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Architecture of the Vessel Wall

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Abstract

The sections in this article are:

1 Classification and Definition of Blood Vessels
2 Components of the Vascular Wall
2.1 Intima
2.2 Media
2.3 Adventitia
Figure 1. Figure 1.

Schematic drawing summarizing major structural characteristics of principal segments of blood vessels in mammals. It is intended to serve as a general guide to the various components of the vascular wall that are analyzed and discussed in this chapter. ρ, Diameter.

Figure 2. Figure 2.

General appearance of luminal surface of vascular endothelium in arteries. Endothelial cells are elongated and their long axis is parallel to that of the blood vessel. Boxed‐in area is detailed in Figure .

Figure 3. Figure 3.

Endothelial junctional areas. A: endothelium from abdominal aorta of squirrel monkey. × 111,000. B and C: endothelium of subepithelial connective tissue capillaries of ureter in the rat. B, × 70,000; C, × 72,000. Vascular lumen (1) is above endothelial cells (2) and basal lamina (3). In the large blood vessels, endothelial cells often show flaplike overlappings (4). Punctate fusions (5) form basis for tight junction, whereas a polygonal lattice of subunits (6) establishes the gap junction. Of the pinocytic (surface) vesicles, some are connected to the luminal cell membrane (7), some to the abluminal cell membrane (8), and some to the junctional cell membrane (9). Many vesicles occur freely in the cytoplasm (10). Other organelles of endothelial cells are cytoplasmic filaments (11), microtubules (12), and ribosomes (13).

Figure 4. Figure 4.

Endothelial vesicles. Pinocytic vesicles of endothelial cells occur in large numbers as seen in this micrograph in which aortic endothelium of squirrel monkey was sectioned in a plane parallel to the basal lamina. The level of the section is just above the abluminal cell membrane. Area of square is 1 μm2; it contains about 130 vesicles. Diagonal dense structure in top left corner is a bundle of cytoplasmic filaments near base of endothelial cell. × 74,000.

Figure 5. Figure 5.

Summary of general appearance of the several types of cell junctions between endothelial cells as seen in sectioned material. The most commonly used markers for testing transendothelial transport in electron microscopy are indicated in box together with approximate molecular diameter (ρ). Tight junction appears as punctate fusions of outer leaflets of apposed cell membranes, and is believed by most investigators to allow penetration of only ionic lanthanum. Gap junction in sections also appears to consist of cell membrane fusions, but has been shown to allow penetration of colloidal lanthanum and heme peptides with an average diameter of about 20 Å, therefore qualifying this junction as a gap junction, 20 Å wide. The close junction does not show a membrane fusion and allows penetration of myoglobin with a molecular diameter up to 40 Å. The pinocytic vesicles have been demonstrated to transport all tracer substances listed, particularly horseradish peroxidase, dextrans, and ferritin particles. Note that size of junctions has been slightly exaggerated in the drawing to explain more readily their substructure.

Figure 6. Figure 6.

Summary of general appearance of the several types of cell junctions between endothelial cells as seen in freeze‐cleaved material. It is assumed that by using this preparation technique, one has been successful in separating the two adjoining cells shown in Figure , so that it is possible to examine the apposing cell surfaces. In reality, the freeze‐cleavage technique splits along the central plane of cell membranes and other membranes. It now appears that the punctate fusions of the tight junction are in fact derived from a system of ridges formed by minute particles within the cell membranes. Large arrows indicate that some investigators have demonstrated penetration of colloidal lanthanum across these junctions in some segments of the cardiovascular system, which is assumed to prove that these ridges are interrupted at some points along the tight junctions, either permanently or momentarily. Gap junctions are, in fact, patchlike areas (maculae) with minute membrane particles forming small linkages, which serve as areas for transfer of ions and metabolites from one cell to the next, indicated by small double‐headed arrows. Double arrows indicate movement of pinocytic vesicles. This movement can also be assumed to occur in the opposite direction.

Figure 7. Figure 7.

