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Proximal and distal convoluted tubules display features that aid in their identification in H&E-stained paraffin sections. Proximal tubules generally have a larger diameter than distal tubules have; cross sections of the lumen often appear stellate. A brush border (apical microvilli) is often visible on the proximal tubule cells. Also, the proximal convoluted tubule is more than twice as long as the distal convoluted tubule; thus, the majority of tubular profiles in the cortical labyrinth will be of proximal tubules.

Mesangial cells and their extracellular matrix constitute the mesangium of the renal corpuscle. They underlie the endothelium of the capillaries of the glomerular tuft and extend to the vascular pole, where they become part of the juxtaglomerular apparatus. The terminal portion of the distal thick segment of the nephron lies close to the afferent arteriole. Tubule epithelial cells closest to the arteriole are thinner, taller, and more closely packed than other tubule cells and constitute the macula densa. Arterial smooth muscle cells opposite the macula densa are modified into juxtaglomerular cells that secrete renin in response to decreased blood NaCl concentration.

Figure 1, kidney, human, H&E x240.

In this figure, an area of cortical labyrinth, there are six distal convoluted (DC) tubule profiles. The proximal tubules (unlabeled) have a slightly larger outside diameter than the distal tubules have. The proximal tubules have a brush border, whereas the distal tubules have a cleaner, sharper luminal surface. The lumen of the proximal tubules

Figure 2, kidney, human, H&E x240.

In this figure, all of the tubular profiles are rounded except for a proximal convoluted tubule (PC) included in the lower right corner of the figure (it belongs to the adjacent cortical labyrinth). Second, the number of proximal (P) and distal (D) tubular profiles are about equal in the medullary

Figures 3 and 4, kidney, human, H&E x360.

The renal corpuscle appears as a spherical structure whose periphery is composed of a thin capsule that encloses a narrow clear-appearing space, the urinary space (asterisks), and a capillary tuft or glomerulus that appears as a large cellular mass. The capsule of the renal corpuscle, known as the renal or Bowman's capsule, actually has two parts: a parietal layer, which is marked (BC), and a visceral layer. The parietal layer consists of simple squamous epithelial cells. The visceral layer consists of cells, called podocytes (Pod), that lie on the outer surface of the glomerular capillary. Except where they clearly line the urinary space, as the labeled cells do in Figure 3, podocytes may be difficult to distinguish from the capillary endothelial cells. To complicate matters, the mesangial cells are also a component of the glomerulus. In general, nuclei of podocytes are larger and stain less intensely than do the endothelial and mesangial cell nuclei.

is often star shaped; this is not the case with distal tubules. Typically, fewer nuclei appear in a cross section of a proximal tubule than in an equivalent segment of a distal tubule.

Most of the above points can also be utilized in distinguishing the straight portions of the proximal and distal thick segments in the medullary rays, as shown in Figure 2.

ray, as is shown by the labeling of each tubule in this figure. Note that, in contrast to the distal tubules, the proximal tubules display a brush border and have a larger outside diameter, with many displaying a star-shaped lumen. The medullary ray also contains collecting tubules (CT). They are considered in Plate 73.

A distal (DC) and two proximal (PC) convoluted tubules are marked in Figure 3. The cells of the distal tubule are more crowded on one side. These crowded cells constitute the macula densa (MD) that lies adjacent to the afferent arteriole.

In Figure 4, both the vascular pole and the urinary pole of the renal corpuscle are evident. The vascular pole is characterized by the presence of arterioles (A), one of which is entering or leaving (double-headed arrow) the corpuscle. The afferent arteriole possesses modified smooth muscle cells with granules, the juxtaglomerular cells (not evident in Fig. 4). At the urinary pole, the parietal layer of Bowman's capsule is continuous with the beginning of the proximal convoluted tubule (PC). Here, the urinary space of the renal corpuscle continues into the lumen of the proximal tubule, and the lining cells change from simple squamous to simple cuboidal or low columnar with a brush border.

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