Schematic of nephrons and collecting duct. This scheme depicts a short‐looped and a long‐looped nephron together with the collecting system. Not drawn to scale. Within the cortex a medullary ray is delineated by a dashed line. 1 = renal corpuscle including Bowman's capsule and the glomerulus (glomerular tuft); 2 = proximal convoluted tubule; 3 = proximal straight tubule; 4 = descending thin limb; 5 = ascending thin limb; 6 = distal straight tubule (thick ascending limb); 7 = macula densa located within the final portion of the thick ascending limb; 8 = distal convoluted tubule; 9 = connecting tubule (CNT); 9^* = CNT of the juxtamedullary nephron that forms an arcade; 10 = cortical collecting duct; 11 = outer medullary collecting duct; and 12 = inner medullary collecting duct. From Kriz and Bankir, with permission, 339.

(A) Schematic drawing, demonstrating the essential structural features of renal transporting epithelia; 1 paracellular route through the tight junction and the lateral intercellular spaces; 2 transcellular route, across the apical plasma membrane, the cytoplasm and the basolateral plasma membrane; the apical membrane area may be augmented by short microvilli, microfolds (not shown) or “ brush border” (long microvilli of uniform dimensions), the basolateral membrane may be augmented by infoldings of the basal plasma membrane or by lateral folding, giving rise to basolateral processes, which narrowly interdigitate with those of neighbouring cells, the lateral folds narrowly enclose large mitochondria. [(B)‐(F)] Salient features of interdigitated epithelia. (B) Freeze fracture electron micrograph of a tight junction (TJ) (e.g., thick ascending limb) consisting of several densely arranged parallel strands. Rabbit (Cooperation with A. Schiller and R. Taugner). (C) TEM of interdigitated lateral folds, enclosing large mitochondria (M), and narrow intercellular spaces (arrows) of regular width (e.g., thick ascending limb epithelium); cell adhesion molecules within the intercellular spaces are contrasted by tannic acid staining; BL, basal lamina; Rat; (cooperation with T. Sakai). (D) Freeze‐fracture electron micrograph of the luminal aspect of a tubule demonstrating the mode of cellular interdigitation (e.g., ascending thin limb and Psammomys obesus) [from Kriz et al., with permission 345]. (E) Freeze fracture electron micrograph of a TJ in noninterdigitated epithelia (e.g., collecting duct cell); the prominent tight junction consists of several anastomosing tight junctional strands. (F) Infolded basal membranes confining extracellular spaces (arrows) which may vary in width with function; BL, basal lamina. (G) Freeze fracture electron micrograph of the basal aspect of a noninterdigitated cell (e.g., cortical collecting duct); the polygonal shape of this cell is recognized by contours of basal slits through which lateral intercellular spaces (white arrows) open into the interstitium; basal infoldings of extracellular spaces (black arrows) are separated from lateral intercellular spaces. Bars ∼1 μm (D, F, and G) and ∼0.1 μm (B, C, and E). Adapted, with permission, from Kaissling and Kriz 291.

Schematic three‐dimensional appearance of a proximal tubule cell from the convoluted part of the rat proximal tubule, illustrating the appearance of the lateral and basal ridges and processes. Adapted, with permission, from Maunsbach 426.

Schematic representation of the typical mouse nephron and collecting duct (CD) organization: Tortuous descending thin limbs (DTL) of long‐loop nephrons (LLN; large arrow); winding course of thick ascending limbs (TAL) of short loop nephrons (SLN) and CD (arrow heads); a piece of TAL inserted in the DTL of LLN (LLNt, small arrow), which forms its bend just beneath the “transitional zone” within the inner medulla (IM); and three different types of SLN bends (SLN1, SLN2, and SLN3). Based on the distribution of the nephron segments, the renal zones are defined, including cortex, outer stripe of outer medulla (OSOM), inner stripe of outer medulla (ISOM), and IM. The postmacula densa segments and the directions of the proximal tubules (PT) arising from their glomeruli are also illustrated. Adapted, with permission, from Zhai et al. 762.

