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.
