Renal Corpuscle and Nephron Borders
Histologically, the nephron in the strict sense includes a double-walled epithelial cup—the Bowman's capsule (capsula glomeruli)—and a long, unbranched tubule extending from it.
A vascular element—the capillary glomerulus—is invaginated into the capsule. Together they form the renal corpuscle (corpusculum renale). It is important to understand a histological nuance: formally, the capillary glomerulus belongs to the circulatory system, but functionally they work as a single apparatus.
This is where primary filtration takes place: about 10% of the blood plasma passes from the capillaries into the capsular lumen, forming the primary filtrate. In addition, the renal corpuscle performs an endocrine function, participating in the production of renin, which is necessary for blood pressure control.
Structural Organization of the Tubules
The tubular part extends from the capsule. Its task is reabsorption (the reuptake of necessary substances from the primary filtrate into the secondary capillary network) and secretion (active transport of metabolites from the blood into the lumen).
Along its length (about 5–5.5 cm), the tubule is divided into five consecutive segments:
- Proximal convoluted tubule—forms loops near its own renal corpuscle within the cortical substance. This segment alone accounts for about 2 cm of the length.
- Proximal straight tubule—a short segment following the convoluted one.
- Thin tubule—the descending limb of the loop of Henle, descending from the cortex into the medulla.
- Distal straight tubule—the ascending limb of the loop of Henle, returning to the cortex.
- Distal convoluted tubule—the final segment. It has a unique topographical feature: it invariably returns and contacts the renal corpuscle of its own nephron in the region of the vascular pole (strictly between the afferent and efferent arterioles).
Histophysiology of the Epithelium and Excretory System
All segments of the nephron are lined with a simple epithelium. The tubular cells are tightly joined at the apical pole, isolating the lumen. At the basal pole, the junctions diverge, forming the lateral intercellular space.
During reabsorption, substances first enter this lateral space, cross the basement membrane, and exit into the peritubular (interstitial) space, from where they are taken up by capillaries. The interstitium itself (connective tissue between the tubules) in the medulla has an endocrine function—it synthesizes prostaglandins.
Passing through the nephron, the secondary urine enters the collecting ducts. They run perpendicularly to the kidney surface: starting in the medullary rays, they descend into the medulla and merge into papillary ducts at the apices of the pyramids. The length of the ducts is about 4.5 cm, and it is important to remember that they are an independent element of the parenchyma and are not part of the nephron.
Classification of Nephrons
All renal corpuscles lie exclusively in the cortical substance, giving it a dark red color. The presence of corpuscles in a histological section is a 100% diagnostic sign of the cortex. The type of nephron is determined by the localization of the corpuscle and the length of the loop of Henle (ansa nephroni):
- Intermediate cortical (80%)—the main type. The loop of Henle is of medium size, reaching the outer zone of the medullary pyramids.
- Long / Juxtamedullary (about 20%)—corpuscles lie at the cortico-medullary junction (juxta meaning near). Their long loop penetrates the pyramids almost to their full thickness.
- Short cortical (from 1% to 15-20%)—corpuscles in the outer cortex. They have a very short loop that practically does not descend deeply, remaining within the medullary rays.
Briefly: answers to topical questions
Which cells form the visceral layer of Bowman's capsule?
The inner (visceral) layer of Bowman's capsule is formed by large epithelial cells called podocytes.
These cells tightly wrap around each capillary of the vascular glomerulus, causing them to merge with it under light microscopy. Slit membranes are located between the podocyte foot processes, acting as one of the layers of ultrafiltration and preventing proteins from passing into the primary urine.
Which structures form the juxtaglomerular apparatus (JGA)?
The juxtaglomerular apparatus is formed by three elements located in the region where the distal tubule contacts the renal corpuscle:
- Macula densa (macula densa)—a specialized region of the distal convoluted tubule wall.
- Juxtaglomerular cells—located in the tunica media of the arterioles, predominantly the afferent one.
- Extraglomerular mesangial cells—Lacis cells, located in the space between the two arterioles and the macula densa.
What cell types make up the epithelium of the collecting ducts?
The epithelium of the renal collecting ducts is represented by two cell types:
- Light (principal) cells—numerically predominant, containing aquaporins; their function is passive water reabsorption, intensively regulated by antidiuretic hormone (ADH).
- Dark (intercalated) cells—perform the secretion of H⁺ and ammonia ions into the urine, participating in pH regulation.
To easily remember the five segments of the tubule in order, use the phrase: "Primary Pedestrian Travels Down Down" (Proximal convoluted, Proximal straight, Thin, Distal straight, Distal convoluted).
Class and exam questions
How to quickly distinguish the renal cortex from the medulla under a microscope?
The main marker is the presence of renal corpuscles (capsules with glomeruli). They are located exclusively in the cortex. If there are no corpuscles in the field of view, you are looking at the medulla.
Why is the capillary glomerulus formally not considered part of the nephron?
The nephron in the strict histological sense consists only of epithelial tissues (capsule and tubules). The capillary glomerulus is an element of the vascular system, although functionally they form a single filtration apparatus.
Are collecting ducts part of the nephron?
No, they are not included. Collecting ducts are an independent epithelial element of the excretory system, into which the nephron merely empties.
What else to study on this topic
- Mechanisms of formation and composition of the lateral intercellular space in the tubular epithelium.
- Topography of the vascular pole of the renal corpuscle and its contact with the distal tubule.
- Endocrine function of the kidney: synthesis of renin and prostaglandins.
- Morphology of medullary rays and papillary ducts of the pyramids.
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