MH 141b Kidney

Nephron

The nephron is the structural and functional unit of the kidney.

• Renal Corpuscle - spherical structures with an average diameter of 200 µm distributed throughout the cortex.
  • Bowman's Capsule - encloses the glomerulus.
    • Parietal Layer - simple squamous epithelium that lines the outer wall of the capsule.
    • Visceral Layer - podocytes cover the glomerular capillaries. These cells have large ovoid nuclei.
    • Bowman's Space - the space between the parietal and visceral layers that receives the ultrafiltrate.
  • Glomerulus - blood flowing through a capillary network (or tuft) undergoes filtration to produce the ultrafiltrate.
    • Glomerular Capillaries - supplied by an afferent arteriole and drained by an efferent arteriole.
    • Podocytes - cover the surface of the glomerular capillaries and form narrow (~25 nm) filtration slits. These cells have large ovoid nuclei.
    • Mesangial Cells - large cells with irregularly shaped nuclei that have phagocytic and contractile function.
  • Vascular Pole - where the afferent and efferent arterioles enter and exit the glomerulus.
  • Urinary Pole - where the ultrafiltrate exits Bowman's space and a proximal convoluted tubule begins.

• Proximal Convoluted Tubule - begins from the urinary pole of a renal corpuscle.
  • Seen only in cross-section (#1 and #2) in the cortex because of their tortuous or convoluted course.
  • Simple cuboidal to columnar epithelium with an irregular, often stellate shaped lumen.
  • Eosinophilic - stain a darker pink than the distal tubules and ducts.
  • 7x the number of profiles as distal tubules
• Proximal Straight Tubule (Thick Descending Limb of Henle's Loop) - descends from the cortex into the medulla.
  • Seen as longitudinal sections (#1 and #2) in medullary rays and cross-section in the medulla.
  • Similar in morphology to proximal convoluted tubule.
• Thin Descending and Ascending Limbs (of Henle's Loop) - continuation of proximal straight tubule within the medulla that makes a hairpin turn and returns to the cortex.
  • Seen in cross-section in the medulla.
  • Simple squamous epithelium.

• Distal Straight Tubule (Thick Ascending Limb of Henle's Loop) - continuation of the thin ascending limb that ascends from the medulla to the cortex.
  • Seen as longitudinal sections (#1 and #2) within medullary rays and in cross-section in the medulla.
  • Simple cuboidal epithelium with an uniform lumen and indistinct boundaries between cells.
  • Neutral staining - lightly stained compared to the eosinophilic, proximal convoluted and straight tubules.
• Macula Densa - tightly packed cells where the distal straight tubule contacts the afferent arteriole of the vascular pole of the renal corpuscle.
• Distal Convoluted Tubule - empty into a collecting tubule.
  • Seen in cross-section (#1 and #2) in the cortex because of their tortuous or convoluted course.
  • Similar in morphology to distal straight tubule.
  • Neutral staining - lightly stained compared to eosinophilic, proximal convoluted and straight tubules.
  • Profiles less frequent than proximal convoluted tubules (7:1)

• Collecting Ducts - drain urine from the nephrons.
  • Collecting Ducts (#1 and #2) - convey urine from nephrons to collecting ducts within medullary rays.
  • Collecting ducts drain into a minor calyx at the apex of a renal pyramid.
  • Simple cuboidal to columnar epithelium with well-defined boundaries between cells.
  • Duct cells are lightly stained.