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The kidney contains 1 - 4 million nephrons.
The nephron is the functional unit of filtration.
Each nephron has 4 main parts:
1. Renal Corpuscle, which can be further divided into a glomerulus and a Bowman's capsule
2. Proximal Tubule
3. Loop of Henle, with thin and thick limbs
4. Distal Tubule
From the distal convoluted tubule, the filtrate flows into a collecting duct.
A nephron and the collecting duct which drains it are collectively called a uriniferous tubule.
The ascending loop of Henle continues as the distal tubule, which is followed by the distal convoluted tubule.
The distal tubule cells are simple cuboidal cells.
They are generally much less eosinophilic than proximal tubule cells.
Also, the lumen is not star shaped.
The cells are smaller and flatter than proximal tubule cells so distal tubule cross sections appear to have larger lumens and more nuclei.
The nuclei of distal tubule cells are more apical than those of the proximal tubule, and distal tubule nuclei often appear to bulge into the lumen.
Because these cells contain no glycocalyx, no debris is visualized in the lumen. This micrograph illustrates some of these contrasting features of the distal and proximal tubules-- note that the distal tubule cells are flatter, the lumen is not star-shaped, there is no glycocalyx and no debris in the lumen.
The proximal tubule consists of three histologically distinct parts, the S1, S2 and S3 segments.
These only roughly correspond with the appellations of proximal convoluted and proximal straight tubule (pars convoluta and pars recta).
These appelations will not be discussed in depth here; it is sufficient to understand that the cells of the proximal tubule become shorter and less eosinophilic, and the tubule straightens out as it approaches the loop of Henle (the pars recta = the descending thick limb of the loop of Henle).
Proximal tubule cells are simple cuboidal.
They are also acidophilic due to a high content of long mitochondria concentrated in the base of the cell.
They have a microvillous brush border that extends into the lumen of the nephron.
The brush border is coated with a glycocalyx. In this PAS stained micrograph of the proximal convoluted tubule, the glycocalyx stains a reddish-pink.
Localized along the basolateral membranes is the Na+/K+ ATPase, which pumps sodium out of the cells.
On electron microscopy, or using special stains with light microscopy, apical vesicles and lysosomes can also be seen.
This specially prepared LM shows lysosomal vesicles.
Because the proximal tubule is the only section of the nephron involved in protein uptake, these vesicles can only be seen here.
A good way to identify the proximal tubule by normal light microscopy is to determine the shape of the lumen.
The lumen is star shaped and appears to contain debris (the glycocalyx) in most H&E sections.
There are generally 5-7 cells per cross section.
Each kidney contains 1-4 million nephrons.
A nephron is the functional unit of filtration.
Each nephron has 4 main parts:
Renal Corpuscle, which can be further divided into a glomerulus and a Bowman's capsule
Renal plasma filtration begins in the glomerulus, which is situated within the renal corpuscle.
The glomerulus is a ball of intermeshed network of highly fenestrated capillaries that protrude from the junction of the afferent and efferent arterioles. (This junction, where the afferent arteriole divides into several glomerular capillaries, is the vascular pole of the renal corpuscle, as seen in this micrograph.)
The 50-100 nanometer fenestrae make up the first, least discriminating part of the filter of the renal corpuscle.
The plasma filtrate leaves the glomerulus through those fenestrae.
It should be noted that the glomerulus is made up of endothelial cells and mesangial cells.
The rest of the renal corpuscle is made up of Bowman's capsule.
Bowman's capsule consists of epithelial cells that envelop the glomerular capillaries and form the outer limit of the renal corpuscle.
Those cells which surround the glomerulus make up the visceral layer and those that delimit the outer edge of Bowman's space comprise the parietal layer. This capsule surrounds the capsular space, the area into which the plasma is filtered.
The point at which the parietal layer of Bowman's capsule becomes the proximal tubule is called the urinary pole of Bowman's capsule.
This specially-stained micrograph shows the urinary pole of the capsule opening into the proximal tubule.
The visceral layer epithelial cells are called podocytes (Gr. pod=foot).
The podocytes extend a network of primary and secondary processes which end as pedicels out and around the glomerulus.
Between the pedicels, there exist slits which are about 20 nanometers wide.
These slits are covered by a slit diaphragm, which comprises a part of the renal corpuscle's filter.
The slit diaphragm is made mostly of an anionic sialoprotein (podocalyxin).
The filtrate enters Bowman's space through these slits and their diaphragm.
At higher magnification, one can appreciate how the pedicels interdigitate to create the slit pores.
2. Proximal Tubule
3. Loop of Henle, with thin and thick limbs
4. Distal Tubule
From the distal convoluted tubule, the filtrate flows into a collecting duct.
A nephron and the collecting duct which drains it are collectively called a uriniferous tubule.
