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Renal corpuscles are restricted to the cortical labyrinth. The medulla contains the thick straight segments of proximal and distal tubules, along with their thin segments, the collecting tubules and ducts, and the blood vessels that run in parallel with them. These structures function as the countercurrent multiplier and countercurrent exchange systems that, ultimately, produce hypertonic urine. The final urine drains from the papillary ducts (of Bellini) into calyces that then empty into the renal pelvis.

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

A section through the outer portion of the medulla is shown in this figure. This region contains proximal and distal thick segments, thin segments, and collecting tubules. All of the tubules are parallel, and all are cut in cross section; thus, they present circular profiles. The proximal (P) thick segments display typical star-shaped lumina and a brush border (or the fragmented apical cell surface from which the brush border has been partially broken). These tubules have outside diameters that are generally larger than those of the distal tubules (D). As mentioned previously and as shown here, the distal tubules display a larger number of nuclei than do comparable segments of proximal tubule cells. Note, also, that the lumen of the distal tubule is more rounded and the apical surface of the cells is sharper. The collecting tubules (CT) have outer diameters that are about the same as those of the proximal tubules and larger than those of distal tubules. The cells forming the collecting tubules are cuboidal and smaller than those of proximal tubules; thus, they also display a relatively larger number of nuclei than do comparable segments of proximal tubule cells. Count them! Finally, boundaries between the cells that constitute the collecting tubules are usually evident (asterisks); this serves as one of the most dependable features for the identification of collecting tubules.

The thin segments (T) have the thinnest walls of all renal tubules seen in the medulla. They are formed by a low cuboidal or simple squamous epithelium, as seen here, and the lumina are relatively large. Occasionally, a section includes the region of transition from a thick to a thin segment and can be recognized even in a cross section through the tubule. One such junction is evident in this figure (the tubule with two arrows in the lumen). On one side, the tubule cell (left-pointing arrow) is characteristic of the proximal segment; it possesses a distinctive brush border. The other side of the tubule (right-pointing arrow) is composed of low cuboidal cells that resemble the cells forming the thin segments. In addition to the renal and collecting tubules, there are many other small tubular structures in this figure. Thin-walled and lined by endothelium, they are small blood vessels.

Figure 2, kidney, human, H&E x20.

This figure shows a renal pyramid at low magnification. The pyramid is a conical structure composed principally of medullary straight tubules, ducts, and the straight blood vessels (vasa recta). The dashed line at the left of the micrograph is placed at the junction between cortex and medulla; thus, it marks the base of the pyramid. Note the arcuate vessels (AV) that lie at the boundary of cortex and medulla. They define the boundary line. The few renal corpuscles (RC), upper left, belong to the medulla. They are referred to as juxtamedullary corpuscles.

The pyramid is somewhat distorted in this specimen, as evidenced by regions of longitudinally sectioned tubules, lower left, and cross-sectioned and obliquely sectioned tubules in other regions. In effect, part of the pyramid was bent, thus the change in the plane of section of the tubules.

The apical portion of the pyramid (arrowhead), known as the renal papilla, is lodged in a cup- or funnel-like structure referred to as the calyx. It collects the urine that leaves the tip of the papilla from the papillary ducts (of Bellini). (The actual tip of the papilla is not seen within the plane of section, nor are the openings of the ducts at this low magnification.) The surface of the papilla that faces the lumen of the calyx is simple columnar or cuboidal epithelium (SCEp). (In places, this epithelium has separated from the surface of the papilla and appears as a thin strand of tissue.) The calyx is lined by transitional epithelium (TEp). Although not evident at the low magnification shown here, the boundary between the columnar epithelium covering the papilla and the transitional epithelium covering the inner surface of the calyx is marked by the diamonds.

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