Elastic artery. Cross section of wall of abdominal aorta of squirrel monkey. Lumen (1) is lined by a thin endothelium. In the intimal layer (2), about 27 μm, there are longitudinally arranged smooth muscle cells (3) and some elastic fibers, while the medial layer (4), about 290 μm, is formed by alternating elastic laminae (5) and circularly arranged smooth muscle cells (6). Only part of adventitial layer (7) is seen in this micrograph. × 540.

Figure 8. Figure 8.

Elastic artery. Cross section of medial layer of abdominal aorta in squirrel monkey, showing pattern of alternating smooth muscle cells (1) and elastic laminae (2). Cytoplasm and nucleus (3) of smooth muscle cells show oblique arrangement with alternating directions, in relation to the elastic laminae, as indicated by arrows, reminiscent of a herringbone pattern. × 2,300.

Figure 9. Figure 9.

The principal arrangement of smooth muscle cells and elastic laminae in elastic arteries is analyzed in this schematic drawing. It is assumed that vessel wall consists of cylinders formed by smooth muscle cells and elastic laminae. Smooth muscle cells are arranged in spirals, which may have a different pitch in different cylinders of the vessel. Thus, the pitch in cylinders A and D may be high, or sometimes with an almost longitudinal arrangement, while the pitch in cylinders B and C may be low or almost transverse. Furthermore, looking at cylinders on end, the smooth muscle cells are arranged at an angle between two elastic laminae; this angle may be different in adjacent cylinders, appearing to have a clockwise arrangement in cylinders A and C, and a counterclockwise arrangement in cylinders B and D. By putting the four cylinders together as A‐B‐C‐D, it becomes apparent how the herringbone pattern of smooth muscle cells of elastic arteries are formed, as well as how the mechanical strength of the wall of an elastic artery is derived.

Figure 10. Figure 10.

Smooth muscle cell. Slightly contracted smooth muscle cell from media of abdominal aorta in squirrel monkey. Cell has an elongated shape. Numerous cytoplasmic processes (1) attach the cell to network of collagenous bundles (2) and elastic laminae (3). Grooves (4) of nucleus are an indication of contraction of cell. × 5,700.

Figure 11. Figure 11.

Summary of major components of the wall of an elastic artery. For comparison, study the actual electron micrographs in Figures and .

Figure 12. Figure 12.

Summary of the major components of the wall of a muscular artery. For comparison, study the actual electron micrograph of the same vascular segment in Figure .

Figure 13. Figure 13.

Muscular artery. Cross section of femoral artery of cat, displaying the typical three layers of this vascular wall: intima (1), 4.7 μm; media (2), 26 μm; adventitia (3), 30 μm. Flat endothelial cells (4) send occasional cytoplasmic processes (5) across internal elastic lamina (6). Smooth muscle cells (7) are circumferentially arranged, probably forming a spiral or helix with a low pitch. There are limited collagenous fibrils (8) between the smooth muscle cells. There is an indistinct external elastic lamina formed by longitudinal elastic fibers (10). Adventitia is rich in collagenous bundles (9) and also has some longitudinally arranged elastic fibers (10) together with fibroblasts (11), Schwann cells (12) with nonmyelinated nerve axons, and macrophages (13). × 1,900.

Figure 14. Figure 14.

Atypical muscular artery. Cross section of a helicine artery of cavernous tissue of penis of the rat. Intimal layer consists of only endothelial cells (1) and some longitudinally arranged elastic fibers (2). In the media, majority of smooth muscle cells are arranged longitudinally (3), whereas outer portion consists of circularly arranged (4) smooth muscle cells forming helices of varying pitch. Connective tissue fibers (5) are sparse between the smooth muscle cells. Adventitia (6) blends with interlacunar tissue. This arrangement of media is commonly seen in coronary arteries of heart as well as in arteries of ovary and uterus. × 2,300.

Figure 15. Figure 15.

Most likely arrangement of smooth muscle cells in most muscular arteries and arterioles. Intima and various layers of media are shown as transparent cylinders. Smooth muscle is indicated as two chains of spindle‐shaped cells. The cells form short helices with a low pitch, starting near the internal elastic lamina, and terminating near the adventitia (not shown). It is conceivable that the cells form a continuous spiral, but it is more likely that there exist, as this diagram shows, many short and regularly spaced helices. Upon contraction or relaxation, the pitch of these helices may vary from near transverse to completely transverse. Top figure depicts the situation in a cross section of most muscular arteries, where almost all profiles of smooth muscle cells are identical, which is expected if the cells are helically arranged as shown in bottom figure.