Electron micrograph of rat kidney cortex showing several proximal tubules with open lumens and evenly arranged brush borders. The micrograph illustrates some of the differences between the two segments of the convoluted part of the proximal tubule. In the first segment (S1) the cells are slightly taller, microvilli of the brush border longer, and apical endocytic vacuoles more numerous than in the second segment (S2). The lysosomes in the first segment have light contents whereas the lysosomes in the second segment show densely stained contents. DCT, distal convoluted tubule. The tissue was fixed by vascular perfusion with glutaraldehyde. X 2,100. Adapted, with permission, from Maunsbach 424.

Electron micrographs and schematic drawings of proximal tubule epithelium from rat and rabbit kidney illustrating the three segments of the proximal tubule. (A) Rat, first segment (S1), (B) rat second segment (S2), (C) rat, third segment (S3), (D) rabbit, third segment (S3), and (E) schematic drawings of the three segments in rat. In segments S1 and S2 the mitochondria (M) are oriented perpendicular to the basement membrane and endocytic vacuoles (E) are numerous. The brush border (BB) is taller in S1 than in S2. The matrix of the lysosomes (L) in S1 is lightly stained but rather homogenous and densely stained in S2. The mitochondria in S3 are more randomly oriented than in S1 and S2 and they are less numerous in the rabbit than in the rat cells. In both species the lysosomes in S3 cells appear smaller and fewer as compared to S1 and S2 of the rat. The brush border of this segment is significantly higher in rat than in rabbit. Part of a cilium (C) is present in the brush border in C and peroxisomes (P) are seen in the cytoplasm. Capillary (CAP). The diagram in E illustrates the differences in cell shapes and the different distributions of mitochondria (M), lysosomes (L), endocytic vacuoles (E), and peroxisomes (microbodies) (P). S1, first segment. S2, second segment. S3, third segment. Adapted, with permission, from Maunsbach 426 and Maunsbach and Christensen 429. [(A) and (B)] X 8,900 and [(C) and (D)] X 9,800.

(A) Tight junction (arrowheads) between two rat proximal tubule cells close to the tubule lumen (LU). The triple‐layered plasma membranes are completely apposed. The tight junction has a depth in the luminal‐basal direction of only 100 to 200 Å and is followed on its peritubular side by an intermediate junction. X 167,000. Adapted, with permission, from Maunsbach 424. (B) Tangential section through luminal part of proximal tubule cells showing an intermediate junction, which extends between the two pairs of arrowheads. When the junction is sectioned at right angle it can be noted that the cytoplasm next to the triple‐layered plasma membranes has an increased density and that a dense material fills the gap between the membranes (compare with normal lateral intercellular space at LIS). Lumen (LU) is seen in the upper right corner. X 145,000. Adapted, with permission, from Maunsbach 426. (C) Higher magnification of gap junction between proximal tubule cells from Necturus maculosus. The 20‐Å wide space between the two plasma membranes is clearly observable (arrowheads). X 204,000. Adapted, with permission, from Maunsbach 426.

Parts of proximal tubule cells from rat and rabbit kidney illustrating smooth surfaced endoplasmic reticulum and peroxisomes. (A) Peroxisomes (P) (microbodies) from rat proximal tubule cell limited by a single membrane and showing a uniformly stained matrix. Smooth‐surfaced endoplasmic reticulum closely surrounds the peroxisomes (arrowheads) and is seen in continuity with the rough endoplasmic reticulum at arrow. X 44,000. Adapted, with permission, from Maunsbach and Christensen 429. (B) Smooth‐surfaced endoplasmic reticulum adjacent to the lateral plasma membrane. This so‐called paramembraneous endoplasmic reticulum (PER) has fenestrated cisternae (arrows) that closely follow the lateral plasma membrane. However, it is never in contact with this membrane, nor is it in continuity with the plasma membrane facing the tubule lumen (LU), endocytic vacuoles (E), or lysosomes. (A) X 95,000. Adapted, with permission, from Maunsbach 428. (C) Peroxisomes (P) in cells of isolated perfused proximal tubule of rabbit. In this species, part of the limiting membrane is straight, thickened, and electron dense. The limiting membrane is closely related to the smooth endoplasmic reticulum. Since this tubule was incubated with horseradish peroxidase (HRP) in the bath, the intercellular spaces, except in gap junction (GJ), are filled with reaction product for HRP. Reaction product is also seen in a small endocytic vesicle (V). X 45,000. Adapted, with permission, from Nielsen and Christensen 470. (D) Demonstration of dextran in rat proximal tubule cells 1 h after termination of intravenous infusion of dextran T‐40. The kidney was fixed in vivo by dripping a lead‐containing fixative on the kidney surface. Small dense particles representing dextran are present in lysosomes (arrows), where they are located adjacent to the dense matrix material. A peroxisome (P) with a fingerlike projection is well stained by the lead‐containing fixative. X 15,800. Adapted, with permission, from Christensen and Maunsbach 118.

