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Functional Ultrastructure of the Proximal Tubule

Arvid B. Maunsbach, Erik Ilsø Christensen

10.1002/cphy.cp080102

Source: Supplement 25: Handbook of Physiology, Renal Physiology

Originally published: 1992

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Location of Proximal Tubule
2 Fixation of Proximal Tubule Cells for Electron Microscopy
3 Ultrastructure of the Mammalian Proximal Tubule Cell
3.1 General Cytology
3.2 Cell Shape
3.3 Segmentation of the Mammalian Proximal Tubule
3.4 Luminal Cell Surface
3.5 Basal and Lateral Cell Surface
3.6 Junctions
3.7 Nucleus
3.8 Mitochondria
3.9 Endoplasmic Reticulum
3.10 Golgi Apparatus
3.11 Vacuolar Apparatus (Lysosomes and Endocytic Vacuoles)
3.12 Peroxisomes
3.13 Cytoskeleton
3.14 Cytoplasmic Ground Substance and Inclusions
4 Structure of Proximal Tubule Cells in Submammalian Species
4.1 Birds
4.2 Reptiles
4.3 Amphibians
4.4 Fishes
5 Ultrastructure of Experimentally Perfused Tubules
6 Quantitative Ultrastructure of Proximal Tubules
7 Transepithelial Transport Routes
7.1 The Transcellular Route
7.2 The Paracellular Route
8 Functions of the Vacuolar Apparatus
8.1 Protein Absorption by the Proximal Tubule Cells
8.2 Transtubular Transport of Proteins
8.3 Basolateral Binding and Uptake of Proteins
8.4 Endocytosis of Nonproteins
8.5 Autophogocytosis
8.6 Lysosomal Accumulation of Metals and Other Substances
Figure 1. Figure 1.

Diagram of the different zones of mammalian kidney showing relationships of segments of nephrons to zones of kidney.

Adapted from Maunsbach 374
Figure 2. Figure 2.

Electron micrograph of rat kidney cortex showing several proximal tubules with open lumens and evenly arranged brush borders. This micrograph illustrates some differences between the two segments of convoluted part of proximal tubule. In the first segment (S1) the cells are taller, microvilli of brush border longer, and apical endocytic vacuoles more numerous than in the second segment (S2). Lysosomes in the first segment have light contents, whereas those in second segment show densely stained contents. Tissue was fixed by vascular perfusion with glutaraldehyde. × 2,100

From Maunsbach 366
Figure 3. Figure 3.

Electron micrographs of proximal tubule epithelium from rat kidney illustrating the two segments in convoluted part of proximal tubule. A: first segment (S1); B: second segment (S2). In both segments mitochondria (M) are oriented perpendicular to basement membrane, and endocytic vacuoles (E) are numerous. The brush border (BB) is higher in S1 than in S2. Matrix of lysosomes (L) in S1 is lightly stained but rather homogeneous and densely stained in S2. × 8,900.
Figure 4. Figure 4.

Survey pictures of proximal tubule cells from third segment (S3) of rat (A) and rabbit (B). Mitochondria in S3 are more randomly oriented than in S1 and S2, and less numerous in rabbit cells than in rat cells. In both species 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 A, and peroxisomes (P) are seen in the cytoplasm. Capillary, CAP. × 9,800.
Figure 5. Figure 5.

Basal regions of cells from the three segments of rat proximal tubule. A: first segment (S1), B: second segment (S2), and C: third segment (S3). A: notice close apposition of mitochondria (M) to plasma membrane of interdigitating processes and basement membrane ridges (arrows) penetrating between small basal villi. B: no basement membrane ridges are seen penetrating basal villi. Endothelial capillary in lower part of micrograph shows many fenestrations. C, basal plasma membrane shows very few interdigitations or basal villi, × 36,800.
Figure 6. Figure 6.

Diagram of ultrastructure of epithelium in the three segments of rat proximal tubule, illustrating differences in cell shapes and different distributions of mitochondria (M), lysosomes (L), endocytic vacuoles (E), and peroxisomes (microbodies) (P). S1, first segment. S2, second segment. S3, third segment.

From Maunsbach 374
Figure 7. Figure 7.

A: cross‐sectioned brush border projections from cells in convoluted part of rat proximal tubule fixed in situ by vascular perfusion with glutaraldehyde. × 37,000. [From Maunsbach 374.] B: at higher magnification, plasma membrane of microvilli is triple‐layered and has irregular, amorphous cell coat attached to its outer leaflet (arrows). Cross‐sectioned thin filaments are present within the microvilli × 118,000. [From Maunsbach 374.] C: cross‐section of rat proximal tubule cells halfway between lumen and basement membrane, illustrating the complex interdigitation of the cells. Nuclei (N) of two cells are seen to the left and right. The border between these two cells can be followed from asterisk to asterisk (except for short parts where the membranes are obliquely sectioned). The mitochondria are generally cut perpendicular to their long axis and are located very close to plasma membranes along at least part of their periphery. Cytoplasm contains numerous ribosomes arranged predominantly as free polysomes; some are attached to rough‐surfaced endoplasmic reticulum. Golgi apparatus (G) consists of flattened cisternae, some large vacuoles, many small uncoated vesicles, and some small, coated vesicles (arrows), × 35,000. [From Maunsbach 374.] D: rat proximal tubule cells showing numerous small basal villi in section cut parallel with and close to basement membrane, × 30,000.