Figure 16. Figure 16.

Arrangement of smooth muscle cells in the wall of some muscular arteries, as described by Fischer in bovine arteries. Smooth muscle cells have a longitudinal arrangement in inner and outer layers of media, whereas the cells in middle layers are arranged perpendicular to the long axis of the vessel. It is conceivable that a similar arrangement of smooth muscle cells also exists in the atypical arteries of the uterus, ovary, coronary arteries of the heart, and helicine arteries of the cavernous tissue of the penis (see Fig. ) where the innermost muscle cells have a longitudinal arrangement, and the outermost have a transverse arrangement. A similar arrangement may also explain the pattern found in the walls of many veins. Top figure depicts variation of profiles of smooth muscle cells that can be seen in a cross section of these arteries, varying from perfectly cross‐sectioned muscle cells in inner and outer layers, to diagonally and exactly longitudinally sectioned muscle cells in middle layer.

Figure 17. Figure 17.

Microcirculatory bed. Three cross‐sectioned components of a typical microcirculatory bed, located between femoral artery and vein in the cat, representing vasa vasorum of this region: arteriole (1), blood capillary (2), and venule (3). Lumen of arteriole (18 μm) is smaller than that of the venule (30 μm), whereas wall of arteriole is thicker than that of venule, because endothelial cells (4) and smooth muscle cells (5) of venule are much flatter than those of arteriole. Other structures present are a myelinated nerve (6), nonmyelinated nerves (7), epineural venous capillary (8), as well as a lymphatic capillary (9). × 2,300.

Figure 18. Figure 18.

Arteriole. Cross section of arteriole forming part of vasa vasorum, which accompany femoral artery (see Fig. ) and femoral vein (see Fig. ) of cat. Wall of arteriole consists of endothelial cells (1), basal lamina (2), and one or two layers of smooth muscle cells (3). Extensions (4) of endothelial cells establish myoendothelial junctions. There is also a delicate layer of connective tissue elements which might be referred to as the adventitial layer (5). × 9,600.

Figure 19. Figure 19.

Small artery. In this cross section of a small interlobular artery of rat kidney, an endothelial cell process (1) penetrates thin internal elastic lamina (2), establishing contact with cell membrane of a smooth muscle cell (3). This is referred to as a myoendothelial junction (*↔*). Smooth muscle cells of the delicate medial layer of the small artery are invested by a thin external lamina (4) except in areas where adjoining cells establish membranous contact points (5). Other organelles present are glycogen particles (6), mitochondria (7), ribosomes (8), and filaments (9). × 72,000.

Figure 20. Figure 20.

Capillary. Cross section of blood capillary in subepithelial connective tissue of ureter in the rat. Luminal diameter averages 4.5 μm, and it is assumed that this capillary is sectioned near its venous segment, since at least one fenestra (arrow) is present. Fenestrations are scarce in connective tissue capillaries, but occur to a limited degree in the venous segment. Capillary wall consists of endothelial cells (1), basal lamina (2), and an occasional pericyte (3) covered with an external lamina (4). Compare this micrograph with diagram of same segment in Figure . In this capillary, endothelial lining of capillary shows two junctional areas (5). Pericyte shows a typical Golgi area (6) near the nucleus. A fibroblast (7) is seen to left of capillary together with some collagenous fibrils (8). Part of the basal cells (9) of transitional epithelium of the ureter are seen to right of capillary, separated from subepithelial space by a basal lamina (10). × 16,000.

Figure 21. Figure 21.

Summary of the gradual transition of components of vascular wall in venous segments of microvascular bed. Endothelial cells border the vascular lumen. In venous capillary and postcapillary venule, cells on outer side of endothelium are pericytes, whereas in the muscular venule and small collecting vein, they are all smooth muscle cells. Pericytes and cells that are intermediate in their structural appearance between pericytes and smooth muscle cells occur only in the collecting venule.

From Rhodin
Figure 22. Figure 22.

