Renal tubules

Cortical tubules

The convoluted parts of the renal tubules are located in the renal cortex (except for the medullary rays). They are represented by two main types that are easily differentiated 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 ultrafiltrate.

• Lumen: narrow, irregular star-shaped or slit-like.
• Cytoplasm: acidophilic, opaque. Appears "foamy" under light microscopy due to the massive number of pinocytotic vesicles required for macromolecule transport.
• Apical pole: covered by a prominent brush border consisting of microvilli, which vastly increases the absorptive surface area.
• Basal pole: exhibits basal striation. Electron microscopy reveals that it is formed by deep folds of the plasmalemma with tightly packed mitochondria between them. Mitochondria generate ATP for active transport against the concentration gradient.

Distal convoluted tubules These are smaller than the proximal ones (diameter 30–50 µm) and carry a lower functional load, being primarily responsible for electrolyte reabsorption.

• The epithelium here is low columnar, with cells shorter than in the proximal segment.
• The lumen is wider, smoother, and clearer.
• The cytoplasm is light and transparent because there are fewer organelles here.
• A crucial distinguishing feature is the absence of a brush border on the apical surface. However, basal striation is preserved in the cells.

Medullary tubules

Cross-sections of the renal medulla visualize components of the loop of Henle (thin tubules 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 have a smooth, wide lumen.

Thin tubules (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.

• In the specimen, the nucleus-containing parts of the squamous cells visibly bulge into the lumen.
• The function of this segment is limited to passive water reabsorption.
• The cells of the thin tubules lack a brush border, have no basal plasmalemma folding, and contain few mitochondria.
• Thin tubules create a "honeycomb structure" in the medulla and visually closely resemble blood capillaries.

Renal collecting ducts

Collecting ducts are the largest structures of the tubular system, with a diameter reaching 80–100 µm. The height of their epithelium increases distally: it is simple cuboidal in the cortex and outer medulla, and becomes simple columnar in the inner medulla (indicating increased functional activity).

The epithelial lining of collecting ducts is heterogeneous and includes two types of cells:

1. Principal (light) cells. They are numerically predominant and determine the light appearance of the tubule in the specimen. They contain specialized water channels—aquaporins. Their main task is passive water reabsorption, which is tightly regulated by antidiuretic hormone (ADH). The apical surface of light cells bears 1–2 cilia; these cells can also synthesize prostaglandins.
2. Intercalated (dark) cells. Responsible for the secretion of hydrogen ions (H⁺) and ammonia into the urine, participating in the regulation of acid-base balance (pH). Ultrastructurally, they strongly resemble the parietal cells of gastric fundic glands.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Ultrastructure of the brush border and mechanism of pinocytosis
• Histophysiology of basal striation
• Mechanism of aquaporin action in principal cells
• Morphology of medullary interstitial cells

Other topics in the "Urinary System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.