Electron micrographs illustrating the vacuolar apparatus in rat proximal tubule cells. (A) A round lysosome (L) with an electron‐dense matrix devoid of inclusions and located close to a mitochondrion (M). The lysosome is limited by a triple‐layered membrane. X 100,000. (B) Coated pit connected to the tubular lumen (LU). The coated pit has a thick cell coat or glycocalyx (arrow) and a cytoplasmic clathrin coat (arrowheads). X 100,000. Adapted, with permission, by A.B. Maunsbach. (C) Dense apical tubules connected to endosomes (S) in apical cytoplasm. X 70,000. Adapted, with permission, from Cui and Christensen 138.

Isolated perfused proximal tubule of rabbit incubated with horseradish peroxidase in the bath for 30 min. The reaction product is seen in the lateral intercellular spaces (LIS) but not in the lumen (LU). Note the reaction product in a multivesicular body (MVB) and in lysosomes (L). Unstained section. X 15,000. Adapted, with permission, from Nielsen and Christensen 470.

Immunofluorescence (mouse kidney) and immunogold (rat proximal tubule) localization of megalin and cubilin. The immunofluorescence reveal strong apical labeling in proximal tubules but no obvious labeling in glomeruli (G). Distal tubules (D) and vessels (A) are unlabeled. Bar, 100 μm. Immunogold labeling, megalin, 10 nm gold and cubilin, 5 nm gold, reveals colocalization of the two receptors in coated pits (Inv), coated vesicles (C), endosomes (E), and in recycling dense apical tubules (arrows). Bar, 0.5 μm.

Schematic drawing of megalin, cubilin, and amnionless (AMN) presenting known domains and motifs. The three receptors colocalize in the renal proximal convoluted tubule (PCT) where they cooperate in clearance of the ultrafiltrate. Megalin binds a variety of filtered molecules (more than 50 ligands have been identified) through its complement type repeats and is able to mediate endocytosis via NPXY motifs in the cytoplasmic tail. Cubilin on the other hand includes multiple binding domains (CUB domains), but only around 15 ligands have been identified. Cubilin is a peripheral membrane protein and is thereby dependent on megalin and/or AMN to assure internalization of its ligands. AMN contains a NPXY motif and is probably assisting cubilin in endocytosis as well as in transport during synthesis. Adapted, with permission, from Christensen et al. 123.

The megalin‐ and cubilin‐mediated uptake of three vitamin carrier protein complexes: DBP‐vitamin D₃, TC‐vitamin B₁₂, and RBP retinol in renal proximal tubule. Likewise the cubilin chaperone protein amnionless, amnionless (AMN) is indicated. Following receptor‐mediated endocytosis via apical coated pits, the complexes accumulate in lysosomes for degradation of the proteins, while the receptors recycle to the apical plasma membrane via dense apical tubules. As illustrated here and detailed in the text, megalin mediates the uptake of cubilin and its ligands. Whether the two receptors are constitutively associated in the plasma membrane and remain associated during recycling in dense apical tubules is not known. Whereas TC and RBP apparently bind exclusively to megalin, DBP binds with similar affinity to both megalin and cubilin. The intracellular processing of the vitamins may include modifications such as hydroxylation of 25‐OH‐D₃ to 1,25‐(OH)₂‐D₃, and metaobolism of B₁₂. The mechanisms for the cellular release of the vitamins remain to be clarified. Adapted, with permission, from Verroust et al. 700.

