- The nephron is a long, winding tubule that extends from the Bowman's Capsule and ends in the renal papillae. The nephron possesses a simple epithelium lined by a single layer of cells which can generically be referred to as tubular epithelial cells. As one traverses along the nephron, the morphology and functionality of the tubular epithelial cells changes, thus defining functional segments which are involved in resorption and secretion of different sets of molecules. Here we only provide a brief histological description of the segments of the nephron.
- The proximal tubule is a convoluted section of the nephron extending from the Bowman's Capsule to the descending arm of the thin loop of Henle. It is composed of a simple cuboidal epithelium with cells possessing many mitochondria, which produce ATP for the energy-intensive resorptive processes that occur here. Subcellular microvilli extend from the tubular epithelial cells here and are observed microscopically as a brush border. This substantially increases the proximal tubule's surface area, thus facilitating resorption.
- Henle's Loop connects the proximal tubule to the distal tubule, first diving toward the renal medulla only to immediately loop toward the cortex, thus being divided into descending and then ascending limbs. Most nephrons, known as "Cortical Nephrons", possess only short loops of Henle which remain within the renal cortex, whereas a minority of nephrons, known juxtamedullary nephrons extend far into the renal medulla.
- Thin Loop of Henle
- Both the descending and ascending arms of the thin loop of Henle are histologically similar and are composed of a simple squamous epithelium. These cells are not very metabolically active, consistent with the fact that they do not participate in any active transport of water or solutes.
- Thick Ascending Loop of Henle
- The thick ascending loop of Henle is lined by a thicker simple cuboidal epithelium and is largely involved in active transport of sodium, chloride, and potassium.
- The distal tubule is lined by a simple cuboidal epithelium very similar to that of the proximal tubule except without a brush border. The distal tubule is functionally separated into the early distal tubule and a section with similar functionality as the collecting ducts termed the late distal tubule and collecting ducts.
- The juxtaglomerular apparatus is specialized patch of tissue that occurs where the cells of the distal tubule come in contact with the same nephron's glomerulus. The specialized cells of the distal tubule which contact the glomerulus are known as the "Macula Densa" and lie in the angle between the renal afferent and efferent arterioles. Specialized smooth muscle cells in the renal afferent and efferent arterioles which contact the macula densa are known as juxtaglomerular cells. The specialized architecture of the juxtaglomerular apparatus allows information about the fluid flow within the nephron to be sent back to the glomerulus which can in turn modify the amount of fluid flow, as described in tubuloglomerular feedback.
|Collecting Tubule and Duct|
- The end of the distal tubule continues into the collecting tubule that in turn merges with several other collecting tubules, thus forming a collecting duct. The late distal tubule, collecting tubule, and collecting ducts form a unified functional segment referred to as the " Distal Tubule and Collecting Duct". While the collecting tubule is lined by simple cuboidal epithelium, the tubular epithelial cells grow progressively taller throughout the segment until those in the collecting duct form a simple columnar epithelium. Importantly, the collecting tubule and collecting ducts have two types of functionally distinct tubular epithelial cells (Intercalated Cells and Principal Cells) which look highly similar but can be distinguished based on some nuanced features.
- The renal papillae are the extreme bottom portion of the renal pyramids. Tapering of the renal papillae is achieved through the progressive unification of collecting ducts which expand in diameter. The last remaining and largest collecting ducts at the tips of the papillae are known as the "Ducts of Bellini". Any tubular fluid which remains in the Duct of Bellini drips into the minor calyces and is excreted as urine.