Proximal and Distal Convoluted Tubules Histology > Urogenital System > Urogenital System 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) Powered by TCPDF (www.tcpdf.org) 2 / 2.