From Maunsbach 374
Figure 8. Figure 8.

Diagram of three‐dimensional appearance of proximal tubule cell from convoluted part of rat proximal tubule, illustrating appearance of lateral and basal ridges and processes.

Adapted from Maunsbach 374
Figure 9. Figure 9.

Brush borders of two adjacent proximal tubules microperfused simultaneously in the same rat kidney for 5 min but at different flow rates. A: tubule perfused at rate of 45 nl/min and showing separation of microvilli. B: tubule perfused at rate of 5 nl/min and showing more densely packed microvilli, × 25,000

From Maunsbach et al. 386
Figure 10. Figure 10.

A: tight junction (arrowheads) between two rat proximal tubule cells close to tubule lumen (LU). Triple‐layered plasma membranes are completely apposed. Tight junction has a depth in the luminal‐basal direction of only 100–200 A and is followed on its peritubular side by an intermediate junction, × 167,000. [From Maunsbach 366.] B: tangential section through luminal part of proximal tubule cells showing intermediate junction, which extends between two pairs of arrowheads. When junction is sectioned at ight angles, cytoplasm next to the triple‐layered plasma membranes has increased density, and dense material fills gap between the membranes (compare with normal lateral intercellular space at LIS). Lumen (LU) is seen in upper right corner, × 145,000. [From Maunsbach 374.] C: higher magnification of gap junction between proximal tubule cells from Necturus maculosus. The 20 Å space between the two plasma membranes is clearly observable (arrowheads), × 204,000.

From Maunsbach 374
Figure 11. Figure 11.

A and B: parts of proximal tubule cells from rat kidney illustrating smooth‐surfaced endoplasmic reticulum adjacent to 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. Cells illustrated in B were incubated for glucose‐6‐phospha‐tase (dense reaction product, arrows), associated with smooth‐surfaced endoplasmic reticulum, including the PER. A, × 95,000; B, × 34,500.

From Maunsbach 378
Figure 12. Figure 12.

A: part of rat proximal convoluted tubule cell injected by micropuncture into tubule lumen with canonized ferritin 20 sec before fixation. Ferritin appears in small endocytic vesicles or invaginations (EI), but dense apical tubules (AT) generally do not contain ferritin. Large endocytic vacuole connected to several dense apical tubules (arrows) contains ferritin throughout, × 56,000. [From Christensen 98.] B: apical region of rat proximal tubule cells fixed 3 h after start of i.v. infusion of low molecular weight dextran. Dextran is visualized as an electron‐dense precipitate located in large endocytic vacuoles (E). Dextran is absent from dense apical tubules (AT) but is present in some profiles, which represent endocytic invaginations or small endocytic vesicles covered with a clathrin coat (long arrows). Short arrows delineate transverse periodic pattern seen in apical tubules, × 52,000.

From Christensen and Maunsbach 107
Figure 13. Figure 13.

A and B illustrate variable ultrastructural appearances of lysosomes in rat proximal tubule cells. A: electron micrograph showing round lysosome with electron‐dense matrix devoid of inclusions and located close to Golgi apparatus. Lysosome is limited by a triple‐layered membrane. × 49,000. B: electron micrograph showing lysosome of irregular shape with different types of inclusion materials. Lysosomes of this type presumably represent old lysosomes, or telolysosomes. × 34,000. [From Maunsbach 370.] C: lysosomes in rat proximal tubule cells demonstrated by cytochemistry. Electron‐dense precipitate was formed during incubation for acid phosphatase and is located in lysosomes of different ultrastructure. × 48,000.

From Maunsbach 370
Figure 14. Figure 14.

Electron micrographs illustrating cytoplasmic components presumed to be involved in autophagocytosis in proximal tubule cells. A: horseshoe‐shaped structure consisting of two membranes similar to those of endoplasmic reticulum and closely apposed to form a five‐layered structure. This structure is probably in the process of enclosing part of the cytoplasm, × 98,000. [From Maunsbach 370.] B: cytoplasmic body enclosed by two membranes apposed to a five‐layered structure. This body is interpreted as newly formed autophagosome that encloses part of the cytoplasm, × 80,000. [From Maunsbach 374.] C: cytoplasmic body limited by single membrane and containing different inclusions, including an altered mitochondrion. This organelle represents an autolysosome. × 45,000.

From Maunsbach 374
Figure 15. Figure 15.