Proximal tubular uptake of albumin A and B and insulin C, in rat A and C and in mouse B. (A) Double immunogold labeling of endogenous albumin (18 nm gold) and cathepsin B (6 nm gold) on ultrathin cryosection from rat kidney. The albumin labeling is highly concentrated in a lysosome (L). X 43,000 [adapted, with permission, from Christensen et al. 123]. (B) Immunofluorescence of paraffin section of cortex from kidney‐specific megalin mosaic KO mice by using sheep anti‐rat megalin (green), rabbit anti‐human albumin (red). Proximal tubule cells not expressing megalin (arrows) have no labeling for albumin. X 550 [adapted, with permission, from Christensen et al. 123]. (C) Electron microscope autoradiograph of ¹²⁵I‐labelled insulin uptake in rat proximal tubule, fixed 25 min after start of microinfusion of the insulin. Grains are mainly located over lysosomes in the apical part of the cytoplasm (arrows). Endocytic vacuoles (E) and some lysosomes (L) located deep in the cytoplasm do not contain ¹²⁵I‐insulin. X 16,000. Adapted, with permission, from Hellfritzsch et al. 248.

Epon section from border between the outer and inner stripe of the outer zone of the medulla of rat labeled for AQP‐1 labeled by immunoperoxidase by method described, with permission, by Zhai et al. 761. Segment 3 proximal tubules show an intense brush border labeling and basolateral labeling (arrowheads). Two transitions into nonlabeled, thin‐walled DTL type 1 epithelium of short‐loop nephrons (arrows) are shown. X 400.

Schematic presentation of the tubular‐vascular relationship in the outer part of the inner stripe of outer medulla (ISOM). Tl: thick ascending limb of long‐looped nephrons. Ts: thick ascending limb of short‐looped nephron. Dl: descending thin limb of long‐looped nephron. Ds: descending thin limb of short‐looped nephron. C: collecting duct. VB: vascular bundle. IB: interbundle region. Kindly provided, with permission, by Dr. X.Y. Zhai].

(A) Schematic of thin limb ultrastructure. Type 1 is found in descending thin limb (TL) of short loop nephrons (SLNs). Type 2 is found in upper part of descending TLs of long loop nephrons (LLNs); in most species (e.g., rat, mouse, and Psammomys), it is complexely organized, in others (e.g., rabbit and guinea pig), it is more simple. Type 3 occurs in lower parts of descending TL of LLNs. Type 4 is found at the loop bend and in ascending TL of LLNs. Modified, with permission, from Kaissling and Kriz 291. (B) Four distinct epithelia along the TL of SLN and LLN in mouse. The figure comprises two columns of images representing lower and higher magnifications (right). Type 1 is a cross section of the DTL of an SLN (440 μm from the transition of the PT to the DTL); higher magnification of type 1 shows a simple and thin epithelium with few microprojections, abundant ribosomes, and sparse mitochondria throughout the cytoplasm. Type 2 is a cross section of the DTL of an LLN (370 μm from the transition of the PT to the DTL); higher magnifications of type 2 show a highly specialized epithelium: abundant short and plump microvilli, basolateral infoldings, and highly interdigitated cellular processes with shallow tight junctions (small arrows) and radially oriented mitochondria. Type 3 is a cross section of a DTL of LLN around the boundary between the inner stripe of outer medulla (ISOM) and the inner medulla (IM) (2790 μm from the transition of PT to DTL); higher magnifications of type 3 show the highly differentiated membranes: Numerous relatively long microvilli, abundant finger‐like basolateral infoldings throughout the cytoplasm (large arrows), and the less shallow tight junctions (small arrow) and few mitochondria. Type 4 is a cross section of the ascending thin limb (ATL) of an LLN (700 μm from the transition of the ATL to the TAL); higher magnifications of type 4 show the bold and dense apical membrane, basolateral infoldings, cellular processes with tight junctions (small arrows), and various organelles with sparse ribosomes throughout the cytoplasm. Bars, 1 μm. Adapted, with permission, from Zhai et al. 762.

Peroxidase immunohistochemistry of an Epon section showing the transition (arrows) from UT‐A2‐labeled DTL into the nonlabeled, thick ascending epithelium of a short loop nephron (SLN) of mouse kidney. X 200. Adapted, with permission, from Zhai et al. 762.

Survey on location (left panel) and ultrastructure of the thick ascending limb of Henle's loop [TAL = distal straight tubule, including macula densa (MD)]; C, cortex; IS, inner stripe; OS, outer stripe; IZ, inner zone; the direction of the urinary flow is indicated by white arrows, (A) medullary part, (B) cortical part, and (C) MD. adapted, with permission, from 288,291.