Medium‐sized vein. Cross section of femoral vein of cat. Intima consists of endothelium alone (1), because internal elastic lamina is missing. The media (2) is about 22 μm diam, and in this particular segment of femoral vein in cat it consists of concentrically arranged smooth muscle cells (3) widely separated by thick bundles of collagenous fibers (4). Adventitia (5) is thick and consists of collagenous bundles (6), longitudinally arranged elastic fibers (7), and fibroblasts (8). × 2,100.

Figure 23. Figure 23.

Large vein. Cross section of inferior vena cava of squirrel monkey. Intimal and medial layers (1), about 26 μm, seem inseparable in this segment of the vena cava of this species. Endothelial cells (2) are closely apposed to 4–5 layers of smooth muscle cells (3) of varied directions in relation to longitudinal axis of the vessel. An internal elastic lamina is missing. Bundles of collagenous fibers (4) and elastic fibers (5) are present. The major part of the wall of this vena cava is occupied by an adventitial layer (6) only about one‐half of which is seen in this micrograph. Adventitia consists of widely scattered groups of longitudinally arranged smooth muscle cells (7) separated by very thick masses of collagenous fibrils (8). Lymphatic capillaries (9) and blood capillaries (10) are present in the adventitia. × 2,100.

Figure 24. Figure 24.

Summary of the extensive analyses of the arrangement of smooth muscle cells and the amount of connective tissue in human veins in different locations.

Adapted from Kügelgen
Figure 25. Figure 25.

Summary of major components of the wall of a medium‐sized vein. For comparison, study the actual micrograph of a similar segment in Figure .

Figure 26. Figure 26.

Summary of major components of the wall of a large vein (vena cava). For comparison, study the actual electron micrograph of the same vascular segment in Figure .

Figure 27. Figure 27.

Summary of the principal arrangement of connective tissue in many veins, based on investigations and concepts of Kügelgen and Goerttler . Bundles of collagen fibers form many helices of relatively high pitch which cross each other, originating from either the innermost or outermost layers of the wall of the vein, blending in with the smooth muscle cells and the loose network of elastic fibers. A: spatial arrangement of fiber bundle X as it would appear if one could look down on an imaginary, transparent vascular cylinder. This organization contributes greatly to the support and strengthening of the venous wall.



Figure 1.

Schematic drawing summarizing major structural characteristics of principal segments of blood vessels in mammals. It is intended to serve as a general guide to the various components of the vascular wall that are analyzed and discussed in this chapter. ρ, Diameter.



Figure 2.

General appearance of luminal surface of vascular endothelium in arteries. Endothelial cells are elongated and their long axis is parallel to that of the blood vessel. Boxed‐in area is detailed in Figure .



Figure 3.

Endothelial junctional areas. A: endothelium from abdominal aorta of squirrel monkey. × 111,000. B and C: endothelium of subepithelial connective tissue capillaries of ureter in the rat. B, × 70,000; C, × 72,000. Vascular lumen (1) is above endothelial cells (2) and basal lamina (3). In the large blood vessels, endothelial cells often show flaplike overlappings (4). Punctate fusions (5) form basis for tight junction, whereas a polygonal lattice of subunits (6) establishes the gap junction. Of the pinocytic (surface) vesicles, some are connected to the luminal cell membrane (7), some to the abluminal cell membrane (8), and some to the junctional cell membrane (9). Many vesicles occur freely in the cytoplasm (10). Other organelles of endothelial cells are cytoplasmic filaments (11), microtubules (12), and ribosomes (13).



Figure 4.

Endothelial vesicles. Pinocytic vesicles of endothelial cells occur in large numbers as seen in this micrograph in which aortic endothelium of squirrel monkey was sectioned in a plane parallel to the basal lamina. The level of the section is just above the abluminal cell membrane. Area of square is 1 μm2; it contains about 130 vesicles. Diagonal dense structure in top left corner is a bundle of cytoplasmic filaments near base of endothelial cell. × 74,000.



Figure 5.

