ANNUAL REVIEWS
Further
Quick links to online content
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
Ann. Rev. Med. 1979. 30:1-15 Copyright © 1979 by Annual Reviews Inc. All rights reserved
THE ARTERIAL WALL
.7302
AND ATHEROSCLEROSIS Russell Ross, Ph.D. Department of Pathology SM-30, School of Medicine, University of Washington, Seattle, WA 98195 INTRODUCTION
Although atherosclerosis has been recognized as the principal cause of death in Western man, it is only within the last decade or so that the disease process itself has begun to be understood (1). The disease develops insidi ously before the development of symptoms. Its widespread occurrence at an early age was not g�nerally accepted until studies of American males who died in the Korean and Vietnam wars documented its high prevalence within the second decade of life (2, 3). The lesions of atherosclerosis are limited principally to the innermost layer of the artery wall, the intima. Although there are many variants of the lesion in man, there is general acceptance that the disease process consists of three events, each of which involves the cells of the artery wall. These include (a) intimal smooth muscle cell proliferation, (b) formation of large amounts of connective tissue matrix by the proliferated smooth muscle, lmd (c) deposition of lipids both within the cells and in the connec tive. tissues surrounding them (4-7). The principal biological phenomenon in the development of the lesions of atherosclerosis with which this article is concerned is intimal smooth muscle proliferation. Because of the accumulation of lipid and calcium deposits, atherosclerosis has commonly been considered to be a degenera tive process. However, in the absence of intimal smooth muscle prolifera tion there would probably be few sequelae of clinical significance. Epidemiologic research has identified a list of risk factors that are com monly associated with an increased incidence of atherosclerosis. Each of these risk factors must be explained at the cellular level if we are to under stand the role they play in the induction of lesion formation (1, 8-13). 0066-4219/79/0401-0001$01.00
2
ROSS
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE NORMAL ARTERY WALL
Arteries consist of three morphologically distinct layers: the intima or innermost layer; the middle layer, the media; and the external layer, the adventitia. The intima is bounded on the lumen by a continuous layer of endothelial cells that form a protective barrier between the blood and the artery wall. These cells control the entry of nutrients and have been shown to tum over at differing rates in different parts of the artery wall (14-16). At birth the intima is a narrow layer bounded by endothelium on its innermost aspect and externally by the internal elastic lamina, a fenestrated sheet of elastic tissue. There are relatively few cells within the intima at birth; however, it gradually thickens with progressive age as a result of the accumulation of small numbers of smooth muscle cells and cohnective tissue. The media consists entirely of smooth muscle ·cells oriented in a spiral fashion, surrounded by connective tissues, including collagen and glycosaminoglycan, as well as elastic fibers. The adventitia contains both smooth muscle cells and fibroblasts together with relatively large amounts of collagen, glycosaminoglycan, and elastic fibers. The changes associated
with the lesions of atherosclerosis occur principally within the intima. THE LESIONS OF ATHEROSCLEROSIS
Pathologists have subdivided the lesions of atherosclerosis into three gen eral varieties. These are the fatty streak, the fibrous plaque, and the so-called complicated or advanced lesion (1, 4, 17). The fatty streak is most commonly found in young individuals and is characterized by a focal accumulation of small numbers of smooth muscle cells within the intima, which generally contain deposits of lipid and are also surrounded by lipid extracellularly (18). Clinically it has a yellow sessile appearance and causes no obstruction or symptomatology. Virtually every child has such lesions by the age of ten (1). They are distributed randomly throughout the arterial tree, including those sites where increased shear stress has been shown to occur, namely at junctions and at bifurcations. The fibrous plaque is considered characteristic of the advancing lesions of atherosclerosis and is not as ubiquitous as is the fatty streak. It has a grossly white appearance and is elevated, so that it may protrude into the lumen of the artery. Fibrous plaques consist of an intimal accumulation of proliferated smooth muscle cells surrounded by fairly large amounts of connective tissue, which appear to form a "cap" that covers a deeper accumulation of both intra- and extracellular lipids. Necrosis and cell debris are usually associated with this lipid accumulation. Fibrous plaques gener ally occur at sites where increased shear may be applied to the artery wall,
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
3
