Comprehensive Physiology Wiley Online Library

Ultrastructure of Vascular Smooth Muscle

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Contractile Apparatus
1.1 Filaments
1.2 Dense Bodies and Surface Patches
1.3 Extracellular and Cell‐to‐Cell Connections: Mechanical Coupling
2 Organelles
2.1 Sarcoplasmic Reticulum
2.2 Surface Vesicles
2.3 Golgi Apparatus and Other Intracellular Organelles
3 Cell Junctions: Electrical and Metabolic Coupling
3.1 Structure
3.2 Function
Figure 1. Figure 1.

Transverse (220 nm thick) section of rabbit portal‐anterior mesentric vein. Thick myosin filaments (large arrow) are surrounded by thin actin filaments (double‐headed arrow). Intermediate 10‐nm filaments (small arrows) are associated with dense bodies (DB) or are in bundles (lower left). 200 kV.

From Somlyo et al.
Figure 2. Figure 2.

Chicken gizzard myosin molecules.

From Elliott et al.
Figure 3. Figure 3.

Stereo pair of electron photomicrographs of 160‐nm thick longitudinal section cut near surface of vascular smooth muscle fiber. When three‐dimensional image is viewed in stereo viewer, a 2.3‐μm long myosin filament (ends marked by arrows) is completely included within the section. Note also, below upper arrow, dense body with associated 10‐nm filaments. Microtubule runs along right side of cell adjacent to surface vesicles. Tannic acid 8% in fixative; lead citrate‐stained section. Tilt ± 10°.

From Ashton, Somlyo, and Somlyo
Figure 4. Figure 4.

Stereo pairs of electron photomicrographs of portions of 4 from a series of 8 serial transverse sections 0.47 μm thick from rabbit portal‐anterior mesenteric veins. Thick filaments present in sections labeled A–G. Group of thick filaments, labeled 1–9, starts in sections and 3. Filaments complete in set of 8 sections, ending in sections 6 and 7. Dense body (II) found in sections and 2. Actin filaments seen in subsequent sections in regions continuous with dense body II. Another dense body (III) continuous throughout 8 sections. Note, particularly in section 4, that, due to oblique orientation, the view on right gives appearance of short ribbons. Profile of filament D becomes very small in section 4 and is absent in subsequent section (not shown), showing taper of myosin filaments. Not stained with lead. View on left, tilt + 15°; view on right, tilt + 5°.

From Ashton, Somlyo, and Somlyo
Figure 5. Figure 5.

Six consecutive single (not stereo) serial 140‐nm thick transverse sections of vascular smooth muscle demonstrating tapering of ends of myosin filaments. Three filaments (B, C, D) appear in sections and 3, and another, A, ends in section 5. Fifth filament, E, appears in section 5. Whereas shanks appear to be up to 20 nm wide (slightly obliquely running filaments presenting oversize profiles), ends taper down to approximately 12 nm (C and D) or smaller (B in section ; A in section 5). Surface dense body (DB) is seen at top of each section. Tannic acid in fixative, 2%; sections stained with lead citrate.

From Ashton, Somlyo, and Somlyo
Figure 6. Figure 6.

Stereo electron photomicrograph of 100‐nm thick transverse section showing a thick filament surrounded by 12 thin filaments. When viewed in stereo 3 cross‐bridges in region of arrow occur at different levels and run between thick and thin filaments. There is suggestion of other cross‐bridges lying close to thick filament. Tannic acid 2% in glutaraldehyde. Section stained with aqueous uranyl acetate and lead citrate. 0°, −7°. 200 kV.

From Ashton, Somlyo, and Somlyo
Figure 7. Figure 7.

Tracing of smooth muscle cell from one transverse 0.47‐μm section from series where thick filaments were followed through 8 consecutive sections. Tracing shows only filaments that begin or end in this section. X, filament first appears; O, filament terminates. Note that filaments tend to appear or terminate in small groups. Shaded areas and dots represent, respectively, dense bodies and 10‐nm filaments.

From Ashton, Somlyo, and Somlyo
Figure 8. Figure 8.

gTransverse section of portion of smooth muscle cell from rabbit portal–anterior mesenteric vein. Muscle was fixed in 2% glutaraldehyde for 30 min, frozen in supercooled Freon 22, sectioned at −110°C, picked up from dry knife onto a drop of frozen sucrose solution, allowed to melt, and negatively stained with 3% ammonium molybdate. This technique excludes effects of osmium and dehydration. Rosettes of thick filaments surrounded by thin filaments are present, as well as a few 10‐nm intermediate filaments, large arrows.

