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.