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.
Renal pyramid, kidney, human, H&E ×20.
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 renal column of 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 cupor 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 minor 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.
KEY AV, arcuate vessels CT, collecting tubules D, distal straight tubule P, proximal straight tubule RC, renal corpuscle SCEp, simple columnar epithelium T, thin segment TEp, transitional epithelium arrowhead, location of apex of pyramid asterisks, boundaries between cells of a collecting tubule diamonds, boundary between a transitional and a columnar epithelium left-pointing arrow, proximal tubule cell right-pointing arrow, thin segment cell