A, Peroxisomes (P) (microbodies) from rat proximal tubule cell limited by single membrane and showing uniformly stained matrix. Smooth‐surfaced endoplasmic reticulum closely surrounds peroxisomes and is seen in continuity with rough endoplasmic reticulum at arrow, × 44,000. B: Peroxisomes (P) in cells of isolated perfused proximal tubule of rabbit. In this species part of limiting membrane is straight, thickened, and electron‐dense. Limiting membrane is closely related to smooth endoplasmic reticulum. Since this tubule was incubated with horseradish peroxidase (HRP) in the bath, intercellular spaces, except in gap junction (GJ), are filled with reaction product for HRP. Reaction product is also seen in small endocytic vesicle (V). × 45,000.

B from Nielsen and Christensen 427
Figure 16. Figure 16.

A: longitudinally sectioned microvilli from rat proximal tubule showing thin actin filaments inside microvilli (arrows). × 90,000. B: thin section cut almost parallel to basement membrane (BM) of proximal tubule cells and demonstrating a bundle of thin actin filaments (arrows), × 160,000.

From Maunsbach 374
Figure 17. Figure 17.

A: proximal tubule wall of Ambystoma tigrinum. The peritubular cell membrane exhibits pronounced surface amplification due to numerous small cytoplasmic folds that project into the basal extracellular labyrinth (BEL). Small cytoplasmic folds also project into lateral intercellular space (LIS) but in smaller numbers. Notice that cytoplasmic processes (*), like pillars, reach down to basement membrane (BM). Mitochondria (M) and lysosomes (L) are predominantly present in middle regions of cytoplasm. Some mitochondria are also located close to basolateral membrane but not in small basal membrane folds. CAP, peritubular capillary, × 6,200. B: basal part of proximal tubule cell in Ambystoma showing basal extracellular labyrinth (BEL). Thin cytoplasmic folds (bold arrows) project from all directions into BEL. Footlike cytoplasmic processes (F) separate BEL from peritubular space (PS), but several openings devoid of junctions (light vertical arrows) are observed between these processes. Mitochondria (M) are located close to BEL but never in cytoplasmic folds. Endothelium of peritubular capillary (CAP) shows some fenestrae (arrowheads), × 24,100.

From Maunsbach and Boulpaep 382
Figure 18. Figure 18.

Electron micrographs of isolated proximal tubules from Ambystoma tigrinum (outer tubule) and rabbit perfused in vitro. Tubules are shown at same magnification (x 1,800) and mounted concentrically to illustrate dimensional differences between tubules from the two species. Both tubules show well‐preserved ultrastructure.

Ambystoma tubule from Tripathi et al. 613; rabbit tubule from Nielsen and Christensen 426
Figure 19. Figure 19.

Part of isolated proximal convoluted tubule from rabbit perfused in vitro for 60 min before fixation. Cells show normal architecture with mitochondria oriented perpendicular to basement membrane (BM), endocytic vacuoles (E), and lysosomes (L), regular brush border (BB), and basement membrane (BM). × 16,600.

From Nielsen and Christensen 426
Figure 20. Figure 20.

Diagram illustrating possible transepithelial transport pathways for water and solutes in proximal tubules of (A) mammals, where cells interdigitate extensively; (B) Necturus maculosus, where the basolateral plasma membrane shows only moderate surface amplifications; and (C) Ambystoma tigrinum, where basal part of basolateral membrane is greatly enlarged. Arrows indicate transcellular pathways, that are either translateral 1 or transbasal 2, and a paracellular pathway 3. BB, brush border; TJ, tight junction; LIS, lateral intercellular space; BV, basal villi; BM, basement membrane.
Figure 21. Figure 21.

A: part of the tubule wall in proximal tubule from Necturus maculosus. Oil block was microinjected into most proximal part of tubule, thus creating reversed hydrostatic pressure gradient across epithelium with low luminal pressure. Cells have increased height, and lateral intercellular spaces (arrows) are wider than they are in free‐flow tubules, × 3,900. B: tubule wall from Necturus proximal tubule with increased intraluminal pressure due to oil block in distal part of proximal tubule. Cells are somewhat flattened, as are nuclei, and lateral intercellular spaces (arrows) appear decreased in width as compared to spaces in free‐flow tubules with lower intraluminal pressure, × 3,500.

From Maunsbach and Boulpaep 380
Figure 22. Figure 22.

A: part of tubule wall of isolated, perfused proximal tubule from Ambystoma tigrinum. Tubule was perfused in vitro with substrate‐free Ringer in the lumen and hyperosmolar Ringer in the bath, which creates osmotic water flow from lumen to bath. Lateral intercellular spaces (arrows) are more dilated than in controls. × 5,400. B: parts of tubule wall of isolated, perfused proximal tubule from Ambystoma tigrinum. Tubule was perfused with hyperosmolar Ringer in lumen and substrate Ringer in the bath, which creates osmotic water flow from bath to lumen. Lateral intercellular spaces (arrows) are much narrower than in control tubules, × 5,400.