Overview on the histotopography of the thick ascending limb (T); (A) cross section of the inner stripe; the upper portion shows part of a vascular bundle with descending (a) and ascending vasa recta (v); in the lower half thick ascending limbs are intermingled with descending thin limbs (DL), collecting ducts (CDs), and capillaries from the interbundle capillary plexus. (B) Longitudinal section of the deep cortex in rat; the thick ascending limb (TAL) profiles reveal a much thinner epithelium than in the inner stripe; they are grouped around the CDs and surrounded by profiles of proximal tubules [adapted, with permission, from 291]. Inset: longitudinal section through the sharp transition (arrowheads) from the thin epithelium of the ascending limb (AL) to the TAL (T) in mouse. [(A) and inset] ∼10 μm and (B) ∼50 μm.

Ultrastucture of thick ascending limb cells. (A) Deep level of the inner stripe; rat; (B) cortical part; rat; and (C) macula densa cells; rabbit. Mitochondria (arrow head) in A and B are confined in the interdigitated lateral folds and the intercellular spaces (long arrow) are narrow; in C the mitochondria are scattered in the perinuclear cytoplasm and the intercellular spaces are dilated; the macula densa (MD) cells are affixed to the extraglomerular mesangium (star); [(A) and (B) adapted, with permission, from 291 and (C) from 290]. Bars ∼ 1 μm.

Macula densa (MD). (A) Preparation of a isolated human renal corpuscle (G) together with the cortical radial artery (A), glomerular arterioles (red), and tubules; proximal tubule (P; gray); the thick ascending limb (T, yellow) ascends along the cortical collecting duct (C; violet), bends toward the vascular pole of its glomerulus and passes between the glomerular arterioles; it continues as distal convoluted tubule (DCT, orange); the asterisk indicates the location of the MD. [Slightly modified, with permission, from Karl Peter (1909) 517]. (B) Scanning electron micrograph; the cortical thick ascending limb (CTAL) is open at the level of the MD, the luminal aspect of rabbit MD cell plaque is visible; single cilia are seen on MD cells as well as on CTAL cells; EGM, extraglomerular mesangium [adapted, with permission, from 291]. (C) Epon section of rat MD within the cortical thick ascending limb; the epithelium reveals conspicuously dilated intercellular spaces; those between thick ascending limb cells are narrow; shortly after the MD the epithelium transforms (arrowheads) to that of the DCT (D); arrow, indicates flow direction; (ea), efferent arteriole. (D) Epon section of mouse MD in contact with granulated rennin‐producing cells (GC) in the afferent glomerular arteriole; the MD cells display abundant microvilli on their luminal surface as well as single cilia (arrows); the intercellular spaces are narrow. Bars ∼ 10 μm.

Organization of the transition from the DCT to the collecting duct in the cortex of rabbit. (A) Connecting tubule (CN) of a superficial nephron, opening into the beginning of a cortical collecting duct (CD) under the renal capsule; the sharp transition in the epithelial lining is indicated by a bar in the lumen. (B) Sharp transition (bar) of DCT (D) to CNT (CN); fusion of several CNTs establishes an arcade that ascends (arrow) in the cortex; note the progressive decrease in height of the CNT epithelium along the CNT. (C) Arcade (CN), ascending in the cortex in close proximity to the cortical radial artery (A), takes up several tributaries. (D) Opening of an arcade (CN) in the upper third of the cortex into a CCD within a medullary ray; the arrows indicate the flow directions; note the proximity of the arcade to an afferent arteriole (white arrow head). (E) Juxtaposition of an arcade (CN) to a lymphatic (L) in the periarterial connective tissue of the cortical radial artery (A). [(A) and (B)] Epon sections of rabbit kidney, (E) of mouse kidney; [(C) and (D)] paraffin sections of rabbit kidney. Bars ∼ 50 μm. [(A) and (B)] [adapted, with permission, from 292]; [(C) and (D)] [adapted, with permission, from 288], (E) [adapted, with permission, from 286].

