Introduction to Renal Histology Part 2 of 2 Anjali Satoskar MD PhD Department of Pathology

Tubules - - Proximal tubule - Loop of Henle - Distal convoluted tubule - Collecting tubule - Collecting duct Tubules

Tubules – Modify the glomerular filtrate by reabsorption and secretion (180 liters to 1.5 liters)

95% of the renal cortex is occupied by tubules (proximal tubules)

Renal medulla contains only tubules, no glomeruli

- Proximal tubule - Loop of Henle - Distal convoluted tubule - Collecting tubule - Collecting duct Tubules

PROXIMAL TUBULE Reabsorption S1 and S2 segments –Reabsorb most of NA +, Cl -, K +, H 2 O and most of HC03 - as well as glucose, amino acids. –Luminal brush border and basolateral membrane infoldings, many mitochondria. S3 segment (or pars recta) –Secretion of various ions, drugs, toxins Susceptible to toxic injury. –Less prominent brush border, less mitochondria, little basolateral infoldings.

Proximal tubule Metabolically active cells. Tall, lots of cytoplasm. Mitochondria at the base. Brush border at the luminal surface, to increase surface area of absorption.

Proximal convoluted tubule – Brush border and basolateral infoldings

Histology of proximal tubules, prominent brush border

Proximal tubules - Basolateral folds with mitochondria

Ultrastructural appearance of proximal tubule

Loop of Henle Small flattened cells

LOOP OF HENLE Urine concentration (Medullary concentration gradient countercurrent multipliers) Thin descending limb –Simple thin cells, few organelles –Water permeable Thin ascending limb –Simple thin cells, few organelles –Water impermeable Thick ascending limb of Henle (TALH) –Cuboidal cells, basolateral infoldings, mitochondria –Relatively water impermeable –Produces Tamm-Horsfall protein –No brush border

Loop of Henle – Maintenance of Medullary concentration gradient

Thin loop of Henle Thin limbs of Loop of Henle

Thick ascending loop of Henle (secrete Tamm-Horsfall protein)

Juxta-glomerular apparatus Tubulo-glomerular feedback

Location of the Juxta- glomerular apparatus

Juxta-Glomerular Apparatus (JGA) 3 major components:- Juxta-glomerular cells - arterioles Macula densa – distal tubule Lacis cells – mesangial cells

Juxta-Glomerular Apparatus (JGA)

Function of Juxtaglomerular apparatus Tubuloglomerular feedback Autoregulation – Adjust renal blood flow and the GFR Long term blood pressure control

JG apparatus seen in a histologic section

Renin granules (electron microscopy)

Questions to Review 1. Where is the JGA apparatus located? 2. Renin is secreted by which cells? 3.Proximal tubule brush border serves what purpose? 4. Where does the proximal tubule arise from?

Distal tubule

DISTAL CONVOLUTED TUBULE Absence of brush border Basolateral infoldings. Similar to TALH Higher N/C ratio than proximal tubular epithelial cells Connecting segment is between the distal convoluted tubule and the cortical collecting duct.

Distal convoluted tubules (arrows)

Distal convoluted tubule

Collecting tubule

COLLECTING DUCT Principal cells (2/3 of cells) –Light cells with few organelles –ADH sensitive and role in K+ secretion Intercalated cells (1/3 of cells) –Dark cells –Acid base regulation –Type A: H + secretion –Type B: HCO3 - secretion

Type A

Type B

Medullary collecting ducts COLLECTING DUCT

RENAL INTERSTITIUM Extracellular space Peritubular capillaries RELATIVE INTERSTITIAL VOLUME 7% in cortex 10-15% medulla 30-40% deep medulla - papilla

Cortex with little interstitium

Outer renal medulla with interstitial space between collecting ducts

Deep renal medulla with more interstitial space between collecting ducts and loops of Henle

FUNCTIONS OF RENAL INTERSTITIUM Tubular oxygen supply Osmoregulation (together with distal tubules) Immune function (interstitial dendritic cells) Some endocrine function (interstitial cells)

Renal vasculature Renal artery Segmental arteries Interlobar arteries Arcuate arteries Interlobular arteries Afferent arterioles Glomeruli Efferent arterioles Peritubular capillaries Venules-veins

Nephron and the surrounding vascular framework

Cross-section of the intra-renal arteries

Arteriole

Renal oxygen supply is provided by the postglomerular capillaries (peritubular capillaries in the cortex, vasa recta in the medulla)

Quiz for revision 1.In which segment of the tubules, does the majority of reabsorption of solutes take place? 2.Aldosterone influences potassium excretion in which segment of the tubules? 3.Where is the renal papilla situated? 4.Where are the slit diaphragms situated?

Summary Kidney Glomerulus Tubules Vasculature Interstitium Pair of bean-shaped organs, retroperitoneal. Capsule, cortex, medullary pyramids, calyces, pelvis, ureter. Functional unit - Nephron Glomerular basement membrane structure. Podocytes foot processes, slit diaphragm. Fenestrated endothelium. Glomerular filtration barrier. Proximal, distal, loop of Henle, collecting ducts. Juxta-glomerular apparatus – Autoregulation Renal arteries directly branch from aorta. Elaborate branching pattern in the kidney Surrounds the nephrons and vasculature Less in cortex, more in the medulla. Important in maintenance of osmotic gradient around the loops of Henle. Oxygen supply of tubules – peritubular capillaries

Thank you, Any questions? Contact me at: Department of Pathology Division of Renal and Transplant pathology

References Basic Histology Lange medical text book Pathologic Basis of Disease Robbins and Cottran

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