Nephron Loop & Collecting Duct

Key Functions and Features

The nephron loop's function is to produce and maintain a high osmotic gradient in the medullary extracellular fluid
The vasa recta, which comprises a looping capillary network, travels in parallel with the nephron loop to effectively participate in the counter-current exchange
The collecting duct further concentrates urine and regulates its acidity to maintain systemic acid-base homeostasis

Anatomical Context

Kidney:

Renal capsule covers the cortex
Medulla comprises the renal pyramids
Cortico-medullary junction is where the cortex and medulla meet

Nephron:

Renal corpuscle gives rise to the proximal convoluted tubule
PCT turns towards the medulla as the thick descending limb (aka, pars recta of the proximal tubule), then the thin descending limb
At bottom of the loop, the thin descending limb becomes the thin ascending limb, then abruptly becomes the thick ascending limb (the thick ascending limb is sometimes called the pars recta of the distal tubule).
The thick limb transitions to the distal convoluted tubule, which drains into a collecting duct via a collecting tubule.

Histological structures

Thin segments of the nephron loop

A thin layer of interdigitating squamous epithelial cells
Intercellular junctions link cells laterally
Nuclei are oval to round in shape, and lie near the lumen of the tubule.
Sparsely populated with organelles.

Thick ascending limb, aka, the diluting segment,

Has features that reflect active transport via the sodium-potassium pump.
Low cuboidal cells with interdigitating basolateral processes; these processes create an extensive intercellular matrix.
Nuclei and the abundant mitochondria support the energetic needs of the sodium-potassium pump.
Thick ascending limb has few, if any, short stubby microvilli; as in the thin limbs, there is no brush border.

Collecting duct

Comprises bulging columnar cells that enclose a large diameter.
Abundant organelles and basal infoldings
Distinct lateral borders are connected via intercellular junctions
Two types of collecting duct cells: principal and intercalated

— Intercalated cells are darker-staining because they have more organelles and basal infoldings than do principal cells — Intercalated cells may also have numerous stubby microvilli (but not enough to constitute a brush border).

Tips to identify these structures in a histological sample

First, identify a portion of thick limb by its low columnar/cuboidal cells and roundish nuclei.
Next, identify portions of the thin limbs by their characteristic squamous epithelium.
Compare this to a portion of the vasa recta, which is also thin-walled; the two can be distinguished by the presence of red blood cells in the vasa recta.
Lastly, identify the tall bulging cells of a collecting duct; notice the wide diameter and dark-staining organelles.

Images:

Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)

Nephron Loop & Collecting Duct

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