particularly in individuals who are hypertensive. Such sites may be at greater risk of endothelial injury (19). The advanced or so-called complicated lesion appears to be a fibrous plaque that has been altered as a result of hemorrhage, calcification, necro sis, thrombosis, and further cell degenerative change mixed with cell prolif eration. Calcification is perhaps the most distinctive change associated with the complicated lesion. Thus the lesions of atherosclerosis are focal and contain newly prolifer ated smooth muscle cells, together with newly formed connective tissue, and deposits of lipid. THE RESPONSE-TO-INJURY HYPOTHESIS OF ATHEROGENESIS
Together with my colleagues at the University of Washington School of ' Medicine, I have been examining a hypothesis termed the "response-to injury hypothesis" (4, 20, 21). This hypothesis evolved because of the strong similarity in appearance between the early lesions of atherosclerosis, the "fibromusculoelastic lesions" (1), and the response of arteries to experimen tally induced injury. It represents a modification of some of the earlier notions of the famed pathologist, Rudolph Virchow (22), which have since been modified by numerous investigators. Simply stated, the response-to-injury hypothesis suggests that alterations occur to the lining endothelial cells, which thus changes the ability of cells to function as a protective barrier. The alterations in the endothelium may be quite subtle, in which case the changes may be manifest by altered permeability characteristics of the cells. On the other hand, the alterations may be sufficiently severe so that the endothelial cells may become detached from one another and possibly even from the underlying connective tissue and be swept into the blood stream. Platelets would be capable of adhering to the exposed subendothelial connective tissue and of undergoing the "platelet release reaction." The loss of the endothelial barrier function and the presence of platelets at the sites of exposed connective tissue would lead to the ingress of material derived from the platelet release reaction from the plasma into the artery wall. The interaction between plasma components and platelet constituents upon the smooth muscle cells of the artery wall is postulated to result in the focal migration of smooth muscle cells from the media into the intima. Together with the proliferation of preexisting intimal smooth muscle cells, this produces a fibromusculoelastic precursor type lesion of atherosclerosis. Such focal sites of smooth muscle cell prolifer ation would be accompanied by the formation of relatively large amounts of connective tissue matrix. The smooth muscle cell is not only contractile
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
4
ROSS
but also quite capable of producing relatively large amounts of all of the known connective tissue matrix macromolecules (23-27). The hypothesis goes on to suggest that if the injury is a single event, the proliferative response that is evoked may be reversible and the lesions will regress. On the other hand, if the injury occurred on numerous occasions over an extended period of time, such cycles of injury, cell proliferation, and connective tissue formation could lead to a chronic progressive lesion that in time could become sufficiently large so as to cause the development of clinical sequelae. This hypothesis has been a valuable aid to a number of investigators because it has indicated those areas in our knowledge that are lacking and has provided new directions for future research. If the hypothesis is to be adequately tested, it will be necessary to fully understand the biology of the two principal cells of the artery, the endothelium and the smooth muscle, as well as that of the platelet and the plasma. Each risk factor will have to be explained in cellular and molecular terms. The effects of stress, hyperten sion, hyperlipidemia, smoking, diabetes, and various genetic factors should be understandable on the basis of endothelial injury and the subsequent
smooth muscle proliferative response if this hypothesis is to have continued merit. This hypothesis is illustrated and further explained in Figure 1. EXPERIMENTAL DATA RELATED TO THE RESPONSE-TO-INJURY HYPOTHESIS
It is possible to disrupt the endothelial barrier in a number of different ways. The endothelium can be injured mechanically, chemically, immunologi cally, and by various toxins. Regardless of the source of injury, it can be experimentally shown that endothelial disruption leads to an immediate response of the platelets. Within just a few moments after the endothelium has been abraded with an intraarterial balloon catheter, platelets adhere to the subendothelial connective tissue at the site of injury (28, 29). After adhering they undergo aggregation and lose their granules during the re lease reaction. Such platelet adherence, aggregation, and release may occur for as long as 48 hours after mechanical injury and possibly intermittently for longeioperiods (Figure 2). Mechanical injury (30, 31) as well as several forms of chemical injury were shown by Harker and his colleagues (32, 33) to result in a decrease in platelet survival when experimental animals were given SlCr�tagged platelets. There appears to be a linear correlation between the amount of endothelium removed with an intraarterial balloon catheter and the extent of the decrease in platelet survival. In like fashion, the return to normal platelet survival correlates with the regeneration of the endo thelium and restoration of the defect. Thus, platelet survival may serve as a useful clinical index of endothelial injury (48).
