Kidney

#50 Kidney, (H&E)

Identify the outer, brighter staining cortex and the central, paler staining medulla. The cortex is characterized by round capillary tufts called glomeruli within the renal corpuscles. The base of the medullary pyramid lies below the cortex, and the apex of the pyramid projects or empties into the renal pelvis. The hilum of the kidney is the site of entrance and exit of the renal artery, vein and ureter. Note the abundance of white fat in this region.

Examine the junction between cortex and medulla. This junction is irregular. The cortex is subdivided into alternating regions: 1) the cortical labyrinth consists of glomeruli and convoluted tubules and 2) the medullary rays consist primarily of radially directed straight segments of the loop of Henle and collecting tubules. A kidney lobule consists of a medullary ray and the portions of the adjacent cortical labyrinth. The medulla is further sub-divided into an outer zone adjacent to the cortex and an inner zone including the tip of the pyramid (which is called the papilla).

With medium power identify the different regions of the nephron, the structural and functional unit of the kidney. A nephron is composed of: 1) renal corpuscle, consisting of the vascular glomerulus and its capsule (Bowman's capsule); 2) proximal convoluted tubule; 3) loop of Henle, consisting of a thick descending segment, a thin U-shaped segment, and a thick ascending segment; and 4) distal convoluted tubule. The excretory portion of the kidney begins with the collecting tubules (which are in continuity with the distal convoluted tubules).

1. Within the renal corpuscle identify the vascular capillary tuft called the glomerulus. Search for a renal corpuscle in which you can identify the vascular pole. This is the site of the entrance and exit of the afferent and efferent arterioles, respectively. Identify the visceral and parietal layers of Bowman's capsule, and try to locate a renal corpuscle in which the parietal layer of Bowman's capsule is in continuity with a proximal convoluted tubule. This junction is the urinary pole of the renal corpuscle, and it lies opposite the site of the vascular pole.
2. Proximal convoluted tubules surround the glomeruli, and are the most abundant tubules of the cortical labyrinth. The cuboidal epithelium of the proximal tubules is strongly acidophilic, in contrast to the lightly stained distal tubules.
3. Search for a glomerulus in which the last portion of the thick ascending limb distal tubule is closely apposed to the vascular pole of the renal corpuscle. This area has cells that are more columnar and have a higher concentration of nuclei, and is called the macula densa. The macula densa, together with the modified muscle cells of the afferent arteriole called the juxta-glomerular apparatus. The juxtaglomerular cells are secretory and release an enzyme called renin into the blood stream.
4. Distal convoluted tubules. Leading from the macula densa, the nephron becomes the distal convoluted tubule. The cuboidal epithelium of the tubules is palely stained with H&E. The distal tubules are much less numerous than the proximal tubules in the cortical labyrinth.

Next, examine the medullary rays adjacent to the labyrinth and the medulla itself.

Identify the collecting tubules in the ray and in the medulla. These latter tubules are pale-staining like the distal tubules, but differ from them in that their epithelium is more columnar, the apex of the epithelial cells tend to bulge into the tubule lumen, and the intercellular boundaries are readily evident as the cells do not form interdigitations.

The following are difficult to distinguish from adjacent surrounding tubules.

1. The radially running thick descending segment of the loop of Henle (cytologically similar in appearance to the proximal convoluted tubules with which they are continuous in the ray)
2. The thin segment of the loop of Henle (in most cases the simple squamous epithelium of these tubules cannot be distinguished from that of a capillary in the inner zone of the medulla)
3. The thick ascending segment of the loop of Henle (cytologically similar in appearance to the distal convoluted tubules with which they are continuous) returns to the glomerulus of origin of the nephron and forms the macula densa (see description above)

Try to visualize the spatial relationships of an entire nephron as you examine the cortical labyrinth and rays, and consider which components you would expect to find in each region.

The renal medulla consists primarily of collecting tubules and larger collecting ducts, thin segments of the loop of Henle, and the thick ascending and descending segments of the loop of Henle. The largest collecting ducts that open on the area cribrosa of the papilla are the papillary ducts (of Bellini). Note the epithelial type lining the renal pelvis. The calyx itself is lined by transitional epithelium.

#103 Kidney, Periodic acid Schiff (PAS) reaction and hematoxylin

Orient yourself as with the previous slide, and then examine the cortical labyrinth with medium power. Locate a region with several renal corpuscles. The staining differences of the general cytoplasm in the two types of convoluted tubules are not as distinct with this stain as with H&E, but the proximal convoluted tubules can be readily identified by the PAS-positive brush border stained red or magenta at its luminal surface. Glomeruli and proximal and distal convoluted tubules are all sharply demarcated at their outer surfaces by PAS, since this is also an excellent stain for demonstrating basement membranes. Examine the tubules and glomeruli under higher magnification and identify all the components of the cortical labyrinth. Note regions in which the glomeruli have been section through the urinary and/or vascular pole<. At the vascular pole, look for examples of the macula densa and the juxtaglomerular cells. Within the glomerulus, examine the parietal epithelium of Bowman's capsule and the visceral epithelium of the glomerulus. Be sure you understand the cells that form the visceral epithelium and the composition of the glomerular filter.

Examine electron micrographs of the glomeruli, proximal and distal convoluted tubules in your textbook and in the lab, and correlate the PAS-positive structures evident with the light microscope with their ultrastructural counterparts. What is the functional significance of the occurrence of a brush border in the proximal tubule? Be sure you understand the significance of PAS staining.

#49 Kidney, (H&E)

Examine the cortical labyrinth and rays as described previously. Be certain that you understand the blood supply of the renal corpuscle, the convoluted tubules, and the loop of Henle, and the functional significance of these. Consult your textbook and its illustrations. What are the components of the arterial portal system of the kidney?