Tubules of the Renal Cortex
In the renal cortex (excluding medullary rays), the convoluted parts of the renal tubules are located. They are represented by two main types that can be easily distinguished on a histological specimen.
Proximal Convoluted Tubules These are the largest tubules in the cortex, reaching a diameter of about 60 µm. The wall is formed by simple cuboidal brush border epithelium. The main function of this segment is the active reabsorption of most substances from the filtrate.
- Lumen: Narrow, irregular stellate or slit-like shape.
- Cytoplasm: Acidophilic, opaque. Under light microscopy, it looks "foamed" due to a massive number of pinocytotic vesicles required for macromolecule transport.
- Apical pole: Covered by a prominent brush border consisting of microvilli, which vastly increases the surface area for absorption.
- Basal pole: Features basal striation. Electron microscopy shows that it is formed by deep invaginations of the plasmalemma, densely packed with mitochondria in between. Mitochondria generate ATP for active transport against concentration gradients.
Distal Convoluted Tubules These are smaller than the proximal ones (diameter 30–50 µm) and perform a lighter functional load, primarily responsible for electrolyte reabsorption.
- The epithelium here is low columnar, with cells shorter than those in the proximal segment.
- The lumen is wider, smoother, and more open.
- The cytoplasm is pale and transparent because there are fewer organelles here.
- The most critical distinguishing feature is the absence of a brush border on the apical surface. However, basal striation is preserved in these cells.
Tubules of the Renal Medulla
Cross-sections of the renal medulla reveal components of the loop of Henle (thin segments and distal straight tubules) as well as collecting ducts.
Distal Straight Tubules are functionally and morphologically equivalent to the distal convoluted tubules. They are lined by low columnar or cuboidal epithelium and possess a smooth, wide lumen.
Thin Segments (descending and ascending limbs of the loop of Henle) have a very thin wall formed by simple squamous epithelium. Their diameter is only about 15 µm.
- On a specimen, the nucleus-containing parts of the squamous cells visibly bulge into the lumen.
- The function of this segment is restricted to the passive reabsorption of water.
- Cells of the thin segments lack a brush border, have no basal plasmalemma folding, and contain few mitochondria.
- Thin segments create a "honeycomb structure" in the medulla and visually closely resemble blood capillaries.
Collecting Ducts
Collecting ducts are the largest structures of the tubular system, with diameters reaching 80–100 µm. The height of their epithelium increases as fluid moves distally: in the cortex and upper medulla, it is simple cuboidal, whereas in the lower medullary regions, it becomes simple columnar (indicating increasing functional activity).
Clicks and lining of the collecting ducts are heterogeneous, comprising two cell types:
- Principal (Light) Cells. They are numerically dominant and give the duct its pale appearance on specimens. They contain specialized water channels called aquaporins. Their main task is the passive reabsorption of water, which is strictly regulated by antidiuretic hormone (ADH). The apical surface of principal cells bears 1–2 cilia; these cells can also synthesize prostaglandins.
- Intercalated (Dark) Cells. These cells are responsible for the secretion of hydrogen ions (H⁺) and ammonia into the urine, participating in acid-base balance (pH) regulation. Ultrastructurally, they closely resemble the parietal cells of gastric fundic glands.