# The Kidney Under the Microscope

Histology is the branch of biology that studies the microscopic anatomy of biological tissues

## Studying kidney histology - Kidney tissue can be examined at the microscopic level by preparing thin sections and staining them - Stained sections of kidney tissue are viewed under a microscope to reveal the internal structures - These preparations allow for detailed examination and drawing of nephron components - Different stains highlight different cellular structures, making them easier to identify ## Viewing nephrons in tissue sections - Complete nephrons cannot be seen in any single section of kidney tissue - Nephrons are irregular, three-dimensional structures - They do not lie in a single plane within the kidney - Think of trying to see an entire rollercoaster track in one photograph - you'd only capture parts of it from any angle - Tissue sections therefore show parts of many adjacent nephrons - Multiple sections at different angles would be needed to trace a complete nephron pathway

When examining kidney tissue slides, you're seeing cross-sections and longitudinal sections of different nephron parts, not whole nephrons

## Kidney cortex under the microscope The cortex region contains several identifiable structures: - **Glomeruli** - appear as dense clusters of capillaries - Look like tangled balls of blood vessels - Usually stain darker due to the high density of cells - **Bowman's capsule** - surrounds each glomerulus - Appears as a clear space around the glomerulus - The capsule wall is visible as a thin layer of cells - **Proximal convoluted tubule (PCT)** - highly abundant in the cortex - Cells have a distinctive brush border of microvilli - The lumen (central space) appears relatively small - Cells typically stain pink/red with standard stains - Epithelial cells appear cuboidal (roughly cube-shaped) - **Distal convoluted tubule (DCT)** - also found in cortex - Lacks the obvious brush border seen in PCT - Lumen appears larger and clearer than in PCT - Cells are also cuboidal but appear "cleaner" in outline - **Capillaries** - small blood vessels threaded throughout - Appear as tiny circular spaces, often containing red blood cells ## Distinguishing features in cortex sections | Structure | Key identifying features | |-----------|-------------------------| | Glomerulus | Dense network of capillaries; darkly stained; circular arrangement | | Bowman's capsule lumen | Clear space surrounding glomerulus; relatively large | | Proximal convoluted tubule | Brush border visible; smaller lumen; densely packed cells | | Distal convoluted tubule | No brush border; larger, clearer lumen; "cleaner" cell boundaries | | Capillaries | Very small diameter; may contain red blood cells | ## Kidney medulla under the microscope The medulla region has a different appearance from the cortex: - **Loop of Henle** - predominant structure in medulla - Thick ascending limb has cuboidal epithelial cells - Thin descending limb has very flat (squamous) epithelial cells - Thick portions have smaller lumens relative to wall thickness - Thin portions have larger lumens relative to wall thickness - **Collecting ducts** - run through the medulla toward the pelvis - Larger diameter than tubules in the cortex - Lined with pale-staining cuboidal cells - Cells have distinct cell boundaries (like tiles on a wall) - Multiple collecting ducts may be visible running parallel - **Capillaries** - network of blood vessels (vasa recta) - Run alongside loops of Henle and collecting ducts - Create the countercurrent exchange system

The medulla typically looks more organized and "striped" than the cortex due to parallel arrangements of loops of Henle and collecting ducts

## Distinguishing loop of Henle sections | Portion of loop | Wall thickness | Cell type | Lumen appearance | |----------------|----------------|-----------|------------------| | Thick ascending limb | Relatively thick | Cuboidal epithelium | Smaller lumen | | Thin descending limb | Very thin | Squamous epithelium | Larger lumen | | Thick descending limb | Relatively thick | Cuboidal epithelium | Smaller lumen | ## Practical observations - Photomicrographs at low magnification (×30) show: - Overall tissue organization - Distribution of glomeruli and tubules - Distinction between cortex and medulla regions - Photomicrographs at medium magnification (×200) show: - Individual structural components - Glomeruli within Bowman's capsules - Different types of tubules - Central veins in lobules (in liver sections) - Photomicrographs at high magnification (×600) show: - Cellular detail within structures - Brush borders on PCT cells - Individual cells in tubule walls - Capillary endothelium

Tissue preparation can cause artifacts - shrinkage, distortion, or gaps between structures that weren't present in living tissue

## Interpreting kidney histology When examining stained kidney sections: 1. **Identify the region** - cortex or medulla? - Cortex: contains glomeruli; appears more "busy" and disorganized - Medulla: no glomeruli; more parallel, organized appearance 2. **Look for landmark structures** - In cortex: find glomeruli first, then identify surrounding structures - In medulla: look for larger collecting ducts, then smaller loops of Henle 3. **Use cellular features** for identification - Brush borders indicate proximal convoluted tubules - Very thin, flat cells indicate thin limb of loop of Henle - Large clear lumen with regular cuboidal cells indicates collecting duct 4. **Consider the plane of section** - Circular profiles show cross-sections (tubule cut perpendicular to length) - Elongated profiles show longitudinal sections (tubule cut along length) - Oblique angles create oval profiles ## Drawing from kidney sections When producing drawings from microscope observations: - Use clear, continuous lines (not sketchy or hairy lines) - Draw only what you observe - don't add structures from memory - Label structures accurately using straight label lines - Include a scale bar or state the magnification - Add a title indicating the tissue type and stain used - Keep drawings simple but scientifically accurate - Show relative sizes of structures correctly

Practice identifying structures in multiple different sections - the same structure can look quite different depending on the angle of the cut

## Common structures to identify and label **In cortex sections:** - Glomerulus - Lumen of Bowman's capsule - Bowman's capsule epithelium (wall) - Proximal convoluted tubule (with brush border) - Distal convoluted tubule (without brush border) - Capillaries **In medulla sections:** - Collecting duct - Loop of Henle (thick portions) - Loop of Henle (thin portions) - Capillaries (vasa recta) ## Why study kidney histology? Understanding microscopic kidney structure helps explain function: - The extensive capillary network in glomeruli enables high filtration rates - Brush borders in PCT massively increase surface area for reabsorption - The thin walls of the loop of Henle's thin limb allow efficient water movement - The arrangement of structures in the medulla creates the concentration gradient needed for water reabsorption - The parallel flow of blood and filtrate enables countercurrent multiplication Histological examination can also reveal pathology: - Damaged glomeruli appear disrupted or collapsed - Protein deposits may be visible in tubules - Inflammatory cells indicate infection - Tissue scarring shows chronic damage

Always be prepared to identify kidney structures in unfamiliar orientations or at different magnifications in exam questions