Summary
This lecture continues from the loop of Henle and completes the nephron and the conducting/storage parts of the urinary tract: the distal convoluted tubule, the juxtaglomerular apparatus, the collecting tubules and ducts, the renal papilla, then the ureters, bladder and urethra. The same structure-to-function logic applies throughout. The second half introduces transitional epithelium (urothelium), which is unique to the urinary system and is built for distensibility and impermeability. (This material is on slides numbered 1 to 22 within the second half of the combined PDF.)
Distal convoluted tubule (DCT)
- Simple cuboidal epithelium. Structure-to-function:
- Basolateral interdigitations for active Na+ transport.
- Abundant mitochondria because active transport is energy dependent.
- DCT cells are similar to PCT cells but have no microvilli, so there is no glucose or amino acid absorption here.
Juxtaglomerular apparatus (JGA)
- Sits where the DCT contacts the vascular pole of its own corpuscle. Know its three components:
- Macula densa (MD): modified DCT cells that sense tubular fluid.
- Lacis cells (L): extra-glomerular mesangium.
- Juxtaglomerular (granular) cells (J): modified smooth muscle cells in the afferent arteriole wall.
- Understand it as the structure linking tubular fluid sensing to control of glomerular blood flow (and renin release).
Collecting tubules and ducts
- Collecting tubule: simple cuboidal. Two cell types:
- Principal cells (about 66%): Na+ reabsorption, K+ secretion, ADH-dependent water reabsorption.
- Intercalated cells: K+ reabsorption and acid-base balance.
- Collecting tubule cells resemble DCT cells but with particular emphasis on active Na+ transport.
- Collecting duct (duct of Bellini): simple columnar epithelium, same two cell populations and functions as the collecting tubule. Epithelium gets taller (cuboidal to columnar) as the system converges toward the papilla.
- In the medulla, collecting ducts are surrounded by thin limbs of the loop of Henle and vasa recta.
Renal papilla and proximal ureter
- The ducts of Bellini open onto the cribriform surface of the renal papilla, delivering urine into the pelvicalyceal space.
- At the papilla the lining transitions to transitional epithelium, with underlying smooth muscle, marking the start of the conducting system.
Ureters and bladder (conduction and storage)
- Ureters are muscular tubes for peristalsis, lined by the same epithelium as the bladder (transitional). Built for distensibility and impermeability to fluid loss; in the relaxed state the mucosa is thrown into folds.
- Ureter wall layers (TS): transitional epithelium, lamina propria, smooth muscle (inner longitudinal and outer circular), adventitia. The distal third gains a third muscle layer: inner longitudinal, middle circular, outer longitudinal.
- Bladder wall: the detrusor muscle is three interwoven smooth muscle layers (inner longitudinal, outer circular, outer longitudinal) within elastin-rich connective tissue.
Transitional epithelium (urothelium)
- Unique to the urinary system. Stratified-looking epithelium whose surface cells are umbrella cells (not squamous, unlike true stratified epithelia which have squamous surface cells).
- Understand distensibility: the epithelium flattens from many-layered (relaxed) to few-layered (stretched) while keeping its barrier. Tissue elasticity comes from elastic fibres in the lamina propria and surrounding smooth muscle.
- Umbrella cell apical specialisations (TEM) allow this: inflexible plaques that permit membrane folding, and surface vesicle invaginations that add membrane on stretch. SEM surface view shows the plaque mosaic.
Urethra (epithelial transition along its length)
- Epithelium becomes progressively less specialised distally as the need for distensibility falls:
- Prostatic urethra (near bladder): transitional.
- Membranous urethra: pseudostratified / columnar.
- Spongy (penile) urethra: stratified columnar.
- External urethral orifice: stratified squamous.
- The prostate is where the urinary and reproductive systems meet.
Self-test checklist
- Can you explain why the DCT has no microvilli but the PCT does, and what that means functionally?
- Can you name the three components of the JGA and what each does?
- Can you distinguish principal from intercalated cells by function?
- Can you state how the epithelium changes from collecting tubule to collecting duct, and why?
- Can you describe where the ducts of Bellini open?
- Can you list the wall layers of the ureter and what is different about its distal third?
- Can you explain how transitional epithelium achieves distensibility at the cell level (umbrella cells, plaques, vesicles)?
- Can you trace the epithelial changes along the male urethra from bladder to orifice?