Coupon Accepted Successfully!



Nephron cytology
  1. Glomerulus
    1. Blood is fed, via an afferent arteriole, under pressure into groups of capillaries, tufting out as loops from the vascular pole, and ensheathed in visceral squamous epithelium.
    2. Glomerular wall of
    3. Fenestrated endothelium,
    4. Thick basal lamina (two laminae fused together),
    5. Podocytes’ pedicels (visceral epithelial cells’ feet), separated by filtration slits of controllable width, permit
    6. The filtration of water and solutes, with a molecular mass less than 30 kDa, into a capsular space between
    7. glomerular/visceral epithelium and the parietal squamous epithelium and BL of Bowman’s capsule.
    8. The altered blood is collected from the capillary tufts, and passes out via the narrower efferent arteriole.
    9. Between the capillaries at their base lie mesangial cells, synthesizing and maintaining the glomerular basal lamina, and also probably phagocytic and contractile.
    10. Mesangial cells are significantly involved in renal disease, e.g., in diabetes and glomerular nephritis.
  2. Proximal tubule (40-50 µm diameter)
    1. Most common of those tubules seen in the sectioned cortex, since it is longer than the distal tubule.
    2. Simple, acidophilic, cuboidal, epithelial lining cells with: large round nuclei ;
    3. Many microvilli (brush border*), and a surface glycoprotein coat containing peptidases to reduce polypeptides;
    4. Vesicles and lysosomes just below the microvilli, and involved in endocytotic protein uptake and breakdown to amino acids;
    5. Marked lateral membrane infoldings and interdigitation with adjacent cells, to which they attach with junctional complexes.
  3. Thin segment (15 µm diameter)
    1. Squamous epithelial lining on a BL.
    2. Cells are pale, tightly fastened, with small, short microvilli, and a few mitochondria scattered randomly.
    3. The lack of red blood corpuscles in the lumen, and plumper nuclei, distinguish thin segments from capillaries.
  4. Distal tubule (20-50 µm diameter)
    1. Weakly acidophilic, cuboidal epithelial cells enclose large lumens.
    2. No brush border is seen because only a few short microvilli are present.
    3. Basal infoldings and interdigitations, with very many long mitochondria, give a basal striation.
    4. Cells lie on a BL, also supporting fenestrated endothelial cells of the surrounding capillaries.
    5. Macula densa is a specialized, more nucleated region of the epithelium, where it attaches to the arterioles of the glomerulus to form part of the juxtaglomerular apparatus. It senses the [Cl–] locally in the distal tubule and signals, via mesangial cells, for renin release, and arteriolar and mesangial contraction.
  5. Juxtaglomerular apparatus
    1. Afferent arteriole, nearing the JGA, loses its elastica interna.
    2. Smooth muscle cells change to epithelioid with
    3. Secretory granules and some GER.
    4. The juxtaglomerular secretory cells are in contact with the endothelium of the arteriole and, indirectly, with the macula densa of the distal tubule: for sensing (i) renal tubular chemistry, and (ii) stretch, indicating blood pressure.
      The cells’ sympathetic innervation is another element in the control matrix.
    5. Granules are the enzyme renin for release into the blood, where it cleaves a potentially hypertensive polypeptide (angiotensin I) from angiotensinogen.
    6. A juxtaglomerular interaction with the adrenal cortex and Na+ excretion also occurs.
    7. Polkissen/Goormatigh/lacis cells lie in the angle between the afferent and efferent vessels and the attached distal tubule.
  6. Collecting duct (40-200 µm diameter)
    1. Pale cuboidal cells, with the lateral cell membranes prominent because lateral interdigitation is lacking.
    2. Principal collecting-duct cells, and, set between them, alpha/A and beta/B intercalated cells, all differing in their ion-transport roles.
    3. Principal cells have few microvilli, and few mitochondria, but are tightly connected by occluding junctions. Aquaporin 2 constructs the channels making the luminal cell membrane permeable to water in the presence of vasopressin/ADH, so that the cells reabsorb water. Baso laterally, a membrane Na, K-ATPase lets the cells secrete potassium, while absorbing sodium.
    4. Intercalated cells have darker cytoplasm, and more and darker mitochondria, than principal cells. The number of vesicles is highly variable, because they function to insert or remove ion pumps into the cell membrane, in a similar way to the gastric parietal cell.
    5. Type A intercalated cells bear a luminal-membrane H, K-ATPase to secrete hydrogen ions and reabsorb potassium; type B cells have a luminal Cl/HCO3– counter transporter to secrete bicarbonate and recover chloride.
    6. A simple columnar epithelium lines the final papillary ducts of Bellini, and covers the papillae.
  7. Renal interstitium
    1. Lies between the kidney tubules and vessels.
    2. It comprises: (a) reticular fibres, (b) a little ground substance, and (c) interstitial fibroblasts, looking after the matrix and secreting erythropoietin.
    3. The interstitial elements are more prominent in the medulla than the cortex.
  8. Renal blood vessels
    Renal artery branches to form Interlobar arteries (interpyramidal), extending to the cortico-medullary junction, where they branch and turn as arching Arcuate arteries, giving off outward branches called Interlobular arteries ; from which Intralobular arteries provide Afferent arterioles to Glomeruli; from the capillaries of which the blood is taken via Efferent arterioles to serve one or both of Two capillary beds - around the convoluted tubules, and between the straight medullary tubules. The blood collected in stellate, deep cortical, and interlobular veins, traces back the arterial path to the renal vein. The sympathetic nervous supply to the kidney goes mainly to the renal vasculature, including the juxtaglomerular cells. Vasa recta is a collective name for arteriolar, capillary, and venous straight blood vessels in the medulla. They participate in the counter-current exchange.

