The thin limb receives urine from the proximal straight tubule and also contributes to the loop of Henle. It contains descending and ascending parts, which are both key components of the countercurrent multiplication system that promotes concentration of urine (details on Plate 3-12).
The transition from the proximal straight tubule to the thin limb involves a sharp change from cuboidal and low columnar cells to simple, largely ﬂat epithelium. It occurs at the border of the outer and inner stripes of the outer medulla.
The length of the thin segment differs depending on nephron type. In short-looped nephrons, the descending thin limb reaches the border of the outer and inner zones of the medulla and then transitions to the thick ascending limb. Meanwhile, in long-looped nephrons, the descending thin limb continues deep into the inner zone of the medulla, makes a hairpin turn, becomes the ascending thin limb, and then transitions to the thick ascending limb at the border between the outer and inner zones of the medulla. Thus, although both nephron types feature a descending thin limb, only juxtamedullary nephrons feature an ascending thin limb.
Four morphologically distinct types of cells have been described in thin limbs of several mammals, and each cell type appears to have its own physiologic signiﬁcance (discussed on Plate 3-15). It is unclear if these same cell types exist in humans.
Type I Cells
Type I cells are found throughout the descending thin limbs of short-looped nephrons. They are short, with few microvilli or basolateral interdigitations, as well as scant mitochondria or other organelles. Their nuclei bulge into the tubular lumen. Neighboring cells are joined by multistranded tight junctions and desmosomes, which restrict paracellular transport.
Type II-IV Cells
Type II-IV cells are found in the thin limbs of long-looped nephrons.
Type II cells are seen in the descending thin limb in the outer zone of the medulla. They are taller than type I cells, with more numerous microvilli and basolateral interdigitations. In addition, the tight junctions are single-stranded and thus somewhat leaky, permitting paracellular transport. Of the four cell types, type II cells show the most interspecies variation.
Type III cells are seen in the descending thin limb in the inner zone of the medulla. These cells are shorter than type II cells, with fewer microvilli and interdigitations. Their tight junctions are well-developed, restricting paracellular transport.
Type IV cells are seen just before the hairpin turn of the descending thin limb and are present for the remainder of the ascending thin limb. These cells are completely ﬂattened and have no microvilli, like type I epithelium, but they have an increased number of basolateral interdigitations. Their tight junctions are leaky, permitting paracellular transport.