Complete Histology of the Urinary System and Prostate

PART 1: THE KIDNEY

Overview and Gross Organization

• Cortex - reddish-brown outer region containing renal corpuscles and convoluted tubules
• Medulla - lighter inner region containing straight tubules (loops of Henle) and collecting ducts arranged into renal pyramids. The apex of each pyramid forms the renal papilla, which opens at an area cribrosa into a minor calyx.

The Nephron - Functional Unit

1. Renal corpuscle
2. Proximal tubule (convoluted + straight)
3. Thin segment (loop of Henle)
4. Distal tubule (straight + convoluted)
5. Connecting tubule → collecting duct

Renal Corpuscle

• Parietal layer - simple squamous epithelium lining the outer wall; continuous with the proximal tubule at the urinary pole
• Visceral layer - composed of podocytes (modified epithelial cells) whose processes wrap around glomerular capillaries; their nuclei are large and pale-staining compared to endothelial/mesangial nuclei

• Vascular pole - where the afferent arteriole enters and efferent arteriole exits; the juxtaglomerular (JG) apparatus sits here
• Urinary pole - where the urinary space is continuous with the proximal convoluted tubule lumen; epithelium transitions from squamous to cuboidal/columnar with brush border

1. Fenestrated endothelium
2. Glomerular basement membrane (GBM) - type IV collagen + laminin + heparan sulfate
3. Podocyte foot processes with slit diaphragms (nephrin protein)

Juxtaglomerular Apparatus (JGA)

• Juxtaglomerular (JG) cells - modified smooth muscle cells in the afferent arteriole wall; contain secretory granules loaded with renin; they are the renin-secreting cells
• Macula densa - a cluster of densely-packed cells in the distal convoluted tubule where it returns to lie adjacent to its own glomerulus; nuclei appear crowded and apical cytoplasm is reduced; monitors NaCl concentration in tubular fluid
• Extraglomerular mesangial cells (lacis cells) - fill the angle between afferent and efferent arterioles

Proximal Convoluted Tubule (PCT)

• Epithelium: simple cuboidal to low columnar
• Brush border (microvilli): prominent, stains deeply eosinophilic - the defining feature; gives a "fuzzy" luminal border; greatly increases absorptive surface area
• Lumen: appears small and irregular (star-shaped cross-section) due to the thick brush border
• Cytoplasm: intensely eosinophilic; abundant mitochondria (basolateral infoldings interdigitate with mitochondria for active Na⁺-K⁺-ATPase-driven transport)
• Nucleus: large, round, centrally placed; only 2-3 nuclei visible per tubule cross-section
• Function: reabsorbs ~65% of filtered Na⁺, water, glucose, amino acids, HCO₃⁻, phosphate; secretes organic acids, drugs, toxins

Loop of Henle

Distal Convoluted Tubule (DCT)

• Epithelium: simple cuboidal
• No brush border - lumen appears more open than PCT
• Smaller cells with less cytoplasm; more nuclei visible per cross-section than PCT
• Basolateral membrane: highly infolded with mitochondria (active ion transport)
• Function: regulated reabsorption of Na⁺ (aldosterone-sensitive), Ca²⁺ (PTH-sensitive), Mg²⁺

Collecting Duct

• Principal (light) cells: pale, few organelles; respond to ADH (aquaporin-2 insertion → water reabsorption) and aldosterone (Na⁺ reabsorption)
• Intercalated (dark) cells: rich in mitochondria; stain darker; regulate acid-base balance (H⁺ secretion, HCO₃⁻ reabsorption)

Renal Interstitium and Blood Supply

PART 2: EXCRETORY PASSAGES

General Plan

Transitional Epithelium (Urothelium)

1. Basal cells - small, round/polygonal; minimal cytoplasm; resting on the basement membrane
2. Intermediate (umbrella precursor) cells - medium-sized; 2-3 layers thick
3. Superficial (umbrella/dome) cells - the hallmark - very large, dome-shaped; often binucleate; have a thick, rigid asymmetric unit membrane (AUM) on their apical surface

• Relaxed/empty bladder: 4-6+ cell layers visible
• Distended bladder: as few as 2-3 layers (cells spread out, not actually lost)

Renal Calyces and Renal Pelvis

• Epithelium: transitional (2-3 cell layers in minor calyces, thicker in pelvis)
• Lamina propria: fibroelastic connective tissue
• Muscularis: thin smooth muscle, inner longitudinal and outer circular layers
• Adventitia: merges with perinephric fat

Ureter

• Length: ~24-34 cm
• Mucosa: transitional epithelium (4-5 cells thick when relaxed); underlying lamina propria has dense fibroelastic connective tissue; no muscularis mucosae; wall folds create a star-shaped lumen in cross-section when empty
• Muscularis:
  • Upper 2/3: two layers - inner longitudinal and outer circular (note: opposite of gut arrangement)
  • Lower 1/3: three layers - inner longitudinal + middle circular + outer longitudinal
• Adventitia: retroperitoneal adipose tissue with vessels and nerves
• The oblique passage through the bladder wall creates a valve effect preventing vesicoureteral reflux

Urinary Bladder

• Mucosa: transitional epithelium (up to 6-10 cell layers when contracted; 2-3 when distended); prominent mucosal folds when empty; lamina propria is highly vascular with loose fibroelastic connective tissue
• Trigone region: embryologically derived from the mesonephric ducts; lacks mucosal folds; no muscularis mucosae or submucosa; smooth and flat at all times
• Muscularis (Detrusor muscle): three poorly-defined interdigitating layers of smooth muscle (inner longitudinal, middle circular, outer longitudinal); bundles are interlaced rather than forming pure sheets; contraction empties the bladder
• Internal urethral sphincter: ring of smooth muscle at the bladder neck (involuntary)
• Serosa (superior surface) or Adventitia (inferior/lateral surfaces)

Urethra

• Upper half (internal): transitional epithelium continuous with the bladder
• Lower half: pseudostratified or stratified columnar epithelium → stratified squamous epithelium at the external meatus
• Muscularis: inner longitudinal smooth muscle + outer circular smooth muscle; the external urethral sphincter is skeletal muscle (voluntary)
• Paraurethral (Skene's) glands: small mucous glands opening near the external meatus

PART 3: THE PROSTATE

Anatomy and Zones

Histological Structure of Normal Prostate

1. Mucosal glands - innermost, small, adjacent to urethra
2. Submucosal glands - middle layer
3. Main (peripheral) glands - outermost; largest; drain into the posterior urethra via excretory ducts

Prostatic Stroma

• Smooth muscle (fibromuscular stroma) - contracts during ejaculation to expel secretions
• Dense fibrous connective tissue - collagen bundles
• Blood vessels, lymphatics, nerves

Hormonal Dependence

Summary Comparison Table