Survey on organization of the cortical distal segments and collecting ducts (CDs; left panel) and on ultrastructure of the segment‐specific cells (right panel); C ‐ cortex, OS, outer stripe, IS, inner stripe of the outer medulla; IM, inner medulla; dashed line, delimits the medullary ray; (a) distal convoluted tubule (DCT) and (A) DCT‐cell; (b) connecting tubule (CNT) and (B) CNT‐cell; (c) CCD and (C) CCD cell; (d) inner medulla (IM), and (D) IMCD cell; the black semicircles indicate the occurrence of intercalated (IC) cells in the segment; adapted, with permission, from 291.

(A) The thick ascending limb (T) transforms shortly after the macula densa (MD), affixed to the granulated portion of the afferent glomerular arteriole (aa), to the distal convoluted tubule 1 (D); the abrupt change in the epithelial structure (arrowheads) coincides with the marked increase in contrast; the appearance of an intercalated cell (asterisk) in the DCT epithelium indicates the transition (arrowheads) to the transitional segment (D2), which shows a slightly lower density of mitochondria; mouse; epon section; Bar ∼ 10 μm. (B) DCT1 cell in rat; characteristic are the tall mitochondria enclosed in lateral interdigitating cell processes, the apical position and apical flattening of the nucleus and the narrow mitochondria‐free cytoplasmic rim on top of the nucleus, containing many small vesicles; arrowhead, single cilium. Epon section; bar ∼ 1 μm. Adapted, with permission, from 341.

Comparison of salient features of DCT, CNT, and collecting duct segments and cells in the renal cortex. (A) Mouse renal cortex showing side by side a profile of the distal convoluted tubule 2 (D2), of a connecting tubule (CN), and a collecting duct (CCD); the decrease of density in mitochondria, their change in cellular position from basal infoldings to the apical cytoplasm, the decrease in cell height, and the increase in the frequency of intercalated cells (asterisks) in the epithelial lining is evident. Epon section; bar ∼ 10 μm. [(B)‐(D)] Higher magnification of a DCT cell (B), a CNT cell (C), and a CCD cell (D); compare with (A); the arrows point to interdigitating cell processes, narrowly enclosing large mitochondria (B), abundant deep infolding of the basal plasma membrane reaching between tall mitochondria (C) and to infoldings of the basal plasma membrane restricted to a narrow rim in the basal cell portion in the noninterdigitating CCD cell (D); all mitochondria and cell organelles are located above this basal rim. [(B) and (C)] From Psammomys obesus; (D) rat; fixation by reduced osmium. TEM, bar ∼ 1 μm. [(B)‐(D)] Modified, with permission, from 341.

Connecting tubule (CNT) in rat. (A) Longitudinal section through renal cortex, showing profiles of CNT, forming arcades, ascending (arrows) in the cortex in close juxtaposition to the cortical radial vessels (CRV) and glomerular afferent arterioles (aa); bar ∼ 50 μm. (B) The CNT epithelium is composed of CNT cells and intercalated cells (arrows); bar ∼ 10 μm. (C) Characteristic CNT cell with abundant infoldings of the basal plasmalemm extending deeply into the cytoplasm; mitochondria are found between the infolded membranes and in the apical cytoplasm on top of the nucleus; bar ∼ 10 μm. (D) The infolded plasma membranes reveal numerous caveolae; bars ∼ 0.01 μm. Adapted, with permission, from 291.

Cortical [(A) and (B)] and outer medullary [(C) and (D)] collecting duct. (A) Collecting duct in the upper cortex of rat; the epithelium is composed by collecting duct (CD) cells and intercalated cells (IC cells; asterisks); T, thick ascending limb: P, proximal tubule. (B) Cortical collecting duct (CCD) cell; infoldings of the basal plasma membrane are restricted to the basal cell portion; all mitochondria and cell organelles are located above the infolded membranes. (C) Collecting duct in the inner stripe of mouse; the epithelial organization is similar as in the cortex; note the different aspect of IC cells (asterisks); (T) thick ascending limb. (D) Outer medullary conduction duct (OMCD) cell in the inner stripe; the organization is similar as in the CCD; the infoldings of the basal plasmalemm and the amount of mitochondria decrease toward the medulla. Bars: [(A) and (C)] ∼ 10 μm; [(B) and (D)] ∼ 1 μm. [(B) adapted, with permission, from 291 and (D) adapted, with permission, from 341].

