# 138 - Kidney, human, 1.5 mm, h-e, sec.

# 139 - Kidney, rabbit, 1.5 mm, h-e, sec.

# 140 - Kidney, monkey, Mallory stain for connective tissue

Observe the outer zone (cortex) and the more regular appearance of the inner zone (medulla). Scan the entire slide at the lower magnification, noting the connective tissue capsule and the differences between cortex and medulla. At the corticomedullary junction, note the presence of arcuate arteries and veins. Note the medullary rays extending toward the capsule. Observe the renal corpuscles; are these structures present in the medullary regions?

Increase the magnification and find a renal corpuscle that shows the entrance of the arteriole at its vascular pole. You may be fortunate to find a section through the renal corpuscle that shows the vascular pole as well as the urinary pole, the portion that is continuous with the proximal convoluted tube. In the region of the vascular pole, the smooth muscle cells of the afferent arteriole are replaced by the juxtaglomerular cells which secrete renin. Renin acts on a substrate in the blood leading to the formation of angiotensin, a vasoconstrictor.

Observe the outer or parietal layer of Bowman's capsule. What type of cells are found here? The parietal layer is reflected over the capillaries of the glomerulus to form the visceral layer of Bowman's capsule (composed of podocytes). In your sections you cannot distinguish between capillary endothelium and visceral capsular epithelium. The electron microscope has enabled us to understand the intimate relationship between these two cell types. Using your text or an atlas of ultrastructure compare your observations with the light microscope with electron micrographs of sections through the renal corpuscle. What structures must be traversed by substances passing from capillary lumen to urinary space?

Now find a section through a renal corpuscle that shows a continuity of the parietal layer of Bowman's capsule with the proximal convoluted tubule. What do you observe about the epithelial cells in this transitional area? Note well the continuity of the subcapsular space (Bowman's space) with the lumen of the proximal convoluted tubule. Before leaving the structure of the renal corpuscle be certain that you understand its function.

Study the organization and function of the tubular portions of the nephron and collecting ducts. Identify the proximal convoluted tubule and observe its histological characteristics: eosinophilic, cuboidal or pyramidal shaped cells, rounded nuclei and the presence of a brush border on the luminal surface of the cells. The cells rest on a basal lamina of connective tissue. Since the cells are rather thick, there are relatively few nuclei per given cross section. Also observe the indistinct cell boundaries. Relate the presence of the brush border with an ultrastructural feature of the surface of this cell type. Do the same for the lack of distinct intercellular borders. Electron micrographs in your text or in an atlas of ultrastructure will help you to do this. Can you now correlate the structure of this tubule with its function?

Locate a section through a distal convoluted tubule (DCT). The lumen of DCT is larger than that of PCT. Compare the histological features of the DCT with that of the proximal convoluted tubule. Notice that the cells of the distal convoluted tubule do not possess a brush border on their luminal surface. Does this infer that these cells do not possess any microvilli?

The cells of the distal convoluted tubule are cuboidal in shape, but are thinner than those of the proximal tubule. As a result you will see more nuclei through a given cross section. Cell boundaries are indistinct in the DCT and their cells rest on a basement membrane. Basal striations (involutions of basal cell membrane and mitochondria in a palisade arrangement) are found in the cells of the distal tubules. Be certain that you understand the function of this tubule. Why are so many mitochondria present near the basal membrane?

As the distal tubule approaches its own glomerulus, it passes near the afferent and efferent arteriole at the vascular pole. Here the distal tubule is modified to form the macula densa ("dense spot"), an area where human DCT cells are taller, their nuclei closer together, and they have a darker appearance (in the rat kidney, the cells of the macula densa are flatter). They look like a "string of beads" next to the renal corpuscle and are believed to monitor sodium levels in the filtrate in the distal tubule. In the same area are juxtaglomerular cells, modified endothelial cells of the afferent arteriole which are responsible for producing renin. Specialized mesangial cells called lacis cells (extraglomerular mesangial cells) at the vascular pole produce erythropoietin which stimulates RBC production in the bone marrow.

The descending limb (or proximal straight tubule) of Henle's loop has histological characteritics similar to the proximal convoluted tubule while the ascending limb (or distal straight tubule) is histologically similar to the distal convoluted tubule. The loop of Henle can be found in the medullary area of the kidney. The walls are lined with squamous epithelium and are barely thicker than those of a capillary. However, the lumen is larger than the capillary lumen and does not contain blood. Search for longitudinal and cross sections through Henle's loop. What is the function of this region of the nephron?

