KIDNEY:
• Cortex (outer) — contains glomeruli + convoluted tubules.
• Medulla (inner) — straight tubules + collecting ducts; arranged in pyramids.
• Nephron = functional unit:
– Renal corpuscle (glomerulus + Bowman's capsule).
– Proximal tubule (brush border).
– Loop of Henle (descending + ascending).
– Distal tubule.
– Connecting + Collecting duct.

URETER + BLADDER:
• Transitional epithelium (urothelium) — stretches when full.
• Loose lamina propria.
• Smooth muscle (3 layers in bladder).

URINARY SYSTEM HISTOLOGY

I. KIDNEY:
Outer cortex + inner medulla. Each kidney has thousands to millions of nephrons.

NEPHRON STRUCTURE (functional unit):

1. Renal Corpuscle:
• Glomerulus — tuft of fenestrated capillaries between afferent + efferent arterioles.
• Bowman's capsule — double-walled cup:
– Visceral layer = podocytes with foot processes wrapping capillaries (slit diaphragms = filtration barrier).
– Parietal layer = simple squamous epithelium.
• Filtration barrier = fenestrated endothelium + glomerular basement membrane + podocyte slit diaphragm.
• Mesangial cells — between capillary loops; phagocytic + contractile.

2. Proximal Convoluted Tubule (PCT):
• Cuboidal cells with apical BRUSH BORDER (microvilli) + many mitochondria → eosinophilic.
• Active reabsorption of glucose, amino acids, Na⁺ + H₂O (~70% of filtrate).

3. Loop of Henle:
• Thin descending + thin ascending — simple squamous epithelium (look almost like capillaries).
• Thick ascending — cuboidal cells; impermeable to water; pumps Na⁺/Cl⁻ → concentration gradient in medulla.

4. Distal Convoluted Tubule (DCT):
• Cuboidal; smaller cells than PCT, more nuclei per cross-section, NO brush border.
• Site of macula densa — dense cluster of cells where DCT touches its parent afferent arteriole; senses NaCl.

5. Connecting tubule + Collecting duct (CD):
• Cuboidal/columnar; principal cells (water reabsorption under ADH) + intercalated cells (acid-base balance).

JUXTAGLOMERULAR APPARATUS (JGA):
Three components — at the vascular pole of glomerulus:
• Macula densa (specialised DCT cells).
• JG cells (modified smooth muscle of afferent arteriole) — secrete RENIN in low BP/Na.
• Extraglomerular mesangial cells.

SPECIES-SPECIFIC NOTES:
• OX kidney — multipyramidal, lobulated; renal pelvis ABSENT (calyces drain directly).
• Horse, dog and cat — unipyramidal/unipapillary. Pig — smooth externally but multipyramidal/multipapillate internally.

II. URETER:
• Lumen — star-shaped (folds when relaxed).
• Mucosa: transitional epithelium + lamina propria.
• Muscularis: 2-3 layers smooth muscle (inner longitudinal + outer circular ± additional outer longitudinal in distal third).
• Adventitia: CT.

III. URINARY BLADDER:
• Mucosa with transitional epithelium (urothelium) — 3-7 cell layers; surface cells dome-shaped when relaxed, flatten when stretched. Top cells often binucleate. Special tight junctions (lots of claudins) prevent urine leak.
• Lamina propria — loose CT.
• Muscularis (Detrusor) — 3 layers smooth muscle (inner longitudinal + middle circular + outer longitudinal); circular layer thickened at neck = internal urethral sphincter.
• Serosa / Adventitia.

IV. URETHRA:
• Female: short; lined by transitional → stratified squamous near opening.
• Male: long; pelvic part lined by transitional/stratified columnar; penile part by stratified squamous.