A. V. Somlyo and F. T. Ashton, unpublished observations
Figure 9. Figure 9.

Three stereo pairs of electron photomicrographs of transverse sections of dense bodies from vascular smooth muscle cells, demonstrating association of actin with dense bodies. A and B, dense bodies with some substructure and amorphous material surrounded by 10‐nm filaments. When viewed in stereo, profiles approximately size of actin filaments are visible (small arrows). Sections 100 nm thick stained with aqueous uranyl acetate and lead citrate. Tilt ± 4°. C, dense body is shown with latticelike structure whose spacing is similar to that of actin filaments. Actin size profiles are visible (small arrow), as well as 10‐nm filaments (double arrow). Section 100 nm thick stained with lead citrate. Tilt ± 3.5°.

From Ashton, Somlyo, and Somlyo
Figure 10. Figure 10.

Photomicrograph of 80‐nm thick longitudinal section of 2 smooth muscle cells showing actin filaments (arrows) passing over surface vesicles and attaching to surface dense body (DB). Section stained with lead citrate.

From Ashton, Somlyo, and Somlyo
Figure 11. Figure 11.

Transverse section of part of smooth muscle cell from rabbit main pulmonary artery. Four portions of sarcoplasmic reticulum (SR) approach surface membrane associated with surface vesicles (SV). Dense bodies (DB) alternate with peripheral SR and SV. Some microtubules (arrows) are also present. M, mitochondrion.

From Somlyo and Somlyo
Figure 12. Figure 12.

Freeze‐fracture photo of several smooth muscle cells from small mesenteric artery of guinea pig, showing general longitudinal orientation of rows of surface vesicles (arrows). Typical planes of fracture through cell membrane are shown revealing P faces (with cytoplasm beneath membrane as viewed) and E faces (with extracellular space beneath membrane as viewed). Openings of surface vesicles can be seen as depressions on P face; whereas broken‐off necks of surface vesicles are present on E face. Fractured collagen fibrils (C) are present between smooth muscle cells. Unetched.

From Devine and Rayns
Figure 13. Figure 13.

a, Stereo pair of electron photomicrographs of 50‐nm thick longitudinal section of vascular smooth muscle cell showing actin filaments actually entering dense bodies rather than coursing above or beneath them. This point can only be observed when viewed in stereo. Thin filaments attach to 2 lower portions of dense bodies (DB), and there are several prominent intermediate filaments (arrows) around upper portions. b, Lighter print of lower portion of a. Arrowheads indicate points where parallax measurements were made: when viewed in stereo arrowheads to left indicate thickness of section; those to upper right indicate top and bottom of dense body; lower arrowheads show that the 2 thin filaments measured run into rather than above or below the dense body. Tannic acid 2% in glutaraldehyde. Section stained with 5% uranyl acetate in absolute ethanol and lead citrate. Stereo angle ± 20°; 150 kV.

From Ashton, Somlyo, and Somlyo
Figure 14. Figure 14.

Portions of 2 smooth muscle cells from chicken gizzard incubated before embedding in Epon‐Araldite with anti‐α‐actinin followed by sheep anti‐rabbit IgG conjugated with peroxidase (indirect method). Dark reaction product is located both at periphery of cell (double arrow), corresponding to surface dense bodies and within cytoplasm (single arrow), in regions of dense bodies, ecs, Extracellular space. This section counterstained with lead citrate.

From Schollmeyer et al.
Figure 15. Figure 15.

Transverse section from rabbit main pulmonary artery showing smooth muscle cells between elastic lamellae (el) and collagen (col). Note many cell processes and cell contacts (arrows) within lamellae but not across lamellae. Connective tissue staining is greatly enhanced by 2% tannic acid.

Figure 16. Figure 16.

Longitudinal section from rabbit mesotubarium stretched twice rest length in both directions. Cell‐to‐cell attachments (large arrows) and elastin microfibrils (ef, small arrows) appear to connect smooth muscle cells (see lower 2 cells). col, Collagen. Note prominent staining of basement membrane (bm) and connective tissue due to use of 2% tannic acid following fixation.

Figure 17. Figure 17.

