 Type II epithelium, found in the thin descending limb of long-looped nephrons in the cortical labyrinth, consists of taller epithelium. These cells possess abundant organelles and have many small, blunt microvilli. The extent of lateral interdigitation with neighboring cells varies by species.
 Type III epithelium, found in the thin descending limb in the inner medulla, consists of a thinner epithelium. The cells have a simpler structure and fewer microvilli than type II epithelial cells. Lateral interdigitations are absent.
 Type IV epithelium, found at the bend of long-looped nephrons and through the entire thin ascending limb, consists of a low, flattened epithelium without microvilli. The cells possess few organelles.
The specific functional roles of the four cell types in the thin segment are not yet clear, although this segment is part of the countercurrent exchange system that functions in concentrating urine. Morphologic differences, such as microvilli, mitochondria, and degree of cellular interdigitation, probably reflect specific active or passive roles in this process.
The thin descending and ascending limbs of the loop of Henle differ in structural and functional properties.
Studies of ultrafiltrate that enters the thin descending limb and leaves the thin ascending limb of the loop of Henle reveal dramatic changes in ultrafiltrate osmolality. The ultrafiltrate that enters the thin descending limb is isosmotic, whereas the ultrafiltrate leaving the thin ascending limb is hyposmotic to plasma. This change is achieved by reabsorbing more salts than water. The two limbs of the loop of Henle have different permeabilities and thus different functions:  The thin descending limb of the loop of Henle is highly permeable to water and much less permeable to solutes like NaCl and urea. Because the interstitial fluid in the medulla is hyper-osmotic, water diffuses out of this nephron segment. In addition, a small amount of NaCl and urea enters the nephron at this site. The cells of this limb do not  actively transport ions; thus the increased tubular fluid  osmolality that occurs in this nephron segment is caused in large part by the passive movement of water into the peritubular connective tissue.
FIGURE 20.20 • Schematic diagram of loop of Henle thin-limb epithelial cells. Roman numerals (I–IV) identify the various segments of the epithelium and the region where they are found in the thin limb of the short and long loops of Henle. The diagrams of the epithelium do not include nuclear regions of the epithelial cells. (Modified from Madsen KM, Tisher CC. Physiologic anatomy of the kidney. In: Fisher JW (Ed). Kidney Hormones, London, UK: Academic Press 1986;3:45–100.)  The thin ascending limb of the loop of Henle does not actively transport ions, but it is highly permeable to NaCl and thus allows passive diffusion of NaCl into the interstitium. Cl diffuses into the interstitium following its concentration gradient through Cl conducting channels. Although the energy from ATP is required to open these channels, the movement of Cl is not an example of active transport and does not require Cl -stimulated ATPase activity. Counter ions, in this case Na (the majority) and K, follow passively to maintain electrochemical neutrality. The hyperosmolarity of the interstitium is directly  related to the transport activity of the cells in this limb of the loop of Henle.