.
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
5
Examination of sites where platelets h�ve adhered and undergone the release reaction showed that after three to seven days smooth muscle cells had migrated from the media into the intima between fenestrae of the internal elastic lamina and had begun to proliferate within the intima at these sites. In monkeys made hyperlipidemic by a high fat diet, not only does smooth muscle proliferate and new connective tissue form, but lipid deposition is found as well. If the monkeys are normocholesterolemic, the intimal smooth muscle proliferative response appears to be reversible. In contrast, catheter-induced Injury
Repeated or chron ic injury (chronic hypercholes terolemia, for example)
Figure 1
In the response-to-injury hypothesis, two different cyclic events may occur. The
outer, or regression, cycle may represent cornmon single occurrences in all individuals: Endo thelial injury leads to desquamation, platelet adherence, aggregation, and release, followed by intimal smooth muscle proliferation and connective tissue formation. If the injury is a single event, the lesions may go on to heal and regression occur. The inner, or progression, cycle demonstrates the possible consequences of repeated or chronic endothelial injury as may occur in chronic hyperlipidemia. In this instance, lipid deposition
as
well
as
continued smooth
muscle proliferation may occur after recurrent sequences of proliferation and regression, and these may lead to complicated lesions that calcify. Such lesions could go on to produce clinical sequelae such
as
thrombosis and infarction. Reproduced from
Science (21).
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
6
ROSS
lesions in hypercholesterolemic monkeys appear to progress and show no signs of regression (Figure 3). In addition, endothelial regeneration appears to be defective in the chronic hypercholesterolemic monkeys. Thus the sequence of events noted in the response-to-injury hypothesis can be shown to occur in animals that are injured experimentally with various forms of intraarterial mechanical devices. A similar sequence of events can be shown to occur as a result of chemi cally induced injury. Such injury can occur in baboons with chronic homo cysteinemia (32, 33), as a result of chronic hypercholesterolemia (21), or through immune injury that results from circulating antigen-antibody com plexes (34, 35). In both chronic homocysteinemia and chronic hypercholes-
Figure 2
Twenty-four hours after endothelial removal, platelets adhere to the denuded
surface and are seen in this electron micrograph to be closely apposed to the microfibrils
(arrows) of the elastic fibers as well as
to the basement membrane-like material. Fibrin is seen ·
on the luminal side of the vessel between the platelets.
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
7
terolemia, nonhuman primates can be shown te have decreased platelet survival. After six days of chronic homocysteinemia baboons have as much as a 50% decrease in platelet survival. Autopsy revealed that as much as 10% of the endothelium of the thoracic and abdominal aorta was missing. In fact, Harker and his colleagues (32, 33) showed a linear correlation between the level of homocysteinemia and the amount of missing endo-
Figure 3
This low power electron micrograph represents a small segment of a three-month
lesion in a pigtail monkey that was continuously hyperlipemic for the entire duration of the experiment. The deeper portion of the lesion contains numerous fat-filled smooth muscle cells. The extensive lipid deposits fill the cells and severely extend them. Many of these lipid deposits have been partially extracted and appear as central clear areas. In the more superficial portion of the lesion, numerous small deposits of extracellular lipid can be seen in the connective tissue matrix
(arrows).