Transverse section of a bundle of smooth muscle fibers of rabbit portal‐anterior mesenteric vein, illustrating regular spacing of thick filaments (large arrows) and the relatively large number of thin myofilaments (small arrows). Several groups of intermediate filaments (some shown by double arrows) associated with dense bodies are present. Elements of sarcoplasmic reticulum (SR), arrow‐heads, occur at periphery of cells. Smooth SR continuous with rough endoplasmic reticulum can be seen in cell in upper left quadrant.

From Somlyo et al.
Figure 18. Figure 18.

Transverse sections through peripheral portions of portal‐anterior mesenteric vein smooth muscle cells incubated for 90 min in Krebs solution containing ferritin. Ferritin is in extracellular space (ecs) and in the surface vesicles (v). No ferritin is in sarcoplasmic reticulum (SR), arrows, that lies between surface vesicles. m, Mitochondrion. Not lead stained.

From Somlyo and Somlyo
Figure 19. Figure 19.

Transverse sections through peripheral portions of portal‐anterior mesenteric vein smooth muscle cells incubated for 90 min in Krebs solution containing ferritin. Ferritin is in extracellular space (ecs) and in the surface vesicles (v). No ferritin is in sarcoplasmic reticulum (SR), arrows, that lies between surface vesicles. m, Mitochondrion. Not lead stained.

From Somlyo and Somlyo
Figure 20. Figure 20.

Section through portal‐anterior mesenteric vein smooth muscle fibers that were exposed to extracellular marker colloidal lanthanum during osmium fixation, after primary fixation with glutaraldehyde. Some lanthanum deposits are in free‐floating vesicles (arrows) not visibly connected to cell membrane; connections are out of plane of section. Sarcoplasmic reticulum does not contain lanthanum and is therefore not in direct communication with extracellular space.

From Somlyo and Somlyo
Figure 21. Figure 21.

Longitudinal sections showing couplings of sarcoplasmic reticulum (SR) with surface membrane. Dense periodic structures (bars) are present across 15–20‐nm junctional gap between SR and the plasma membrane. Tannic acid 2% post osmium; microtubule (arrow).

Figure 22. Figure 22.

Longitudinal sections showing couplings of sarcoplasmic reticulum (SR) with surface membrane. Dense periodic structures (bars) are present across 15–20‐nm junctional gap between SR and the plasma membrane. Tannic acid 2% post osmium; microtubule (arrow).

Figure 23. Figure 23.

High‐magnification view of longitudinal section of portal–anterior mesenteric vein, illustrating surface vesicle (SV) and sarcoplasmic reticulum (SR) relationship. SR forms fenestrated network running between and sometimes encircling (arrows) surface vesicles.

From Somlyo and Somlyo
Figure 24. Figure 24.

Transversely sectioned main pulmonary artery smooth muscle cell with elements of central and peripheral sarcoplasmic reticulum (SR), arrows. Compare more extensive SR in this artery with that of portal vein (Fig. ). el, Elastin; col, collagen.

Figure 25. Figure 25.

Section through portion of smooth muscle cell surface membrane from vena cava of the diamondback turtle, showing several striated vesicles. In some longitudinally sectioned vesicles the striations extend completely across vesicle (arrows). ecs, Extracellular space.

From Somlyo et al.
Figure 26. Figure 26.

Longitudinal section of vascular smooth muscle showing flattened sacs and vesicles of normal Golgi apparatus (G). Rabbit main pulmonary artery incubated for 30 min in normal Krebs solution.

From Somlyo et al.
Figure 27. Figure 27.

Longitudinal section of vascular smooth muscle, showing marked swelling of Golgi system. Vacuolation is at nuclear pole. Rabbit main pulmonary artery incubated for 30 min in X537A, 5 μg/ml.

From Somlyo et al.
Figure 28. Figure 28.

Gap junction between 2 smooth muscle cells from rabbit portal‐anterior mesenteric vein. Gap is stained, giving rise to pentalayered structure. Note cytoplasmic densities in region of gap junction. Tannic acid 2% post osmium. Section stained with lead citrate.

Figure 29. Figure 29.

Gap junction between 2 longitudinally oriented portal‐anterior mesenteric vein smooth muscle cells. Long regions of cell membranes are closely apposed and form a gap junction in one region (arrow). Tannic acid 2% post osmium. Section stained with lead citrate.

Figure 30. Figure 30.

Model of vertebrate gap junction. A, cross‐sectional profile. B, profile after lanthanum staining (section). C, face view after lanthanum staining (section). D, freeze fracture. Junction is fractured in steps. E, diagram of path followed by fracture plane.

From Peracchia
Figure 31. Figure 31.