From Maunsbach et al. 391
Figure 23. Figure 23.

Electron microscope autoradiograph of rat proximal tubule cells that were fixed 10 min after microperfusion of homologous 125I‐labeled albumin into tubule lumen (LU). Location of absorbed 125I‐labeled albumin is indicated by irregular silver grains present in apical region of cells and predominantly located over large and small apical endocytic vacuoles. Lysosomes and other cytoplasmic structures deeper in cells are not labeled, × 15,000.

From Maunsbach 365
Figure 24. Figure 24.

A: part of rat proximal convoluted tubule microperfused with cationized ferritin 20 sec before fixation. Ferritin molecules are attached to microvilli (MV) and present in endocytic invaginations (EI). Ferritin is not seen in intercellular space (LIS) beyond tight junction (TJ) or in dense apical tubules (arrow). × 60,000. [From Christensen 98.] B: apical part of proximal tubule cell from kidney fixed 5 min after intravenous injection of 125I‐labeled cytochrome c. Labeled protein is present in an endocytic vacuole and appears preferentially associated with inner side of its limiting membrane, × 36,500. [From Christensen and Maunsbach 105.] C: electron microscope autoradiograph of 125I‐labeled insulin uptake in rat proximal tubule, fixed 25 min after start of microinfusion of insulin. Grains are mainly located over lysosomes in apical part of cytoplasm (arrows). Endocytic vacuoles (E) and some lysosomes (L) located deep in the cytoplasm do not contain [125I]insulin. × 16,000.

From Hellfritzsch et al. 239
Figure 25. Figure 25.

Thin section from segment S2 of rabbit proximal tubule perfused in vitro with native insulin at a concentration of 58 ng/ml for 30 min, fixed in glutaraldehyde, and embedded at low temperature in Lowicryl K4M. The section was incubated first with insulin antiserum and then protein A‐coated gold particles. Gold particles (6 nm in diameter) are located over an endocytic vacuole (arrows) and a lysosome (L). × 60,000.

From Nielsen and Christensen 430
Figure 26. Figure 26.

Isolated, perfused proximal tubule of rabbit exposed to both canonized ferritin (CF) and HRP in the perfusate for 30 min and then fixed after 30 min chase period. Clusters of ferritin‐like particles are visible in basolateral intercellular spaces (arrows). Vacuoles (V) with CF and HRP reaction product are seen not far from intercellular spaces. Lysosome (L) contains both CF and HRP reaction product. B, bath. Unstained section, × 48,000.

From Nielsen et al. 428
Figure 27. Figure 27.

Immunocytochemical localization of folic acid binding protein in cryoultramicrotomy section of rat renal proximal tubule. The protein is localized to the limiting membrane of an endocytic vacuole (E) and in dense apical tubules (arrows) with the aid of a polyclonal antibody that is detected by means of 10 nm gold particles covered with protein A. × 60,000.
Figure 28. Figure 28.

A: isolated perfused proximal tubule of rabbit, incubated with cationized ferritin in the bath for 30 min. Cationized ferritin molecules have penetrated the basement membrane (BM) into the intercellular spaces (arrow) and are present in an endocytic invagination (EI), × 100,000. B: isolated perfused proximal tubule of rabbit incubated with horseradish peroxidase in the bath for 30 min. Reaction product is seen in lateral intercellular spaces (LIS) but not in lumen (LU). Note reaction product in a multivesicular body (MVB) and in lysosomes (L). Unstained section, × 15,000.

From Nielsen and Christensen 427
Figure 29. Figure 29.

Demonstration of dextran in rat proximal tubule cells 1 h after termination of intravenous infusion of dextran T‐40. Kidney was fixed in vivo by dripping a lead‐containing fixative onto kidney surface. Small dense particles representing dextran are present in tubule lumen (arrowheads), in endocytic vacuoles (vertical arrows), and in lysosomes (horizontal arrows), where dextran is located adjacent to dense matrix material. A peroxisome (P) with a fingerlike projection is well stained by the lead‐containing fixative, × 15,800.

From Christensen and Maunsbach 104
Figure 30. Figure 30.