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
8
ROSS
thelium in the aorta. There is also a correlation between the amount of missing endothelium and the decrease in platelet survival observed in these animals (Figure 4). If the animals were kept homocysteinemic for 90 days instead of 6 days, not only did they continue to manifest a decrease in platelet survival, but at the end of the 90 days there was a marked intimal smooth muscle proliferation at the sites of missing endothelium. The smooth muscle cells that had proliferated in these lesions were surrounded by large amounts of newly formed connective tissue. In addition, some of the lesions contained accumulations of intracellular lipid deep within the lesions, a characteristic commonly associated with an early fibrous plaque, even though the animals were not hypercholesterolemic (Figure 5). Interestingly, the decrease in platelet survival was clearly correlated with atherosclerotic lesion formation, as demonstrated with pharmacologic agents inhibitory to platelet function. One of these, dipyridamole, returned platelet survival to normal levels in a group of chronically homocysteinemic baboons. Inhibition of platelet function with dipyridamole did not affect the amount of endothelium that was missing, yet it was specifically able to inhibit intimal smooth muscle proliferation. These studies were first to demonstrate that decreased platelet survival was associated with intimal 30
20
\:.
\.
B
\
6 '.\6 66 6 6 •
:\
001L---��--�---� 4 2 6
2
4
6
Plalelet survival (days)
Figure 4
The relationships of platelet survival and plasma homocystine level and endothe
lial cell loss. (A) Platelet survival time correlated with the logarithm of the plasma homo cystine concentration
(dots) with r
=
0.965, P < 0.001,
and regression line represented by
y = 0.453(-O·368x). (0) Measurements of platelet survival time also correlated with the logarithm of endothelial cell loss by y
=
(dots) with r
=
0.958, P < 0.001,
and a regression line represented
44.O
Further
Quick links to online content
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
Ann. Rev. Med. 1979. 30:1-15 Copyright © 1979 by Annual Reviews Inc. All rights reserved
THE ARTERIAL WALL
.7302
AND ATHEROSCLEROSIS Russell Ross, Ph.D. Department of Pathology SM-30, School of Medicine, University of Washington, Seattle, WA 98195 INTRODUCTION
Although atherosclerosis has been recognized as the principal cause of death in Western man, it is only within the last decade or so that the disease process itself has begun to be understood (1). The disease develops insidi ously before the development of symptoms. Its widespread occurrence at an early age was not g�nerally accepted until studies of American males who died in the Korean and Vietnam wars documented its high prevalence within the second decade of life (2, 3). The lesions of atherosclerosis are limited principally to the innermost layer of the artery wall, the intima. Although there are many variants of the lesion in man, there is general acceptance that the disease process consists of three events, each of which involves the cells of the artery wall. These include (a) intimal smooth muscle cell proliferation, (b) formation of large amounts of connective tissue matrix by the proliferated smooth muscle, lmd (c) deposition of lipids both within the cells and in the connec tive. tissues surrounding them (4-7). The principal biological phenomenon in the development of the lesions of atherosclerosis with which this article is concerned is intimal smooth muscle proliferation. Because of the accumulation of lipid and calcium deposits, atherosclerosis has commonly been considered to be a degenera tive process. However, in the absence of intimal smooth muscle prolifera tion there would probably be few sequelae of clinical significance. Epidemiologic research has identified a list of risk factors that are com monly associated with an increased incidence of atherosclerosis. Each of these risk factors must be explained at the cellular level if we are to under stand the role they play in the induction of lesion formation (1, 8-13). 0066-4219/79/0401-0001$01.00
2
ROSS
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE NORMAL ARTERY WALL
