Proximal and Distal Convoluted Tubules Histology > Urogenital System > Urogenital System

Home , Distal convoluted tubule, Nephron, Proximal tubule

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

1 / 2 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.

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