Summary
This lecture covers the basic histological organisation of the kidney, working from low-power architecture down to the cells of the nephron. It establishes where filtration happens (the renal corpuscle, in the cortex only), how the corpuscle is built and how it filters blood, and then walks through each tubular segment, in every case linking a structural feature to a functional consequence. The recurring theme is that epithelial type (squamous vs cuboidal) and cell specialisations (microvilli, basolateral interdigitations, mitochondria) directly predict the segment’s transport role. Slides 26 to 32 in this file (numbered 19 to 25) repeat the loop of Henle and collecting segment material already introduced.
Low-power structure: cortex and medulla
- The kidney divides into an outer cortex and inner medulla (medullary pyramids). Gross landmarks: capsule, cortex, medullary pyramid, calyx, hilum, renal artery/vein, renal pelvis, renal papilla, renal sinus, ureter.
- The functional blood-urine barrier sits in the renal corpuscles, which are located exclusively in the cortex. This is why corpuscles (glomeruli) are a marker of cortex on a slide.
- A lobe = one medullary pyramid plus its overlying cortex.
- Understand the nephron’s spatial layout: afferent arteriole, glomerulus and Bowman’s capsule/space, proximal convoluted tubule, loop of Henle (pars recta, descending thin, ascending thin, ascending thick limbs), distal convoluted tubule, collecting tubule, collecting duct (duct of Bellini), drained by the cortico-medullary vasculature (cortical radial/interlobar/arcuate arteries).
Embryological origin of the corpuscle
- Understand how the corpuscle forms: the developing glomerulus (a capillary tuft) pushes into the blind end of a primitive renal tubule, invaginating it. The invaginated (inner) layer becomes the visceral layer = podocytes; the outer layer becomes the parietal layer of Bowman’s capsule. The space between is Bowman’s space.
- The metanephric duct (from the ureteric bud) forms the calyx, then the collecting tubules, and fuses with the metanephric mesenchyme (blastema). This gives the dual origin of the kidney: ureteric bud (collecting system) and metanephric mesenchyme (nephron).
The renal corpuscle as a blood filter
- Identify corpuscle components on histology: endothelium (E), glomerular capillaries (C), podocytes (P), mesangium (M), Bowman’s space (BS), and the glomerular basement membrane (GBM).
- The corpuscle has a vascular pole (afferent arteriole enters, efferent arteriole leaves) and a urinary pole (filtrate exits into the PCT).
- Know the role of afferent and efferent arteriole diameter: to maintain glomerular blood (hydrostatic) pressure for filtration.
The filtration barrier and the basement membrane
- The three layers fluid crosses: fenestrated capillary endothelium to glomerular basement membrane to podocyte filtration slits.
- Understand from the scanning EM: the capillary endothelium has fenestrations; podocytes wrap the capillary with primary then secondary processes (foot processes), and the gaps between adjacent foot processes are filtration slits (FS) that permit rapid passage of fluid.
- The GBM is made of type IV collagen and acts as the ultra-filter. Podocytes and endothelium share this single basement membrane.
Proximal convoluted tubule (PCT)
- Simple cuboidal epithelium. Structure-to-function table:
- Microvilli (brush border) increase surface area, increasing rate of absorption of glucose and amino acids.
- Basolateral interdigitations house the Na+ pump.
- Abundant mitochondria power active transport (very energy dependent).
- On histology the PCT has a prominent brush border (BB) and acidophilic cytoplasm; recognise it surrounding corpuscles in the cortex.
Loop of Henle
- Segments: pars recta (thick descending limb), thin descending limb, thin ascending limb, thick ascending limb.
- Pars recta of proximal tubule: simple cuboidal, still has microvilli (absorbs glucose and amino acids) but fewer basolateral interdigitations than the PCT.
- Thin limbs: simple squamous epithelium. No microvilli, no basolateral interdigitations, few mitochondria, therefore no active transport; they work by passive movement and support countercurrent exchange.
- Thick ascending limb: simple cuboidal with basolateral interdigitations and no microvilli (less rapid diffusion needed); performs active Na+ transport.
Self-test checklist
- Can you explain why renal corpuscles identify the cortex, not the medulla?
- Can you describe how the visceral and parietal layers of Bowman’s capsule arise embryologically?
- Can you name the two embryological tissues that form the kidney and what each becomes?
- Can you list the three layers of the filtration barrier and what each contributes?
- Can you state what the GBM is made of and its role?
- Can you explain the role of afferent vs efferent arteriole diameter?
- Can you match each nephron segment to its epithelial type (squamous vs cuboidal) and its transport function?
- Can you explain why the PCT has abundant mitochondria and a brush border, but the thin limbs do not?