Embryology | Development of the Urinary System 

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1. Once the mesoderm develops, it is segmented into 3 parts (medial to lateral): paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm (2 layers: somatic layer going with ectoderm, splanchnic layer with endoderm) 2. Folding of the trilaminar disc occurs the intermediate mesoderm starts to condense and invaginates the splanchnic mesoderm. The divot created is called the urogenital ridge. The condensed intermediate mesoderm (in the core of urogenital ridge) is called nephrogenic cord. 3. The nephrogenic cord is located behind the gut tube. At the bottom of the gut tube the cloaca is found which will later become the anal canal, bladder and urethra. 4. During week 4, around the cervical region of nephrogenic cord, the pronephros starts to develop. Which is a Duct with smaller nephrotomes (tubules). But by the end of week 4 it already starts to degenerate. 5. The mesonephros develops off the degenerating pronephros but in the thoracolumbar region. Also made of mesonephric duct and small mesonephric tubules coming out of it. 6. The mesonephros is the primitive urinary system until week 10. 7. The mesonephric tubules grow towards the aorta, and glomeruli grow from the aorta via angiogenesis. The tubules form capsules around glomeruli (bowmans capsule). Primitive urinary system. Week 5-10 8. The mesonephric duct collects the filtrate from the tubules and disposes it into the cloaca. Both hindgut and mesonephric duct empty into cloaca. The allantois empties the cloaca. 9. In the pelvic region, intermediate mesoderm condenses infront of mesonephric duct to form metanephric mesoderm. It releases growth factors to stimulate mesonephric duct to grow ureteric bud out of it. The ureteric bud also releases growth factors to stimulate metanephric mesoderm (blastema) to grow. Reciprocal induction. 10. The ureteric bud grows into the blastema and develops a ureteric stalk (ureter) connected to mesonephric duct. 11. The ureteric bud grows inside the blastema and develops the renal pelvis and major calyces from which the minor calyces develop. Millions of collecting tubules grow from the minor calyces. Collecting system. 12. Each collecting tubule has metanephric mesoderm cap around it, the collecting tubules secretes growth factors making the mesoderm condense and form metanephric vesicle and then forms the metanephric tubule which is connected to the collecting tubule. 13. The metanephric tubule then becomes the DCT and keeps growing to form the PCT and then makes a cup shape structure called bowmans capsule. The common iliac arteries form glomerular capillaries and interact with bowmans capsule. Loop of henle also formed between PCT & DCT. Bowmans capsule from PCT from DCT from collecting tubule from minor calyces… 14. The common iliac artery to the kidney degenerates and the kidney ascends and blood vessels from the top part of aorta develop and invade the kidney (renal arteries). 15. By week 10 the ureter is emptying into mesonephric duct then into cloaca. Distal part of mesonephric duct is absorbed by cloaca and then ureter is emptying directly into it. The mesonephric duct becomes a part of reproductive system. 16. The cloaca develops a urorectal septum that divides it into anterior portion (bladder) and posterior portion (anal canal). The anterior portion is called urogenital sinus which has 3 parts. Proximal part > bladder, middle part > urethra (prostatic & membranous in males), distal part > penile urethra.

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14:30 he looks so enthusiastic when he says bowman's capsule, it makes me happy too :)

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around 41:56 when you re-reviewing the development of the male urethra you said the most distal portion becomes the prostatic urethra but did you mean to say penile urethra (since prostatic is part of the medial section of the primitive bladder/urethra)

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