Inner medullary collecting duct. (A) Tubular profile of a medullary collecting duct in the transitional zone from the inner stripe of the outer medulla to the inner zone in rabbit; the inner medullary conducting duct (IMCD) cells in the homogenous epithelium possess a dense apical cytoskelettal web under the luminal plasmalemma, indicated by a star in two cells, under the luminal plasmalemm; the cytoplasm reveals a paucity of cell organelles; the big round nucleus has a rather homogeneous chromatin structure. (B) Tip of the renal papilla in rat with openings of papillary CDs (paraffin section) into the renal pelvis; the urothelium of the pelvic wall is seen on the right. (C) Epithelium of the middle portion of an IMCD in rat. Within the epithelium three zones are seen: a basal zone with basal infoldings, a middle zone containing Golgi fields, mitochondria and lysosomal elements, and a thin apical zone with abundant tubular and vesicular profiles. Note the deep tight junction. (D) IMCD cells in the deep inner medulla; the tall cells have a large nucleus and relative to their size few cell organelles are encountered; lateral intercellular spaces (arrow) are filled with microvilli and microfolds; the cytoplasm immediately beneath the apical plasmalemm is devoid of cell organelles. Bars: (A) ∼ 10 μm; (B) ∼ 100 μm; (C) ∼ 1 μm; and (D) ∼ 10 μm. Adapted, with permission, from 341.

Schematic distribution of the major apical transport proteins (NKCC2, black; NCC, green; TRPM6, turquoise; TRPV5, orange; ENaC, red; and AQP2, blue), along the cortical distal segments in rabbit 1, and in rat, mouse, and human 2; MR, medullary ray; TAL, thick ascending limb; G, renal corpuscle; DCT, distal convoluted tubule; CNT, connecting tubule; CCD, cortical collecting duct; restriction of a transporter to a tubular portion is indicated by a thick vertical bars, the continuation along the collecting duct (CD) by arrows. Modified, with permission, from 341.

Transition from the thick ascending limb to the distal convoluted tubule [(A)‐(C)]: rabbit; [(D)‐(F)]: mouse; [(A) and (D)] 1 μm epon sections, all others cryostat sections; [(B) and (E)] immunofluorescence for NKCC2; (C) in situ hybridization for mRNA of SLC12a3 (coding for NCC), (F) immunofluorescence for NCC, same section as E (double fluorescence); in both species the sharp change in morphology is congruent with the abrupt change from NKCC2 to NCC (arrowheads). Bars: ∼ 50 μm. Adapted, with permission, from 385.

Transitional regions from the distal convoluted tubule (D) to the connecting tubule (CN) and the cortical collecting duct (CCD) in rabbit. [(A) and (E)] 1 μm epon sections, all others cryostat sections. (B) In situ hybridization for mRNA of SLC12a3 (coding for NCC). (C) Same section as (B), immunostaining for the β subunit of epithelial sodium channel (ENaC); no overlap of NCC and ENaC, arrow heads point to the definite stop of NCC mRNA and the abrupt beginning of apical ENaC staining; no overlap of NCC and ENaC in the same cell. (D) Immunostaining of a consecutive section for Calbindin k28D (CB); the end of NCC expression and the beginning of apical ENaC coincides with a marked increase of CB staining in the cytoplasm of tubular cells and the appearance of intercalated cells in the tubular epithelium (asterisks). (E) The transition of the CNT to the CCD is marked by the obvious change in the epithelial structure at the opening of the CNT (CN) to the CCD (arrowheads) indicates the transition of the CNT to the CCD. (F) The structural change coincides with a shift of luminal ENaC immunostaining to the cytoplasm (arrowheads); (G) with a sharp reduction of CB immunostaining (arrowheads); and (H) and the clear‐cut onset of luminal AQP2 immunostaining. Bars: ∼ 50 μm. Adapted, with permission, from 380.