Summary of general appearance of the several types of cell junctions between endothelial cells as seen in sectioned material. The most commonly used markers for testing transendothelial transport in electron microscopy are indicated in box together with approximate molecular diameter (ρ). Tight junction appears as punctate fusions of outer leaflets of apposed cell membranes, and is believed by most investigators to allow penetration of only ionic lanthanum. Gap junction in sections also appears to consist of cell membrane fusions, but has been shown to allow penetration of colloidal lanthanum and heme peptides with an average diameter of about 20 Å, therefore qualifying this junction as a gap junction, 20 Å wide. The close junction does not show a membrane fusion and allows penetration of myoglobin with a molecular diameter up to 40 Å. The pinocytic vesicles have been demonstrated to transport all tracer substances listed, particularly horseradish peroxidase, dextrans, and ferritin particles. Note that size of junctions has been slightly exaggerated in the drawing to explain more readily their substructure.



Figure 6.

Summary of general appearance of the several types of cell junctions between endothelial cells as seen in freeze‐cleaved material. It is assumed that by using this preparation technique, one has been successful in separating the two adjoining cells shown in Figure , so that it is possible to examine the apposing cell surfaces. In reality, the freeze‐cleavage technique splits along the central plane of cell membranes and other membranes. It now appears that the punctate fusions of the tight junction are in fact derived from a system of ridges formed by minute particles within the cell membranes. Large arrows indicate that some investigators have demonstrated penetration of colloidal lanthanum across these junctions in some segments of the cardiovascular system, which is assumed to prove that these ridges are interrupted at some points along the tight junctions, either permanently or momentarily. Gap junctions are, in fact, patchlike areas (maculae) with minute membrane particles forming small linkages, which serve as areas for transfer of ions and metabolites from one cell to the next, indicated by small double‐headed arrows. Double arrows indicate movement of pinocytic vesicles. This movement can also be assumed to occur in the opposite direction.



Figure 7.

Elastic artery. Cross section of wall of abdominal aorta of squirrel monkey. Lumen (1) is lined by a thin endothelium. In the intimal layer (2), about 27 μm, there are longitudinally arranged smooth muscle cells (3) and some elastic fibers, while the medial layer (4), about 290 μm, is formed by alternating elastic laminae (5) and circularly arranged smooth muscle cells (6). Only part of adventitial layer (7) is seen in this micrograph. × 540.



Figure 8.

Elastic artery. Cross section of medial layer of abdominal aorta in squirrel monkey, showing pattern of alternating smooth muscle cells (1) and elastic laminae (2). Cytoplasm and nucleus (3) of smooth muscle cells show oblique arrangement with alternating directions, in relation to the elastic laminae, as indicated by arrows, reminiscent of a herringbone pattern. × 2,300.



Figure 9.

The principal arrangement of smooth muscle cells and elastic laminae in elastic arteries is analyzed in this schematic drawing. It is assumed that vessel wall consists of cylinders formed by smooth muscle cells and elastic laminae. Smooth muscle cells are arranged in spirals, which may have a different pitch in different cylinders of the vessel. Thus, the pitch in cylinders A and D may be high, or sometimes with an almost longitudinal arrangement, while the pitch in cylinders B and C may be low or almost transverse. Furthermore, looking at cylinders on end, the smooth muscle cells are arranged at an angle between two elastic laminae; this angle may be different in adjacent cylinders, appearing to have a clockwise arrangement in cylinders A and C, and a counterclockwise arrangement in cylinders B and D. By putting the four cylinders together as A‐B‐C‐D, it becomes apparent how the herringbone pattern of smooth muscle cells of elastic arteries are formed, as well as how the mechanical strength of the wall of an elastic artery is derived.



Figure 10.

Smooth muscle cell. Slightly contracted smooth muscle cell from media of abdominal aorta in squirrel monkey. Cell has an elongated shape. Numerous cytoplasmic processes (1) attach the cell to network of collagenous bundles (2) and elastic laminae (3). Grooves (4) of nucleus are an indication of contraction of cell. × 5,700.



Figure 11.

Summary of major components of the wall of an elastic artery. For comparison, study the actual electron micrographs in Figures and .



Figure 12.

Summary of the major components of the wall of a muscular artery. For comparison, study the actual electron micrograph of the same vascular segment in Figure .



Figure 13.

