Proximal Tubule
The proximal tubule receives urine from Bowman’s space.
It plays a major role in the transport of material from the urine back into the
blood (reabsorption) and vice versa (secretion). In humans, the entire proximal
tubule is approximately 14 mm long. It is divided into two sections: the
proximal convoluted tubule (pars convoluta) and the proximal straight tubule
(pars recta). The latter forms the first part of the loop of Henle.
In rats, the proximal tubule is often subdivided into S1
(first two thirds of the convoluted part), S2 (last third of the convoluted part
and initial portion of the straight part), and S3 (remainder of the straight
part); however, these distinctions are generally not made in humans.
The proximal tubule contains cuboidal to low columnar
cells arranged over a tubular basement membrane. These cells possess an
eosinophilic cytoplasm, and their round nuclei are usually situated near the
cell base. Their other histologic features differ according to the particular
region under consideration.
Proximal Convoluted Tubule
The proximal convoluted tubule (PCT) is the major site
of solute reabsorption in the nephron. An extensive microvillous brush border
on the apical plasma membrane projects into the lumen and dramatically
increases the available surface area for solute transport. On light microscopy,
the lumen often appears collapsed or indistinct owing to the presence of the
brush border, which should be readily seen. Distal tubules and collecting
ducts, in contrast, lack a brush border and thus appear more widely patent.
The lateral and basal borders of PCT cells are thrown
into extensive processes that interdigitate with infoldings of adjoining cells;
as a result, lateral cell borders are indistinct on light microscopy sections.
These basolateral processes increase the surface area available for transport
across the basolateral cell membrane. They are replete with additional
mitochondria to support active transport processes. The complex extracellular
area between these folds is known as the basolateral intercellular space. It is
closed by the tubular basement membrane, which separates the tubular epithelium
from the interstitium and peritubular capillaries.
Junctional complexes connect neighboring cells near
their apical surface. These consist of a tight junction (zonula occludens) and
an intermediate junction (zonula adherens). Although tight junctions are
critical for maintaining the barrier between the tubular lumen and
interstitium, a small number of discontinuities permits some molecules to be reabsorbed
through a paracellular route.
PCT cells are rich with mitochondria, which provide
energy for solute transport. These are arranged perpendicular to the cell base
and resemble vertical striations on some histologic sections. These cells also
possess a prominent rough endoplasmic reticulum and Golgi apparatus near the
apical membrane.
PCT cells contain evidence of extensive endocytosis near
the apical plasma membrane, including coated pits, invaginations, and
endosomes. Numerous lysosomes are also present to process and degrade a subset
of the incoming material. Endocytosis appears to be most important for the
reabsorption of filtered proteins, as it is up-regulated in conditions that
damage the normal glomerular filtration barrier.
Proximal Straight Tubule
The cells of the proximal straight tubule differ from
those of the PCT in several important respects. The microvilli are shorter and
sparser, endocytotic figures are less frequent, mitochondria are sparser, and
the basolateral processes and infoldings are smaller and less elaborate. These
morphologic differences reflect the smaller amount of reabsorption that occurs
across these cells.