Diagram illustrating relationships between different components of the vacuolar apparatus in proximal tubule cells. Unbroken arrows designate proven pathways and broken arrows indicate routes suggested by indirect evidence. Merging arrows indicate fusion of structures. The heterophagic pathway is shown in right part of diagram and begins with formation of endocytic vesicles from invaginations of luminal plasma membrane. Invaginations are coated with clathrin (dots) and take up protein molecules (smaller peptides represented by crosses, larger proteins by asterisks) from the tubule lumen and pinch off to form small vesicles. The latter then fuse to larger endocytic vacuoles, which transfer protein to lysosomes for digestion. Membrane of endocytic vacuoles is probably recycled back to apical cell membranes as dense apical tubules. Small amounts of protein may be carried to lateral intercellular spaces (transcytosis). Small molecules, in particular amino acids, formed as result of intralysosomal digestion, presumably diffuse through the lysosome membrane into cytoplasm and may in part pass the basolateral plasma membrane. Heterolysosomes have a long half‐life and fuse with many endocytic vacuoles, including a small number of vesicles derived from basolateral endocytosis, until they are eventually worn out and set aside as telolysosomes and finally extruded from the cell into the tubule lumen. Acid hydrolases are fed continuously into the heterolysosomes through Golgi vesicles, which represent primary lysosomes. Probable relationship between autophagic components of the vacuolar apparatus is shown in lower left part of diagram. Smooth‐surfaced endoplasmic reticulum probably participates in initial stages of formation of autolysosomes. Degradation of small peptides takes place at brush border by means of peptidases localized in brush border cell membrane.

Adapted from Maunsbach 374


Figure 1.

Diagram of the different zones of mammalian kidney showing relationships of segments of nephrons to zones of kidney.

Adapted from Maunsbach 374


Figure 2.

Electron micrograph of rat kidney cortex showing several proximal tubules with open lumens and evenly arranged brush borders. This micrograph illustrates some differences between the two segments of convoluted part of proximal tubule. In the first segment (S1) the cells are taller, microvilli of brush border longer, and apical endocytic vacuoles more numerous than in the second segment (S2). Lysosomes in the first segment have light contents, whereas those in second segment show densely stained contents. Tissue was fixed by vascular perfusion with glutaraldehyde. × 2,100

From Maunsbach 366


Figure 3.

Electron micrographs of proximal tubule epithelium from rat kidney illustrating the two segments in convoluted part of proximal tubule. A: first segment (S1); B: second segment (S2). In both segments mitochondria (M) are oriented perpendicular to basement membrane, and endocytic vacuoles (E) are numerous. The brush border (BB) is higher in S1 than in S2. Matrix of lysosomes (L) in S1 is lightly stained but rather homogeneous and densely stained in S2. × 8,900.



Figure 4.

Survey pictures of proximal tubule cells from third segment (S3) of rat (A) and rabbit (B). Mitochondria in S3 are more randomly oriented than in S1 and S2, and less numerous in rabbit cells than in rat cells. In both species 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 A, and peroxisomes (P) are seen in the cytoplasm. Capillary, CAP. × 9,800.



Figure 5.

Basal regions of cells from the three segments of rat proximal tubule. A: first segment (S1), B: second segment (S2), and C: third segment (S3). A: notice close apposition of mitochondria (M) to plasma membrane of interdigitating processes and basement membrane ridges (arrows) penetrating between small basal villi. B: no basement membrane ridges are seen penetrating basal villi. Endothelial capillary in lower part of micrograph shows many fenestrations. C, basal plasma membrane shows very few interdigitations or basal villi, × 36,800.



Figure 6.

Diagram of ultrastructure of epithelium in the three segments of rat proximal tubule, illustrating differences in cell shapes and different distributions of mitochondria (M), lysosomes (L), endocytic vacuoles (E), and peroxisomes (microbodies) (P). S1, first segment. S2, second segment. S3, third segment.

From Maunsbach 374


Figure 7.

A: cross‐sectioned brush border projections from cells in convoluted part of rat proximal tubule fixed in situ by vascular perfusion with glutaraldehyde. × 37,000. [From Maunsbach 374.] B: at higher magnification, plasma membrane of microvilli is triple‐layered and has irregular, amorphous cell coat attached to its outer leaflet (arrows). Cross‐sectioned thin filaments are present within the microvilli × 118,000. [From Maunsbach 374.] C: cross‐section of rat proximal tubule cells halfway between lumen and basement membrane, illustrating the complex interdigitation of the cells. Nuclei (N) of two cells are seen to the left and right. The border between these two cells can be followed from asterisk to asterisk (except for short parts where the membranes are obliquely sectioned). The mitochondria are generally cut perpendicular to their long axis and are located very close to plasma membranes along at least part of their periphery. Cytoplasm contains numerous ribosomes arranged predominantly as free polysomes; some are attached to rough‐surfaced endoplasmic reticulum. Golgi apparatus (G) consists of flattened cisternae, some large vacuoles, many small uncoated vesicles, and some small, coated vesicles (arrows), × 35,000. [From Maunsbach 374.] D: rat proximal tubule cells showing numerous small basal villi in section cut parallel with and close to basement membrane, × 30,000.

From Maunsbach 374


Figure 8.

Diagram of three‐dimensional appearance of proximal tubule cell from convoluted part of rat proximal tubule, illustrating appearance of lateral and basal ridges and processes.

Adapted from Maunsbach 374


Figure 9.