Interendothelial cleft (arrowhead) from rabbit main pulmonary artery. Tannic acid reacted with osmium has acted as extracellular marker and has penetrated surface vesicles, but has not permeated tight junction (inset, arrow). Both tight junctions and gap junctions (not shown) are found between neighboring endothelial cell membranes . sm.m., Smooth muscle; end., endothelial cell; e.c.s., extracellular space; el, elastin.



Figure 1.

Transverse (220 nm thick) section of rabbit portal‐anterior mesentric vein. Thick myosin filaments (large arrow) are surrounded by thin actin filaments (double‐headed arrow). Intermediate 10‐nm filaments (small arrows) are associated with dense bodies (DB) or are in bundles (lower left). 200 kV.

From Somlyo et al.


Figure 2.

Chicken gizzard myosin molecules.

From Elliott et al.


Figure 3.

Stereo pair of electron photomicrographs of 160‐nm thick longitudinal section cut near surface of vascular smooth muscle fiber. When three‐dimensional image is viewed in stereo viewer, a 2.3‐μm long myosin filament (ends marked by arrows) is completely included within the section. Note also, below upper arrow, dense body with associated 10‐nm filaments. Microtubule runs along right side of cell adjacent to surface vesicles. Tannic acid 8% in fixative; lead citrate‐stained section. Tilt ± 10°.

From Ashton, Somlyo, and Somlyo


Figure 4.

Stereo pairs of electron photomicrographs of portions of 4 from a series of 8 serial transverse sections 0.47 μm thick from rabbit portal‐anterior mesenteric veins. Thick filaments present in sections labeled A–G. Group of thick filaments, labeled 1–9, starts in sections and 3. Filaments complete in set of 8 sections, ending in sections 6 and 7. Dense body (II) found in sections and 2. Actin filaments seen in subsequent sections in regions continuous with dense body II. Another dense body (III) continuous throughout 8 sections. Note, particularly in section 4, that, due to oblique orientation, the view on right gives appearance of short ribbons. Profile of filament D becomes very small in section 4 and is absent in subsequent section (not shown), showing taper of myosin filaments. Not stained with lead. View on left, tilt + 15°; view on right, tilt + 5°.

From Ashton, Somlyo, and Somlyo


Figure 5.

Six consecutive single (not stereo) serial 140‐nm thick transverse sections of vascular smooth muscle demonstrating tapering of ends of myosin filaments. Three filaments (B, C, D) appear in sections and 3, and another, A, ends in section 5. Fifth filament, E, appears in section 5. Whereas shanks appear to be up to 20 nm wide (slightly obliquely running filaments presenting oversize profiles), ends taper down to approximately 12 nm (C and D) or smaller (B in section ; A in section 5). Surface dense body (DB) is seen at top of each section. Tannic acid in fixative, 2%; sections stained with lead citrate.

From Ashton, Somlyo, and Somlyo


Figure 6.

Stereo electron photomicrograph of 100‐nm thick transverse section showing a thick filament surrounded by 12 thin filaments. When viewed in stereo 3 cross‐bridges in region of arrow occur at different levels and run between thick and thin filaments. There is suggestion of other cross‐bridges lying close to thick filament. Tannic acid 2% in glutaraldehyde. Section stained with aqueous uranyl acetate and lead citrate. 0°, −7°. 200 kV.

From Ashton, Somlyo, and Somlyo


Figure 7.

Tracing of smooth muscle cell from one transverse 0.47‐μm section from series where thick filaments were followed through 8 consecutive sections. Tracing shows only filaments that begin or end in this section. X, filament first appears; O, filament terminates. Note that filaments tend to appear or terminate in small groups. Shaded areas and dots represent, respectively, dense bodies and 10‐nm filaments.

From Ashton, Somlyo, and Somlyo


Figure 8.

gTransverse section of portion of smooth muscle cell from rabbit portal–anterior mesenteric vein. Muscle was fixed in 2% glutaraldehyde for 30 min, frozen in supercooled Freon 22, sectioned at −110°C, picked up from dry knife onto a drop of frozen sucrose solution, allowed to melt, and negatively stained with 3% ammonium molybdate. This technique excludes effects of osmium and dehydration. Rosettes of thick filaments surrounded by thin filaments are present, as well as a few 10‐nm intermediate filaments, large arrows.

A. V. Somlyo and F. T. Ashton, unpublished observations


Figure 9.

