Collecting Duct
The collecting ducts receive urine from the connecting
segments (or tubules). The ducts extend from cortex to medulla, and they are
customarily divided into cortical, outer medullary, and inner medullary
regions. As the ducts course toward the medulla, they fuse into progressively
larger conduits that ultimately terminate at the cribriform area of the renal
papillae, where urine drains into the minor calices.
The collecting duct develops from the ureteric bud (see
Plate 2-1) and is thus technically not part of the nephron. Nonetheless, these
ducts play a key role in determining the final composition of urine and do not
serve as mere conduits to the renal papillae.
The collecting ducts are easily distinguished on light
microscopic sections because their cells have distinct and straight borders, no
apical brush border, round and central nuclei, and light to clear cytoplasms.
In cross section, the collecting ducts have large, patent lumina, which can be
distinguished from the narrow, collapsed lumina of proximal tubules.
Cortical Collecting Duct
The cortical collecting duct contains two major cell
types: principal (“light”) cells and intercalated (“dark”) cells. Principal
cells transport salt and water, while intercalated cells participate in
acid-base homeostasis. Although these cell types can be distinguished using
electron microscopy, they often appear similar on light microscopy sections.
Principal cells have relatively few intracellular organelles
and thus their cytoplasm appears “light” using some staining techniques. These
cells are cuboidal in appearance. Their apical membranes have sparse, short
micro-villi. Their basal surface contains few short invaginations, while the
lateral surfaces contain very few small processes and infoldings. Unlike in the
proximal and distal tubules, mitochondria are not localized to the basolateral
processes and are instead scattered throughout the cytoplasm. Extensive water
reabsorption occurs across these cells, as described on Plate 3-15, which
causes the appearance of a prominent intercellular space. The cells are
connected to one another by tight junctions and desmosomes.
Intercalated cells, in contrast, are dense with mitochondria
and other organelles, which cause their cytoplasm to appear “dark” using some
staining techniques. These cells are generally subclassified into type A and B
intercalated cells (see Plates 3-21 and 3-22).
Type A intercalated cells secrete protons into urine and
reabsorb bicarbonate into the interstitium. Their apical surface is dotted with
numerous microvilli. The cells also contain numerous vesicles near their apical
membrane that contain proton transporters. In acidotic conditions, these
vesicles fuse with the apical plasma membrane to increase proton secretion. In
chronic acidotic states, these cells become hypertrophic, especially at their
apical aspect.
Type B intercalated cells, meanwhile, secrete bicarbonate
into urine and pump protons into the interstitium. These cells usually lack the
apical features characteristic of type A cells, such as microvilli and a dense
vesicle population, which may reflect the reversed polarity of proton pumping.
In chronic alkalotic states, these cells become hypertrophic.
The different intercalated cell types may sometimes be
distinguished by morphologic characteristics; however, the most reliable means
of classifying a particular cell is to examine the population and distribution
of its ion transporters using immunostaining. Type A cells express basolateral
AE1 HCO3-/Cl3- exchangers, whereas type B cells express
apical pendrin HCO3-/Cl3- exchangers. Although recent work in some
animals points to the existence of a third population of cells, known as non-A
non-B cells, their function is not well understood at present.
Outer And Inner Medullary Collecting Duct
The outer medullary collecting duct (OMCD) consists of
principal cells and a smaller population of type A intercalated cells. Both
appear taller and less dense than their equivalents in the cortical segment.
The inner medullary collecting duct (IMCD) is often
subdivided into initial and terminal segments. The initial segment contains
principal cells that appear similar to those in the OMCD. The terminal segment
contains IMCD cells, which resemble principal cells but are taller and have
denser microvilli, a lighter staining cytoplasm, and fewer basal invaginations.
Both IMCD segments lack intercalated cells.