Urinary Passages

The kidney’s calyces and pelvis, and the passages to the urethra are lined by transitional epithelium
  1. Transitional epithelium/urothelium
    1. Multilayered, with large surface/umbrella cells, intermediate cells and basal cuboidal cells on a thin BL.
    2. The surface cells have unique properties of:
      1. Making a barrier impermeable to urine;
      2. Changing their shape and extent during bladder distension.
    3. For 2(a), the luminal umbrella cell membrane is asymmetrically thickened (to 12 nm) and has unusual lipids and proteins, including uroplakins
    4. For 2(b), the Golgi complex forms fusiform vacuoles, bounded by thick membranes. During bladder dilation, the vesicles attach to the thick luminal membrane and become part of it, thus increasing its extent and allowing the cell to flatten. No cell-over-cell sliding occurs, the cells being joined by tight and adhaerens junctions and desmosomes.
    5. Large lysosomes destroy defective membrane.
    6. The rate of cell turnover is very low for an epithelium.
  2. Ureter
    1. Transitional epithelium lies on a collagenous lamina propria.
    2. Mucosa has several longitudinal folds, giving the lumen a stellate shape in the cross-section.
    3. Two smooth muscle coats: outer, circular ; inner, longitudinal ; (the terminal ureter has an extra, outer longitudinal one).
    4. CT adventitia, rich in vessels and nerves.
  3. Urinary bladder
    1. Transitional epithelium, on a wide collagenous lamina propria without glands, constitutes the mucosa.
    2. Three smooth muscle tunics interweave in the muscularis, in a pattern to squeeze the bladder empty. Retention of urine invites infection.
    3. A CT adventitia has blood and lymphatic vessels, nerve fibres and ganglion cells. The part of the bladder facing the pelvic cavity has a serosa.
    4. The ureters enter obliquely, with mucosal flaps to prevent reflux; smooth muscle forms a sphincter at the urethral outlet.
  4. Urethra (male)
    1. Epithelium lies on a very loose, elastic, vascular, distensible lamina propria. The lumen is stellate in cross-section.
    2. Epithelium is transitional changing to pseudostratified columnar, stratified columnar, and finally stratified squamous, as it traverses the three sections : prostatic, membranous (short) and penile/cavernous (long).
    3. Branching out in the penile mucosa are Littre’s small tubular mucous glands.
    4. There is a meagre smooth muscle muscularis, except at
    5. The smooth and skeletal muscle sphincters
    6. Female urethra is much shorter than the male; structurally it is similar, but, ending in the pelvic floor, has a skeletal muscle sphincter at its terminus.

Test Your Skills Now!
Take a Quiz now
Reviewer Name