Shared characteristics of the proximal and distal convoluted tubules

• Both are key sites of reabsorption and secretion, which is necessary for fine-tuning the ultrafiltrate to form urine
• Abundant mitochondria support high levels of cellular activity
• Both have plasma membrane infoldings that increase the surface area for optimal diffusion
• Though at opposite ends of the nephron, both reside within the renal cortex, near their renal corpuscles, due to the winding nature of nephrons

Anatomical Review

Kidney:

• Renal capsule covers the cortex
• Medulla comprises the renal pyramids.
• The cortico-medullary junction is where the cortex and medulla meet.
  Nephron:
• Arises from the renal corpuscle in the cortex as the proximal tubule
• Descends and ascends through the medulla as the nephron loop, becomes the distal tubule, then drains into a collecting duct.
  — As we learn elsewhere, collecting ducts drain urine through the renal pyramids to the renal calyxes, from which it exits the kidney.

Histological Features

Proximal convoluted tubule, aka, PCT.

• Bulging cuboidal/low columnar cells
• Basal membrane has infoldings with their own mitochondria.
• Microvilli that make up the brush border that fills the lumen; give the lumen a characteristic “fuzzy” appearance.
  — The basal membrane infoldings and brush border increase the surface area for diffusion; approximately 65% of reabsorption and secretion occurs within the PCT.
• Lateral processes are cytoplasmic extensions that form lateral intercellular space; held together by intercellular junctions.
• Large roundish euchromatic nucleus
  — It has several light-staining areas of euchromatin that reflect genome activity; know that the the dark-staining areas are heterochromatin, which comprises transcriptionally inactive portions of the genome.
• Abundant mitochondria, which produce visible basal striations; mitochondria support the energetic requirements of the sodium-potassium pump, which plays a key role in resorption of water and nutrients from the PCT.
• Abundance of dark-staining organelles, including the vesicles and mitochondria, give PCT cells a darker hue.

Distal convoluted tubule

• Cuboidal and uniform cells
• Lateral processes and intercellular junctions
• Basal membrane infoldings
• Luminal surface does not have a brush border, so the lumen appears wider and clearer than the PCT.
• Euchromatic nuclei that they tend to lie close to the lumen, even bulging into it.
• Numerous mitochondria and vesicles to support their high cellular activity, though not as much as the PCT; hence, these cells appear lighter in histological samples.
• Macula densa is a tightly packed region of the DCT that lies near the renal corpuscle and afferent arteriole of the nephron.

Identification tips:

• First, because we know that both the PCT and the DCT can be found nearby, identify the renal corpuscle.
• Then, identify a proximal convoluted tubule by its fuzzy lumen, which is created by the microvilli brush border. For clarity, we’ve outlined a portion of the brush border in yellow.
• Close by, identify a distal convoluted tubule by its wider, clearer lumen; we’ve used green lines to indicate the macula densa, which appears as a neat row of closely packed cuboidal cells near the mesangium of the renal corpuscle.