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:
The urinary system develops craniocaudally in three evolutionary distinct stages:
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 develops into:
The metanephric blastema develops into the nephrons, including:
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:
Adrenal cortex is derived from the intermediate mesoderm of the mesonephric tubules. Adrenal medulla develops from ectoderm of the neural crest.
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.
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:
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.
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:
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.
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:
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.
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 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:
In males, the mesonephric (Wolffian) ducts become efferent ducts, including:
In females, Wolffian duct remnants include:
In females, the Müllerian (paramesonephric) ducts become:
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.
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