Brush borders of two adjacent proximal tubules microperfused simultaneously in the same rat kidney for 5 min but at different flow rates. A: tubule perfused at rate of 45 nl/min and showing separation of microvilli. B: tubule perfused at rate of 5 nl/min and showing more densely packed microvilli, × 25,000

From Maunsbach et al. 386


Figure 10.

A: tight junction (arrowheads) between two rat proximal tubule cells close to tubule lumen (LU). Triple‐layered plasma membranes are completely apposed. Tight junction has a depth in the luminal‐basal direction of only 100–200 A and is followed on its peritubular side by an intermediate junction, × 167,000. [From Maunsbach 366.] B: tangential section through luminal part of proximal tubule cells showing intermediate junction, which extends between two pairs of arrowheads. When junction is sectioned at ight angles, cytoplasm next to the triple‐layered plasma membranes has increased density, and dense material fills gap between the membranes (compare with normal lateral intercellular space at LIS). Lumen (LU) is seen in upper right corner, × 145,000. [From Maunsbach 374.] C: higher magnification of gap junction between proximal tubule cells from Necturus maculosus. The 20 Å space between the two plasma membranes is clearly observable (arrowheads), × 204,000.

From Maunsbach 374


Figure 11.

A and B: parts of proximal tubule cells from rat kidney illustrating smooth‐surfaced endoplasmic reticulum adjacent to 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. Cells illustrated in B were incubated for glucose‐6‐phospha‐tase (dense reaction product, arrows), associated with smooth‐surfaced endoplasmic reticulum, including the PER. A, × 95,000; B, × 34,500.

From Maunsbach 378


Figure 12.

A: part of rat proximal convoluted tubule cell injected by micropuncture into tubule lumen with canonized ferritin 20 sec before fixation. Ferritin appears in small endocytic vesicles or invaginations (EI), but dense apical tubules (AT) generally do not contain ferritin. Large endocytic vacuole connected to several dense apical tubules (arrows) contains ferritin throughout, × 56,000. [From Christensen 98.] B: apical region of rat proximal tubule cells fixed 3 h after start of i.v. infusion of low molecular weight dextran. Dextran is visualized as an electron‐dense precipitate located in large endocytic vacuoles (E). Dextran is absent from dense apical tubules (AT) but is present in some profiles, which represent endocytic invaginations or small endocytic vesicles covered with a clathrin coat (long arrows). Short arrows delineate transverse periodic pattern seen in apical tubules, × 52,000.

From Christensen and Maunsbach 107


Figure 13.

A and B illustrate variable ultrastructural appearances of lysosomes in rat proximal tubule cells. A: electron micrograph showing round lysosome with electron‐dense matrix devoid of inclusions and located close to Golgi apparatus. Lysosome is limited by a triple‐layered membrane. × 49,000. B: electron micrograph showing lysosome of irregular shape with different types of inclusion materials. Lysosomes of this type presumably represent old lysosomes, or telolysosomes. × 34,000. [From Maunsbach 370.] C: lysosomes in rat proximal tubule cells demonstrated by cytochemistry. Electron‐dense precipitate was formed during incubation for acid phosphatase and is located in lysosomes of different ultrastructure. × 48,000.

From Maunsbach 370


Figure 14.

Electron micrographs illustrating cytoplasmic components presumed to be involved in autophagocytosis in proximal tubule cells. A: horseshoe‐shaped structure consisting of two membranes similar to those of endoplasmic reticulum and closely apposed to form a five‐layered structure. This structure is probably in the process of enclosing part of the cytoplasm, × 98,000. [From Maunsbach 370.] B: cytoplasmic body enclosed by two membranes apposed to a five‐layered structure. This body is interpreted as newly formed autophagosome that encloses part of the cytoplasm, × 80,000. [From Maunsbach 374.] C: cytoplasmic body limited by single membrane and containing different inclusions, including an altered mitochondrion. This organelle represents an autolysosome. × 45,000.

From Maunsbach 374


Figure 15.

A, Peroxisomes (P) (microbodies) from rat proximal tubule cell limited by single membrane and showing uniformly stained matrix. Smooth‐surfaced endoplasmic reticulum closely surrounds peroxisomes and is seen in continuity with rough endoplasmic reticulum at arrow, × 44,000. B: Peroxisomes (P) in cells of isolated perfused proximal tubule of rabbit. In this species part of limiting membrane is straight, thickened, and electron‐dense. Limiting membrane is closely related to smooth endoplasmic reticulum. Since this tubule was incubated with horseradish peroxidase (HRP) in the bath, intercellular spaces, except in gap junction (GJ), are filled with reaction product for HRP. Reaction product is also seen in small endocytic vesicle (V). × 45,000.

B from Nielsen and Christensen 427


Figure 16.

A: longitudinally sectioned microvilli from rat proximal tubule showing thin actin filaments inside microvilli (arrows). × 90,000. B: thin section cut almost parallel to basement membrane (BM) of proximal tubule cells and demonstrating a bundle of thin actin filaments (arrows), × 160,000.

From Maunsbach 374


Figure 17.