Muscular artery. Cross section of femoral artery of cat, displaying the typical three layers of this vascular wall: intima (1), 4.7 μm; media (2), 26 μm; adventitia (3), 30 μm. Flat endothelial cells (4) send occasional cytoplasmic processes (5) across internal elastic lamina (6). Smooth muscle cells (7) are circumferentially arranged, probably forming a spiral or helix with a low pitch. There are limited collagenous fibrils (8) between the smooth muscle cells. There is an indistinct external elastic lamina formed by longitudinal elastic fibers (10). Adventitia is rich in collagenous bundles (9) and also has some longitudinally arranged elastic fibers (10) together with fibroblasts (11), Schwann cells (12) with nonmyelinated nerve axons, and macrophages (13). × 1,900.



Figure 14.

Atypical muscular artery. Cross section of a helicine artery of cavernous tissue of penis of the rat. Intimal layer consists of only endothelial cells (1) and some longitudinally arranged elastic fibers (2). In the media, majority of smooth muscle cells are arranged longitudinally (3), whereas outer portion consists of circularly arranged (4) smooth muscle cells forming helices of varying pitch. Connective tissue fibers (5) are sparse between the smooth muscle cells. Adventitia (6) blends with interlacunar tissue. This arrangement of media is commonly seen in coronary arteries of heart as well as in arteries of ovary and uterus. × 2,300.



Figure 15.

Most likely arrangement of smooth muscle cells in most muscular arteries and arterioles. Intima and various layers of media are shown as transparent cylinders. Smooth muscle is indicated as two chains of spindle‐shaped cells. The cells form short helices with a low pitch, starting near the internal elastic lamina, and terminating near the adventitia (not shown). It is conceivable that the cells form a continuous spiral, but it is more likely that there exist, as this diagram shows, many short and regularly spaced helices. Upon contraction or relaxation, the pitch of these helices may vary from near transverse to completely transverse. Top figure depicts the situation in a cross section of most muscular arteries, where almost all profiles of smooth muscle cells are identical, which is expected if the cells are helically arranged as shown in bottom figure.



Figure 16.

Arrangement of smooth muscle cells in the wall of some muscular arteries, as described by Fischer in bovine arteries. Smooth muscle cells have a longitudinal arrangement in inner and outer layers of media, whereas the cells in middle layers are arranged perpendicular to the long axis of the vessel. It is conceivable that a similar arrangement of smooth muscle cells also exists in the atypical arteries of the uterus, ovary, coronary arteries of the heart, and helicine arteries of the cavernous tissue of the penis (see Fig. ) where the innermost muscle cells have a longitudinal arrangement, and the outermost have a transverse arrangement. A similar arrangement may also explain the pattern found in the walls of many veins. Top figure depicts variation of profiles of smooth muscle cells that can be seen in a cross section of these arteries, varying from perfectly cross‐sectioned muscle cells in inner and outer layers, to diagonally and exactly longitudinally sectioned muscle cells in middle layer.



Figure 17.

Microcirculatory bed. Three cross‐sectioned components of a typical microcirculatory bed, located between femoral artery and vein in the cat, representing vasa vasorum of this region: arteriole (1), blood capillary (2), and venule (3). Lumen of arteriole (18 μm) is smaller than that of the venule (30 μm), whereas wall of arteriole is thicker than that of venule, because endothelial cells (4) and smooth muscle cells (5) of venule are much flatter than those of arteriole. Other structures present are a myelinated nerve (6), nonmyelinated nerves (7), epineural venous capillary (8), as well as a lymphatic capillary (9). × 2,300.



Figure 18.

Arteriole. Cross section of arteriole forming part of vasa vasorum, which accompany femoral artery (see Fig. ) and femoral vein (see Fig. ) of cat. Wall of arteriole consists of endothelial cells (1), basal lamina (2), and one or two layers of smooth muscle cells (3). Extensions (4) of endothelial cells establish myoendothelial junctions. There is also a delicate layer of connective tissue elements which might be referred to as the adventitial layer (5). × 9,600.



Figure 19.

