syndrome. Low amniotic fluid increases pressure on developing organs, leading to Potter syndrome.

It manifests as:

• Hypoplasia of the lungs and gut
• Sloped forehead
• Shortened fingers
• “Parrot beak” appearance of the nose

Pelvic kidney

This occurs due to arrested ascent of the kidney. The kidney remains in the pelvic cavity rather than reaching its normal location in the posterior upper abdomen.

Horseshoe kidney

In this condition, both kidneys fuse together, forming a “U” (horseshoe)-shaped organ. During development, the horseshoe kidney becomes trapped in the lower abdomen by the IMA, which prevents further ascent.

It has been associated with:

• Alcohol consumption in pregnancy
• Thalidomide
• Maternal diabetes

It may be asymptomatic or present with UTIs, abdominal pain, and a mass. It is often accompanied by anomalies of the renal artery and vein. There is a higher risk of Wilms tumor, transitional cell carcinoma, and carcinoid tumors in a horseshoe kidney.

PUJ obstruction

Incomplete recanalisation of the pelviureteric junction during intrauterine development leads to obstruction. It is the most common cause of pediatric hydronephrosis.

Sometimes the obstruction is due to extrinsic causes such as:

• Aberrant vessels
• Fibrotic bands crossing the PUJ

Renal coloboma syndrome

Also known as papillorenal syndrome, this condition is characterised by hypoplastic kidneys and optic nerve coloboma. It may be associated with vesicoureteral reflux (VUR), renal cysts, and .

Mutations in the PAX2 gene, which codes for a transcription factor, are seen. It is inherited as an autosomal dominant condition.

Disorders of the urachus

During embryonic development, the urachus extends from the bladder to the umbilicus. Normally, it seals off and becomes the median umbilical ligament.

Abnormal persistence can present in several ways:

• The entire urachus may remain patent, presenting postnatally as discharge of urine from the umbilicus.
• The ventral end may remain patent, forming a urachal sinus. A sinus may become infected, causing local pain and discharge.
• If only the distal end remains patent, it forms a diverticulum of the bladder, which may cause UTIs.
• Urachal remnants may form a cyst.

Reproductive system

The gonads arise from the intermediate mesoderm of the urogenital ridge. The gonads are indifferent until 7 weeks of intrauterine life.

Sex is determined at fertilisation, but differentiation of an XY embryo into a phenotypically male fetus is determined by SRY (sex determining region Y) on the Y chromosome. SRY promotes the development of testes, including Sertoli and Leydig cells.

Key hormonal effects:

• Testosterone secreted by Leydig cells supports growth of the mesonephric ducts.
• DHT (dihydrotestosterone) supports growth of the prostate gland, penis, and scrotum.
• Sertoli cells produce anti-Müllerian hormone (AMH), also called MIS, which induces regression of the paramesonephric (Müllerian) ducts.

In males, the mesonephric (Wolffian) ducts become efferent ducts, including:

• Epididymis
• Rete testis
• Vas deferens
• Seminal vesicles

In females, Wolffian duct remnants include:

• Epoophoron
• Paroophoron
• Gartner’s ducts

In females, the Müllerian (paramesonephric) ducts become:

• Fallopian tubes
• Uterus
• Cervix
• Upper ⅓ of the vagina

In males, Müllerian remnants are seen in the prostatic utricle.

The prostate gland begins developing laterally as epithelial buds from the urogenital sinus wall. These buds branch into solid cords, which canalize to form the ducts and acini. The surrounding urogenital sinus mesenchyme forms the interfascicular fibroblasts and the smooth muscle of the prostate.

Bulbourethral glands, vestibule, urethra, and the lower ⅔ of the vagina develop from the urogenital sinus (endoderm). The Wolffian ducts regress in females.

Proliferation of ectoderm and mesoderm forms the genital tubercles, folds, and swellings.

• Genital tubercle gives rise to the body and glans of the penis, corpora cavernosa, and corpus spongiosum in males, and the clitoris in females.
• Genital folds become the ventral aspect of the penis and penile raphe in males, and the labia minora in females.
• Genital swellings become the scrotum and scrotal raphe in males, and the labia majora and mons pubis in females.