Arteries consist of three morphologically distinct layers: the intima or innermost layer; the middle layer, the media; and the external layer, the adventitia. The intima is bounded on the lumen by a continuous layer of endothelial cells that form a protective barrier between the blood and the artery wall. These cells control the entry of nutrients and have been shown to tum over at differing rates in different parts of the artery wall (14-16). At birth the intima is a narrow layer bounded by endothelium on its innermost aspect and externally by the internal elastic lamina, a fenestrated sheet of elastic tissue. There are relatively few cells within the intima at birth; however, it gradually thickens with progressive age as a result of the accumulation of small numbers of smooth muscle cells and cohnective tissue. The media consists entirely of smooth muscle ·cells oriented in a spiral fashion, surrounded by connective tissues, including collagen and glycosaminoglycan, as well as elastic fibers. The adventitia contains both smooth muscle cells and fibroblasts together with relatively large amounts of collagen, glycosaminoglycan, and elastic fibers. The changes associated
with the lesions of atherosclerosis occur principally within the intima. THE LESIONS OF ATHEROSCLEROSIS
Pathologists have subdivided the lesions of atherosclerosis into three gen eral varieties. These are the fatty streak, the fibrous plaque, and the so-called complicated or advanced lesion (1, 4, 17). The fatty streak is most commonly found in young individuals and is characterized by a focal accumulation of small numbers of smooth muscle cells within the intima, which generally contain deposits of lipid and are also surrounded by lipid extracellularly (18). Clinically it has a yellow sessile appearance and causes no obstruction or symptomatology. Virtually every child has such lesions by the age of ten (1). They are distributed randomly throughout the arterial tree, including those sites where increased shear stress has been shown to occur, namely at junctions and at bifurcations. The fibrous plaque is considered characteristic of the advancing lesions of atherosclerosis and is not as ubiquitous as is the fatty streak. It has a grossly white appearance and is elevated, so that it may protrude into the lumen of the artery. Fibrous plaques consist of an intimal accumulation of proliferated smooth muscle cells surrounded by fairly large amounts of connective tissue, which appear to form a "cap" that covers a deeper accumulation of both intra- and extracellular lipids. Necrosis and cell debris are usually associated with this lipid accumulation. Fibrous plaques gener ally occur at sites where increased shear may be applied to the artery wall,
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
3
particularly in individuals who are hypertensive. Such sites may be at greater risk of endothelial injury (19). The advanced or so-called complicated lesion appears to be a fibrous plaque that has been altered as a result of hemorrhage, calcification, necro sis, thrombosis, and further cell degenerative change mixed with cell prolif eration. Calcification is perhaps the most distinctive change associated with the complicated lesion. Thus the lesions of atherosclerosis are focal and contain newly prolifer ated smooth muscle cells, together with newly formed connective tissue, and deposits of lipid. THE RESPONSE-TO-INJURY HYPOTHESIS OF ATHEROGENESIS
Together with my colleagues at the University of Washington School of ' Medicine, I have been examining a hypothesis termed the "response-to injury hypothesis" (4, 20, 21). This hypothesis evolved because of the strong similarity in appearance between the early lesions of atherosclerosis, the "fibromusculoelastic lesions" (1), and the response of arteries to experimen tally induced injury. It represents a modification of some of the earlier notions of the famed pathologist, Rudolph Virchow (22), which have since been modified by numerous investigators. Simply stated, the response-to-injury hypothesis suggests that alterations occur to the lining endothelial cells, which thus changes the ability of cells to function as a protective barrier. The alterations in the endothelium may be quite subtle, in which case the changes may be manifest by altered permeability characteristics of the cells. On the other hand, the alterations may be sufficiently severe so that the endothelial cells may become detached from one another and possibly even from the underlying connective tissue and be swept into the blood stream. Platelets would be capable of adhering to the exposed subendothelial connective tissue and of undergoing the "platelet release reaction." The loss of the endothelial barrier function and the presence of platelets at the sites of exposed connective tissue would lead to the ingress of material derived from the platelet release reaction from the plasma into the artery wall. The interaction between plasma components and platelet constituents upon the smooth muscle cells of the artery wall is postulated to result in the focal migration of smooth muscle cells from the media into the intima. Together with the proliferation of preexisting intimal smooth muscle cells, this produces a fibromusculoelastic precursor type lesion of atherosclerosis. Such focal sites of smooth muscle cell prolifer ation would be accompanied by the formation of relatively large amounts of connective tissue matrix. The smooth muscle cell is not only contractile