Transitional regions from the distal convoluted tubule to the connecting tubule and the cortical collecting duct (CCD); mouse. [(A) and (E)] 1 μm epon sections, all others, cryostat sections. (A) The most upstream occurrence of intercalated cells (IC cells; asterisks) indicates the transition from the DCT1 (D1) to the transitional segment of the DCT, the DCT2 (D2); the frequency of IC cells increases toward and along the CNT (CN), note the marked decrease of epithelial lining along the CNT segment. [(B) and (C)] Double immunostaining for NCC and the β subunit of epithelial sodium channel (ENaC); NCC, and ENaC overlap in the cells of DCT2 (D2); (D) consecutive section, immunostained for the vasopressine‐sensitive water channel aquaporin 2 (AQP2); its onset coincides with the disappearance of NCC expression and defines the beginning of the CNT. (E) Transition from the CNT (CN) to the CCD. [(F) and (G)] Consecutive sections, immunostained for β subunit of ENaC and AQP2; both are coexpressed by the CNT cells as well as by the collecting duct cells (CD). Bars: ∼ 50 μm. Adapted, with permission, from 385 and 380.

Distribution of NCC, the epithelial calcium channel TRPV5 and the sodium calcium exchanger (NCX) along the distal nephron. Cryostat sections; [(A) and (B)] rabbit; [(C)‐(G)] mouse. [(A)‐(D)] Double immunostaining for NCC and TRPV5; [(A) and (B)] in rabbit NCC in the distal convoluted tubule (D) and TRPV5 in the connecting tubule (CN) are exclusive; [(C) and (D)] In mouse, NCC and TRPV5 are coexpressed in the DCT2 (D2), TRPV5 continues in the CNT (CN), whereas NCC is absent in the CNT. [(E) and (F)] Immunofluorescence for TRPV5 is the highest in the apical cytoplasm in DCT2 and decreases progressively and shifts to the cytoplasm along the CNT (CN); the arrow indicates the urinary flow direction; the segmental distribution of the sodium calcium exchanger (NCX) in the basolateral membrane parallels that of TRPV5; weak NCX staining is present in DCT1 (D). [(G) and (H)] TRPV5 and NCX expressions sharply stop at the transition from the CNT to the cortical collecting duct (CCD). Bars ∼ 50 μm. Adapted, with permission, from Loffing et al. 385 and Loffing and Kaissling 380.

Rat cortical collecting duct with two distinct types of intercalated cells (IC cells). Type A IC cells marked with an asterix, type B IC cells with #. Original magnification ∼2200×. Adapted, with permission, from 291.

Scheme of the typical appearance of the different types of intercalated cells (IC cells). (A) Type A IC cell. (B) Type B IC cell. (C) Medullary IC cell. Adapted, with permission, from 291.

Rat intercalated cells (IC cells). (A) Type A IC cell with abundant apical microfolds and many mitochondria. (B) Type B IC cells cell with a rather narrow apical cell pole, a narrow rim of dense cytoplasm exempt of vesicles under the apical plasma membrane, abundant smooth surfaced vesicles in the apical cytoplasm, a huge Golgi complex, and abundant mitochondria along the basolateral plasma membrane. Transmission electron microscopy ∼7000× magnification.

Expression of H⁺‐ATPase subunits in intercalated cells (IC cells): (A) mouse connecting tubule stained with antibodies against the B1 H⁺‐ATPase subunit (red) and the segment cell specific water channel aquaporin 2 (AQP2) (green). B1 subunits are detected in luminal (cells with arrows) and/or basolateral membranes (cell with asterix). Adapted, with permission, from 189. (B) The a4 H⁺‐ATPase subunit is expressed in various nephron segments including the proximal tubule and also in IC cells. Mouse CNT stained with antibodies against the a4 H⁺‐ATPase (red) and the AQP2 water channel labeling segment specific cells (green).

Expression of intercalated cell (IC cell) specific anion exchangers AE1 and pendrin. (A) Mouse outer medullary collecting duct stained against AE1 (red) and aquaporin 2 (AQP2) water channels (green). AE1 shows a basolateral localization in cells negative for AQP2. (B) Colocalization of the E subunit of apical H⁺‐ATPase (green) and basolateral AE1 (red) in rat outer medullary collecting duct IC cells. (C) The anion exchanger pendrin (red) is localized at the luminal pole of non‐type A (i.e., type B and non‐A/non‐B IC cells) IC cells in mouse CNT. Segment specific cells are identified by the presence of the AQP2 water channel. Some cells are negative for pendrin and AQP2 (asterix), presumably type A IC cells.