In the medulla, find sections through the pale staining collecting tubules. The tubules are composed of cuboidal cells which show prominent intercellular borders. There are a number of rounded nuclei per cross section and an absence of a brush border. Into what structure does the collecting tubule empty?

In the medullary rays are cortical collecting ducts, longitudinally oriented tubules with a wide lumen. In contrast to the collecting ducts in the medulla, their epithelium is relatively flat and composed of two cell types: flat principal cells and plump intercalated cells.

References: Gartner, p. 322, 324 - 331 Ross, p. 559, 563 - 564, 585 - 591

This is a close up image of a Renal Corpuscle. The renal corpuscle is made up of many structures. The major structural component of the corpuscle is the Glomerulus (red arrow). It is composed of a Fenestrated capillaries with Podocytes surrounding the arteries. It is here where the blood is allowed to filter and produce the Initial urine filtrate. The initial filtrate is contained with in Bowman's Capsule, which is a double layered epithelial cup composed of Squamous Cells (blue arrows). The part that the blue arrows are pointing to is known as Parietal Layer of Bowman's Capsule. The Space in which the filtrate is located is called Bowman's space, indicated by the green arrows and it is the white space that goes around the entire glomerulus.

Red arrow - Glomerulus

Blue arrow - Bowman's Capsule a.k.a. Parietal Layer

Green arrow - Bowman's Space

This is another shot of the renal corpuscle showing the Glomerulus, (red arrows) and Bowman's Space (green arrows). In this image you can see the Vasculature associated with the glomerulus. There are two main vessels, the Afferent arteriole, which supplies the glomerulus, and the Efferent arteriole which drains the glomerulus. The yellow arrows are pointing to a vessel which could be either the Efferent or Afferent arteriole. There is no way to tell. The Blue arrow is pointing to the Vascular Pole, which basically is the side of the Renal corpuscle where the vasculature enter. The vascular pole is where the Parietal layer of Bowman's Capsule, reflects back on itself to cover the glomerulus and form Visceral Layer of Bowman's capsule.

Blue arrow - Vascular Pole

Yellow arrow - Efferent or Afferent arteriole

Green arrow - Bowman's space

Red arrow - Glomerulus

This is another image of the Glomerulus (red arrow). The Blue arrow is pointing to the Vascular pole. The Black arrow is pointing to the other pole of the glomerulus called the Urinary pole. The urinary pole is where the filtrate leaves the capsule to enter the Proximal Tubule. In this image, which is very hard to find on the slides, you can actually see the urinary pole, leading into the beginning of the Proximal tubule indicated by the green arrows.

Blue arrow - Vascular Pole

Red arrow - Glomerulus

Black arrow - Urinary Pole

Green arrow - Beginning of proximal tubule

This is an image showing various tubules found near the Renal Corpuscle, (black arrows). The Yellow arrows are showing cross sections of Proximal Convoluted Tubules, which is the first place the filtrate is drained to after bowman's capsule. The Blue arrows are showing cross sections of Distal Convoluted Tubules which is near the end of the Nephron. There are some basic differences between the two that can aid in identifying them. First and most obvious is the staining pattern. When given a field of view like this, you can see that the proximal tubules are stained a DARK RED, compared to the distal tubules which are a LIGHT PINK. Another thing to notice is the amount of nuclei. The cells in the proximal tubule are thick so there aren't that many nuclei which can also be seen in a longitudinal section of prox. tubule (red arrow). In the distal tubules, there are more nuclei present, which can also be seen in longitudinal section indicated by the white arrow.. Another major difference is the presence of a Brush Border in the proximal tubule and not in the Distal tubule (Brush border can not be seen at this magnification).

Black arrow - Renal Corpuscle

White arrow - Long. Sec. Distal tubule

Red arrow - Long. Sec. Proximal tubule

Blue arrow - Cross sec. Distal tubule

Yellow arrow - Cross sec. Proximal tubule.

This is higher magnification of what was shown above. Again notice how the Proximal tubules (blue arrow) are a lot darker staining and less cellular than the Lighter staining Distal tubules, (orange arrow).

Orange arrow - Distal tubule

Blue arrow - Proximal tubule

Renal Corpuscle

Renal Corpuscle (afferent and efferent vessels)

Glomerulus (vascular and efferent arteriole)

Proximal and Distal Tubules

Proximal and Distal Tubules