Small artery. In this cross section of a small interlobular artery of rat kidney, an endothelial cell process (1) penetrates thin internal elastic lamina (2), establishing contact with cell membrane of a smooth muscle cell (3). This is referred to as a myoendothelial junction (*↔*). Smooth muscle cells of the delicate medial layer of the small artery are invested by a thin external lamina (4) except in areas where adjoining cells establish membranous contact points (5). Other organelles present are glycogen particles (6), mitochondria (7), ribosomes (8), and filaments (9). × 72,000.



Figure 20.

Capillary. Cross section of blood capillary in subepithelial connective tissue of ureter in the rat. Luminal diameter averages 4.5 μm, and it is assumed that this capillary is sectioned near its venous segment, since at least one fenestra (arrow) is present. Fenestrations are scarce in connective tissue capillaries, but occur to a limited degree in the venous segment. Capillary wall consists of endothelial cells (1), basal lamina (2), and an occasional pericyte (3) covered with an external lamina (4). Compare this micrograph with diagram of same segment in Figure . In this capillary, endothelial lining of capillary shows two junctional areas (5). Pericyte shows a typical Golgi area (6) near the nucleus. A fibroblast (7) is seen to left of capillary together with some collagenous fibrils (8). Part of the basal cells (9) of transitional epithelium of the ureter are seen to right of capillary, separated from subepithelial space by a basal lamina (10). × 16,000.



Figure 21.

Summary of the gradual transition of components of vascular wall in venous segments of microvascular bed. Endothelial cells border the vascular lumen. In venous capillary and postcapillary venule, cells on outer side of endothelium are pericytes, whereas in the muscular venule and small collecting vein, they are all smooth muscle cells. Pericytes and cells that are intermediate in their structural appearance between pericytes and smooth muscle cells occur only in the collecting venule.

From Rhodin


Figure 22.

Medium‐sized vein. Cross section of femoral vein of cat. Intima consists of endothelium alone (1), because internal elastic lamina is missing. The media (2) is about 22 μm diam, and in this particular segment of femoral vein in cat it consists of concentrically arranged smooth muscle cells (3) widely separated by thick bundles of collagenous fibers (4). Adventitia (5) is thick and consists of collagenous bundles (6), longitudinally arranged elastic fibers (7), and fibroblasts (8). × 2,100.



Figure 23.

Large vein. Cross section of inferior vena cava of squirrel monkey. Intimal and medial layers (1), about 26 μm, seem inseparable in this segment of the vena cava of this species. Endothelial cells (2) are closely apposed to 4–5 layers of smooth muscle cells (3) of varied directions in relation to longitudinal axis of the vessel. An internal elastic lamina is missing. Bundles of collagenous fibers (4) and elastic fibers (5) are present. The major part of the wall of this vena cava is occupied by an adventitial layer (6) only about one‐half of which is seen in this micrograph. Adventitia consists of widely scattered groups of longitudinally arranged smooth muscle cells (7) separated by very thick masses of collagenous fibrils (8). Lymphatic capillaries (9) and blood capillaries (10) are present in the adventitia. × 2,100.



Figure 24.

Summary of the extensive analyses of the arrangement of smooth muscle cells and the amount of connective tissue in human veins in different locations.

Adapted from Kügelgen


Figure 25.

Summary of major components of the wall of a medium‐sized vein. For comparison, study the actual micrograph of a similar segment in Figure .



Figure 26.

Summary of major components of the wall of a large vein (vena cava). For comparison, study the actual electron micrograph of the same vascular segment in Figure .



Figure 27.

Summary of the principal arrangement of connective tissue in many veins, based on investigations and concepts of Kügelgen and Goerttler . Bundles of collagen fibers form many helices of relatively high pitch which cross each other, originating from either the innermost or outermost layers of the wall of the vein, blending in with the smooth muscle cells and the loose network of elastic fibers. A: spatial arrangement of fiber bundle X as it would appear if one could look down on an imaginary, transparent vascular cylinder. This organization contributes greatly to the support and strengthening of the venous wall.

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Johannes A. G. Rhodin. Architecture of the Vessel Wall. Compr Physiol 2014, Supplement 7: Handbook of Physiology, The Cardiovascular System, Vascular Smooth Muscle: 1-31. First published in print 1980. doi: 10.1002/cphy.cp020201