Three stereo pairs of electron photomicrographs of transverse sections of dense bodies from vascular smooth muscle cells, demonstrating association of actin with dense bodies. A and B, dense bodies with some substructure and amorphous material surrounded by 10‐nm filaments. When viewed in stereo, profiles approximately size of actin filaments are visible (small arrows). Sections 100 nm thick stained with aqueous uranyl acetate and lead citrate. Tilt ± 4°. C, dense body is shown with latticelike structure whose spacing is similar to that of actin filaments. Actin size profiles are visible (small arrow), as well as 10‐nm filaments (double arrow). Section 100 nm thick stained with lead citrate. Tilt ± 3.5°.

From Ashton, Somlyo, and Somlyo


Figure 10.

Photomicrograph of 80‐nm thick longitudinal section of 2 smooth muscle cells showing actin filaments (arrows) passing over surface vesicles and attaching to surface dense body (DB). Section stained with lead citrate.

From Ashton, Somlyo, and Somlyo


Figure 11.

Transverse section of part of smooth muscle cell from rabbit main pulmonary artery. Four portions of sarcoplasmic reticulum (SR) approach surface membrane associated with surface vesicles (SV). Dense bodies (DB) alternate with peripheral SR and SV. Some microtubules (arrows) are also present. M, mitochondrion.

From Somlyo and Somlyo


Figure 12.

Freeze‐fracture photo of several smooth muscle cells from small mesenteric artery of guinea pig, showing general longitudinal orientation of rows of surface vesicles (arrows). Typical planes of fracture through cell membrane are shown revealing P faces (with cytoplasm beneath membrane as viewed) and E faces (with extracellular space beneath membrane as viewed). Openings of surface vesicles can be seen as depressions on P face; whereas broken‐off necks of surface vesicles are present on E face. Fractured collagen fibrils (C) are present between smooth muscle cells. Unetched.

From Devine and Rayns


Figure 13.

a, Stereo pair of electron photomicrographs of 50‐nm thick longitudinal section of vascular smooth muscle cell showing actin filaments actually entering dense bodies rather than coursing above or beneath them. This point can only be observed when viewed in stereo. Thin filaments attach to 2 lower portions of dense bodies (DB), and there are several prominent intermediate filaments (arrows) around upper portions. b, Lighter print of lower portion of a. Arrowheads indicate points where parallax measurements were made: when viewed in stereo arrowheads to left indicate thickness of section; those to upper right indicate top and bottom of dense body; lower arrowheads show that the 2 thin filaments measured run into rather than above or below the dense body. Tannic acid 2% in glutaraldehyde. Section stained with 5% uranyl acetate in absolute ethanol and lead citrate. Stereo angle ± 20°; 150 kV.

From Ashton, Somlyo, and Somlyo


Figure 14.

Portions of 2 smooth muscle cells from chicken gizzard incubated before embedding in Epon‐Araldite with anti‐α‐actinin followed by sheep anti‐rabbit IgG conjugated with peroxidase (indirect method). Dark reaction product is located both at periphery of cell (double arrow), corresponding to surface dense bodies and within cytoplasm (single arrow), in regions of dense bodies, ecs, Extracellular space. This section counterstained with lead citrate.

From Schollmeyer et al.


Figure 15.

Transverse section from rabbit main pulmonary artery showing smooth muscle cells between elastic lamellae (el) and collagen (col). Note many cell processes and cell contacts (arrows) within lamellae but not across lamellae. Connective tissue staining is greatly enhanced by 2% tannic acid.



Figure 16.

Longitudinal section from rabbit mesotubarium stretched twice rest length in both directions. Cell‐to‐cell attachments (large arrows) and elastin microfibrils (ef, small arrows) appear to connect smooth muscle cells (see lower 2 cells). col, Collagen. Note prominent staining of basement membrane (bm) and connective tissue due to use of 2% tannic acid following fixation.



Figure 17.

Transverse section of a bundle of smooth muscle fibers of rabbit portal‐anterior mesenteric vein, illustrating regular spacing of thick filaments (large arrows) and the relatively large number of thin myofilaments (small arrows). Several groups of intermediate filaments (some shown by double arrows) associated with dense bodies are present. Elements of sarcoplasmic reticulum (SR), arrow‐heads, occur at periphery of cells. Smooth SR continuous with rough endoplasmic reticulum can be seen in cell in upper left quadrant.

From Somlyo et al.


Figure 18.