A: proximal tubule wall of Ambystoma tigrinum. The peritubular cell membrane exhibits pronounced surface amplification due to numerous small cytoplasmic folds that project into the basal extracellular labyrinth (BEL). Small cytoplasmic folds also project into lateral intercellular space (LIS) but in smaller numbers. Notice that cytoplasmic processes (*), like pillars, reach down to basement membrane (BM). Mitochondria (M) and lysosomes (L) are predominantly present in middle regions of cytoplasm. Some mitochondria are also located close to basolateral membrane but not in small basal membrane folds. CAP, peritubular capillary, × 6,200. B: basal part of proximal tubule cell in Ambystoma showing basal extracellular labyrinth (BEL). Thin cytoplasmic folds (bold arrows) project from all directions into BEL. Footlike cytoplasmic processes (F) separate BEL from peritubular space (PS), but several openings devoid of junctions (light vertical arrows) are observed between these processes. Mitochondria (M) are located close to BEL but never in cytoplasmic folds. Endothelium of peritubular capillary (CAP) shows some fenestrae (arrowheads), × 24,100.

From Maunsbach and Boulpaep 382


Figure 18.

Electron micrographs of isolated proximal tubules from Ambystoma tigrinum (outer tubule) and rabbit perfused in vitro. Tubules are shown at same magnification (x 1,800) and mounted concentrically to illustrate dimensional differences between tubules from the two species. Both tubules show well‐preserved ultrastructure.

Ambystoma tubule from Tripathi et al. 613; rabbit tubule from Nielsen and Christensen 426


Figure 19.

Part of isolated proximal convoluted tubule from rabbit perfused in vitro for 60 min before fixation. Cells show normal architecture with mitochondria oriented perpendicular to basement membrane (BM), endocytic vacuoles (E), and lysosomes (L), regular brush border (BB), and basement membrane (BM). × 16,600.

From Nielsen and Christensen 426


Figure 20.

Diagram illustrating possible transepithelial transport pathways for water and solutes in proximal tubules of (A) mammals, where cells interdigitate extensively; (B) Necturus maculosus, where the basolateral plasma membrane shows only moderate surface amplifications; and (C) Ambystoma tigrinum, where basal part of basolateral membrane is greatly enlarged. Arrows indicate transcellular pathways, that are either translateral 1 or transbasal 2, and a paracellular pathway 3. BB, brush border; TJ, tight junction; LIS, lateral intercellular space; BV, basal villi; BM, basement membrane.



Figure 21.

A: part of the tubule wall in proximal tubule from Necturus maculosus. Oil block was microinjected into most proximal part of tubule, thus creating reversed hydrostatic pressure gradient across epithelium with low luminal pressure. Cells have increased height, and lateral intercellular spaces (arrows) are wider than they are in free‐flow tubules, × 3,900. B: tubule wall from Necturus proximal tubule with increased intraluminal pressure due to oil block in distal part of proximal tubule. Cells are somewhat flattened, as are nuclei, and lateral intercellular spaces (arrows) appear decreased in width as compared to spaces in free‐flow tubules with lower intraluminal pressure, × 3,500.

From Maunsbach and Boulpaep 380


Figure 22.

A: part of tubule wall of isolated, perfused proximal tubule from Ambystoma tigrinum. Tubule was perfused in vitro with substrate‐free Ringer in the lumen and hyperosmolar Ringer in the bath, which creates osmotic water flow from lumen to bath. Lateral intercellular spaces (arrows) are more dilated than in controls. × 5,400. B: parts of tubule wall of isolated, perfused proximal tubule from Ambystoma tigrinum. Tubule was perfused with hyperosmolar Ringer in lumen and substrate Ringer in the bath, which creates osmotic water flow from bath to lumen. Lateral intercellular spaces (arrows) are much narrower than in control tubules, × 5,400.

From Maunsbach et al. 391


Figure 23.

Electron microscope autoradiograph of rat proximal tubule cells that were fixed 10 min after microperfusion of homologous 125I‐labeled albumin into tubule lumen (LU). Location of absorbed 125I‐labeled albumin is indicated by irregular silver grains present in apical region of cells and predominantly located over large and small apical endocytic vacuoles. Lysosomes and other cytoplasmic structures deeper in cells are not labeled, × 15,000.

From Maunsbach 365


Figure 24.

A: part of rat proximal convoluted tubule microperfused with cationized ferritin 20 sec before fixation. Ferritin molecules are attached to microvilli (MV) and present in endocytic invaginations (EI). Ferritin is not seen in intercellular space (LIS) beyond tight junction (TJ) or in dense apical tubules (arrow). × 60,000. [From Christensen 98.] B: apical part of proximal tubule cell from kidney fixed 5 min after intravenous injection of 125I‐labeled cytochrome c. Labeled protein is present in an endocytic vacuole and appears preferentially associated with inner side of its limiting membrane, × 36,500. [From Christensen and Maunsbach 105.] C: electron microscope autoradiograph of 125I‐labeled insulin uptake in rat proximal tubule, fixed 25 min after start of microinfusion of insulin. Grains are mainly located over lysosomes in apical part of cytoplasm (arrows). Endocytic vacuoles (E) and some lysosomes (L) located deep in the cytoplasm do not contain [125I]insulin. × 16,000.

