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, #2 and #3) in the cortex because of their tortuous or convoluted course.
• Simple cuboidal to columnar epithelium with an irregular, often stellate shaped lumen.
• Eosinophilic (compared to distal tubules and ducts).
• 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 (compared to 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, #2 and #3) in the cortex because of their tortuous or convoluted course.
• Similar in morphology to distal straight tubule.

• Collecting Ducts - drain urine from the nephrons.
• Collecting Ducts - 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.