Transverse sections through peripheral portions of portal‐anterior mesenteric vein smooth muscle cells incubated for 90 min in Krebs solution containing ferritin. Ferritin is in extracellular space (ecs) and in the surface vesicles (v). No ferritin is in sarcoplasmic reticulum (SR), arrows, that lies between surface vesicles. m, Mitochondrion. Not lead stained.

From Somlyo and Somlyo


Figure 19.

Transverse sections through peripheral portions of portal‐anterior mesenteric vein smooth muscle cells incubated for 90 min in Krebs solution containing ferritin. Ferritin is in extracellular space (ecs) and in the surface vesicles (v). No ferritin is in sarcoplasmic reticulum (SR), arrows, that lies between surface vesicles. m, Mitochondrion. Not lead stained.

From Somlyo and Somlyo


Figure 20.

Section through portal‐anterior mesenteric vein smooth muscle fibers that were exposed to extracellular marker colloidal lanthanum during osmium fixation, after primary fixation with glutaraldehyde. Some lanthanum deposits are in free‐floating vesicles (arrows) not visibly connected to cell membrane; connections are out of plane of section. Sarcoplasmic reticulum does not contain lanthanum and is therefore not in direct communication with extracellular space.

From Somlyo and Somlyo


Figure 21.

Longitudinal sections showing couplings of sarcoplasmic reticulum (SR) with surface membrane. Dense periodic structures (bars) are present across 15–20‐nm junctional gap between SR and the plasma membrane. Tannic acid 2% post osmium; microtubule (arrow).



Figure 22.

Longitudinal sections showing couplings of sarcoplasmic reticulum (SR) with surface membrane. Dense periodic structures (bars) are present across 15–20‐nm junctional gap between SR and the plasma membrane. Tannic acid 2% post osmium; microtubule (arrow).



Figure 23.

High‐magnification view of longitudinal section of portal–anterior mesenteric vein, illustrating surface vesicle (SV) and sarcoplasmic reticulum (SR) relationship. SR forms fenestrated network running between and sometimes encircling (arrows) surface vesicles.

From Somlyo and Somlyo


Figure 24.

Transversely sectioned main pulmonary artery smooth muscle cell with elements of central and peripheral sarcoplasmic reticulum (SR), arrows. Compare more extensive SR in this artery with that of portal vein (Fig. ). el, Elastin; col, collagen.



Figure 25.

Section through portion of smooth muscle cell surface membrane from vena cava of the diamondback turtle, showing several striated vesicles. In some longitudinally sectioned vesicles the striations extend completely across vesicle (arrows). ecs, Extracellular space.

From Somlyo et al.


Figure 26.

Longitudinal section of vascular smooth muscle showing flattened sacs and vesicles of normal Golgi apparatus (G). Rabbit main pulmonary artery incubated for 30 min in normal Krebs solution.

From Somlyo et al.


Figure 27.

Longitudinal section of vascular smooth muscle, showing marked swelling of Golgi system. Vacuolation is at nuclear pole. Rabbit main pulmonary artery incubated for 30 min in X537A, 5 μg/ml.

From Somlyo et al.


Figure 28.

Gap junction between 2 smooth muscle cells from rabbit portal‐anterior mesenteric vein. Gap is stained, giving rise to pentalayered structure. Note cytoplasmic densities in region of gap junction. Tannic acid 2% post osmium. Section stained with lead citrate.



Figure 29.

Gap junction between 2 longitudinally oriented portal‐anterior mesenteric vein smooth muscle cells. Long regions of cell membranes are closely apposed and form a gap junction in one region (arrow). Tannic acid 2% post osmium. Section stained with lead citrate.



Figure 30.

Model of vertebrate gap junction. A, cross‐sectional profile. B, profile after lanthanum staining (section). C, face view after lanthanum staining (section). D, freeze fracture. Junction is fractured in steps. E, diagram of path followed by fracture plane.

From Peracchia


Figure 31.

Interendothelial cleft (arrowhead) from rabbit main pulmonary artery. Tannic acid reacted with osmium has acted as extracellular marker and has penetrated surface vesicles, but has not permeated tight junction (inset, arrow). Both tight junctions and gap junctions (not shown) are found between neighboring endothelial cell membranes . sm.m., Smooth muscle; end., endothelial cell; e.c.s., extracellular space; el, elastin.

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Avril V. Somlyo. Ultrastructure of Vascular Smooth Muscle. Compr Physiol 2011, Supplement 7: Handbook of Physiology, The Cardiovascular System, Vascular Smooth Muscle: 33-67. First published in print 1980. doi: 10.1002/cphy.cp020202