From Hellfritzsch et al. 239


Figure 25.

Thin section from segment S2 of rabbit proximal tubule perfused in vitro with native insulin at a concentration of 58 ng/ml for 30 min, fixed in glutaraldehyde, and embedded at low temperature in Lowicryl K4M. The section was incubated first with insulin antiserum and then protein A‐coated gold particles. Gold particles (6 nm in diameter) are located over an endocytic vacuole (arrows) and a lysosome (L). × 60,000.

From Nielsen and Christensen 430


Figure 26.

Isolated, perfused proximal tubule of rabbit exposed to both canonized ferritin (CF) and HRP in the perfusate for 30 min and then fixed after 30 min chase period. Clusters of ferritin‐like particles are visible in basolateral intercellular spaces (arrows). Vacuoles (V) with CF and HRP reaction product are seen not far from intercellular spaces. Lysosome (L) contains both CF and HRP reaction product. B, bath. Unstained section, × 48,000.

From Nielsen et al. 428


Figure 27.

Immunocytochemical localization of folic acid binding protein in cryoultramicrotomy section of rat renal proximal tubule. The protein is localized to the limiting membrane of an endocytic vacuole (E) and in dense apical tubules (arrows) with the aid of a polyclonal antibody that is detected by means of 10 nm gold particles covered with protein A. × 60,000.



Figure 28.

A: isolated perfused proximal tubule of rabbit, incubated with cationized ferritin in the bath for 30 min. Cationized ferritin molecules have penetrated the basement membrane (BM) into the intercellular spaces (arrow) and are present in an endocytic invagination (EI), × 100,000. B: isolated perfused proximal tubule of rabbit incubated with horseradish peroxidase in the bath for 30 min. Reaction product is seen in lateral intercellular spaces (LIS) but not in lumen (LU). Note reaction product in a multivesicular body (MVB) and in lysosomes (L). Unstained section, × 15,000.

From Nielsen and Christensen 427


Figure 29.

Demonstration of dextran in rat proximal tubule cells 1 h after termination of intravenous infusion of dextran T‐40. Kidney was fixed in vivo by dripping a lead‐containing fixative onto kidney surface. Small dense particles representing dextran are present in tubule lumen (arrowheads), in endocytic vacuoles (vertical arrows), and in lysosomes (horizontal arrows), where dextran is located adjacent to dense matrix material. A peroxisome (P) with a fingerlike projection is well stained by the lead‐containing fixative, × 15,800.

From Christensen and Maunsbach 104


Figure 30.

Diagram illustrating relationships between different components of the vacuolar apparatus in proximal tubule cells. Unbroken arrows designate proven pathways and broken arrows indicate routes suggested by indirect evidence. Merging arrows indicate fusion of structures. The heterophagic pathway is shown in right part of diagram and begins with formation of endocytic vesicles from invaginations of luminal plasma membrane. Invaginations are coated with clathrin (dots) and take up protein molecules (smaller peptides represented by crosses, larger proteins by asterisks) from the tubule lumen and pinch off to form small vesicles. The latter then fuse to larger endocytic vacuoles, which transfer protein to lysosomes for digestion. Membrane of endocytic vacuoles is probably recycled back to apical cell membranes as dense apical tubules. Small amounts of protein may be carried to lateral intercellular spaces (transcytosis). Small molecules, in particular amino acids, formed as result of intralysosomal digestion, presumably diffuse through the lysosome membrane into cytoplasm and may in part pass the basolateral plasma membrane. Heterolysosomes have a long half‐life and fuse with many endocytic vacuoles, including a small number of vesicles derived from basolateral endocytosis, until they are eventually worn out and set aside as telolysosomes and finally extruded from the cell into the tubule lumen. Acid hydrolases are fed continuously into the heterolysosomes through Golgi vesicles, which represent primary lysosomes. Probable relationship between autophagic components of the vacuolar apparatus is shown in lower left part of diagram. Smooth‐surfaced endoplasmic reticulum probably participates in initial stages of formation of autolysosomes. Degradation of small peptides takes place at brush border by means of peptidases localized in brush border cell membrane.

Adapted from Maunsbach 374
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Article Title: Functional Ultrastructure of the Proximal Tubule
 

How to Cite

Arvid B. Maunsbach, Erik Ilsø Christensen. Functional Ultrastructure of the Proximal Tubule. Compr Physiol 2011, Supplement 25: Handbook of Physiology, Renal Physiology: 41-107. First published in print 1992. doi: 10.1002/cphy.cp080102