Distal Tubule
The distal tubule receives urine from the thin limb.
Like the proximal tubule, the distal tubule is divided into two major sections.
The first is known as the thick ascending limb (also known as the distal
straight tubule, or pars recta), and the second is known as the distal
convoluted tubule (pars convoluta). Just before the transition to the distal
convoluted tubule, the thick ascending limb touches its parent glomerulus, and
the epithelial cells that make direct contact constitute a specialized
structure known as the macula densa.
In short-looped nephrons, the thick ascending limb
accounts for the entire ascending limb of the loop of Henle. In contrast, in
long-looped nephrons, the thin limb accounts for the initial part of the
ascending limb, then transitions to the thick ascending limb at the border of
the inner and outer zones of the medulla.
Thick Ascending Limb
The thick ascending limb (TAL) plays an important role
in the reabsorption of ions and is crucial for maintenance of the
countercurrent multiplication system, described on Plate 3-12.
The cells of the TAL are low cuboidal. Their height
decreases as the tubule progresses from medulla to cortex. Their apical surfaces
are dotted with sparse, short microvilli. In the rat, there is a subset of
“rough” cells, which have sparse microvilli, and “smooth cells,” which lack
them altogether. The relative proportion of “rough” cells increases as the
tubules approach the renal cortex. Because of the scarcity of microvilli, the
distal tubules appear patent on light microscopy, facilitating the distinction
from proximal tubules, which possess a well-developed brush border.
Below the apical surface are numerous small vesicles,
which traffic ion channels and transporters to the plasma membrane. The rough
endoplasmic reticulum and Golgi apparatus, which synthesize these proteins, are
prominent. The nuclei are located near the apical membrane and sometimes bulge
out toward the lumen. The basolateral membranes are thrown into extensive,
interdigitating processes and infoldings, which increase the surface area available
for basolateral transport. As a result of this configuration, the lateral cell
borders appear indistinct on light microscopy sections. The basolateral
processes are filled with mitochondria, which resemble striations on histologic
sections, to provide energy for active transport. Interdigitating processes and
infoldings from neighboring cells are joined together by tight junctions.
Macula Densa
Unlike the rest of the distal tubular cells, the macula
densa cells are columnar and lack extensive interdigitating basolateral processes.
Their high nucleus to cytoplasmic ratio causes them to appear denser than
neighboring cells. Their nuclei are positioned near their apical surface, above
most of the cellular organelles. The basal surfaces of these cells interdigate
with the adjacent extraglomerular mesangium through their shared
basement membrane, reflecting their physiologic connection.
Distal Convoluted Tubule
At some distance after the macula densa, there is an
abrupt transition from the low cuboidal cells of the TAL to the taller cuboidal
cells of the distal convoluted tubule, which takes a tortuous course through a
small area of cortex. The cells of the distal convoluted tubule have more
numerous apical microvilli than those of the TAL, but their other features are
similar.
Connecting Segment
At the end of the distal tubule, just before the
transition to the collecting duct, there is a zone known as the connecting
segment (or tubule). This segment lacks clear boundaries and mixes gradually
with the previous and next segments. In general, however, the cells in this
segment have less prominent interdigitating membrane processes and fewer
mitochondria than those of the DCT. Principal and intercalated cells, which
figure prominently i the collecting duct, begin to appear in this segment.
