KIDNEY – slide 150 (rat)

Look first with low power for an overall view of the structural arrangement of the kidney. Find: cortex, medulla, papilla, calyx, renal connective tissue, capsule, renal corpuscles and medullary rays.  What are the boundaries of a renal lobule?

Next look with low power for the parts of the nephron and be able to distinguish one portion from another.  Essentially the nephron is a long, thin, epithelial tubule with convoluted and straight portions which double back on themselves and ultimately empty into collecting tubules which empty into the renal pelvis.  The epithelial lining of each part of the tubule is highly specialized to perform its particular task in the formation of urine and reabsorption of excess fluid; much of the chemical balance is achieved by the cells of the nephron.  Malfunction of the kidneys can quickly lead to illness, and death.  Read about the work done by the different parts of the nephron and by the capillary glomerulus.

Clues to the identification of parts of the tubule follow.

Bowman’s Capsule:  The thin-walled dilation of the nephron which forms a cup-like shape, within which the glomerular capillary tuft lies.  One sees a slit-like space between the outer (parietal) layer and the inner (visceral) layer which is reflected over the capillaries.  All of this lining is simple squamous epithelium, with the visceral layer further modified into podocytes.  The initial glomerular filtrate goes into this narrow space and from thence proceeds into the rest of the nephron at the urinary pole of the corpuscle.

Proximal Convoluted Tubule: found near the glomeruli:  The epithelial cells are usually pyramidal in shape, are fairly densely stained (pink), and have a definite brush border on their undulating luminal surface.  A brush, or striated, border is indicative of reabsorptive activity, and these cells are known to resorb most of the water and sodium chloride, amino acids and glucose in the glomerular filtrate.  The final segment of the proximal tubule straightens out and becomes the descending thick portion of the loop of Henle; the epithelium continues to have a striated (brush) border in this thick segment of the loop.

Loop of Henle:  found in the medullary ray of the cortex and in the medulla.  It has a descending and an ascending limb which are variously proportioned between thick and thin segments of the loop.  Epithelium is simple cuboidal in the thick segment and simple squamous in the thin portion.  What happens to the urine in the thin portion of the loop of Henle?

Distal Convoluted Tubule:  found near the glomerulus, as the loop of Henle returns to its point of origin.  This portion of tubule lies near the proximal portion and has a similarly darker (pinker) epithelium, but its cells are lower, more cuboidal, and have no brush border.  Water passes easily through them from the hypotonic urine into the bloodstream.  What is the macula densa?  Where is it?  What is its function?

Collecting Tubule:  although not strictly part of the nephron, it conducts urine from the distal convoluted tubule to the papillary ducts.  Look for small ducts in the medullary rays, and follow them as they become larger in the papillae.  The larger the duct, the higher its epithelium (from simple cuboidal to simple columnar near the papilla), but always this epithelium is characterized by its unusually well defined cell boundaries and dark spherical nuclei.  As urine passes along the ducts, water continues to leave it passively.  What hormone is responsible for this?  Why is the surface of the papilla called the area cribrosa?

Now consider the capillary glomerulus and its associated blood vessels.  The capillary loops are so densely packed that identification for individual cells and layers is difficult.

From your reading, however, and from electron micrographs, begin to understand what is meant by: podocytes, pedicels, filtration slits, slit membrane, basal lamina, fenestrated endothelium, mesangial (intercapillary cells), juxtaglomerular cells.  What is the source of the vasopressor substance renin? What is the afferent arteriole? The efferent arteriole? In the “filtration barrier” what structures lie between the blood in the glomerular capillaries and the filtrate in the capsular space of Bowman’s capsule?  What would be the result of abnormal thickening of any components of this barrier?

The kidney is highly vascularized.  Look for blood vessels, both large and small, in your section.  Read about the vascular pattern of the kidney.  What are the vasa recta?  The peritubular capillaries?  From what vessels do they arise?  Where do they lead?  How do they relate to the formation and concentration of urine in the nephrons?