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
4
ROSS
but also quite capable of producing relatively large amounts of all of the known connective tissue matrix macromolecules (23-27). The hypothesis goes on to suggest that if the injury is a single event, the proliferative response that is evoked may be reversible and the lesions will regress. On the other hand, if the injury occurred on numerous occasions over an extended period of time, such cycles of injury, cell proliferation, and connective tissue formation could lead to a chronic progressive lesion that in time could become sufficiently large so as to cause the development of clinical sequelae. This hypothesis has been a valuable aid to a number of investigators because it has indicated those areas in our knowledge that are lacking and has provided new directions for future research. If the hypothesis is to be adequately tested, it will be necessary to fully understand the biology of the two principal cells of the artery, the endothelium and the smooth muscle, as well as that of the platelet and the plasma. Each risk factor will have to be explained in cellular and molecular terms. The effects of stress, hyperten sion, hyperlipidemia, smoking, diabetes, and various genetic factors should be understandable on the basis of endothelial injury and the subsequent
smooth muscle proliferative response if this hypothesis is to have continued merit. This hypothesis is illustrated and further explained in Figure 1. EXPERIMENTAL DATA RELATED TO THE RESPONSE-TO-INJURY HYPOTHESIS
It is possible to disrupt the endothelial barrier in a number of different ways. The endothelium can be injured mechanically, chemically, immunologi cally, and by various toxins. Regardless of the source of injury, it can be experimentally shown that endothelial disruption leads to an immediate response of the platelets. Within just a few moments after the endothelium has been abraded with an intraarterial balloon catheter, platelets adhere to the subendothelial connective tissue at the site of injury (28, 29). After adhering they undergo aggregation and lose their granules during the re lease reaction. Such platelet adherence, aggregation, and release may occur for as long as 48 hours after mechanical injury and possibly intermittently for longeioperiods (Figure 2). Mechanical injury (30, 31) as well as several forms of chemical injury were shown by Harker and his colleagues (32, 33) to result in a decrease in platelet survival when experimental animals were given SlCr�tagged platelets. There appears to be a linear correlation between the amount of endothelium removed with an intraarterial balloon catheter and the extent of the decrease in platelet survival. In like fashion, the return to normal platelet survival correlates with the regeneration of the endo thelium and restoration of the defect. Thus, platelet survival may serve as a useful clinical index of endothelial injury (48).
.
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
5
Examination of sites where platelets h�ve adhered and undergone the release reaction showed that after three to seven days smooth muscle cells had migrated from the media into the intima between fenestrae of the internal elastic lamina and had begun to proliferate within the intima at these sites. In monkeys made hyperlipidemic by a high fat diet, not only does smooth muscle proliferate and new connective tissue form, but lipid deposition is found as well. If the monkeys are normocholesterolemic, the intimal smooth muscle proliferative response appears to be reversible. In contrast, catheter-induced Injury
Repeated or chron ic injury (chronic hypercholes terolemia, for example)
Figure 1
In the response-to-injury hypothesis, two different cyclic events may occur. The
outer, or regression, cycle may represent cornmon single occurrences in all individuals: Endo thelial injury leads to desquamation, platelet adherence, aggregation, and release, followed by intimal smooth muscle proliferation and connective tissue formation. If the injury is a single event, the lesions may go on to heal and regression occur. The inner, or progression, cycle demonstrates the possible consequences of repeated or chronic endothelial injury as may occur in chronic hyperlipidemia. In this instance, lipid deposition
as
well
as
continued smooth
muscle proliferation may occur after recurrent sequences of proliferation and regression, and these may lead to complicated lesions that calcify. Such lesions could go on to produce clinical sequelae such
as
thrombosis and infarction. Reproduced from
Science (21).
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
6
ROSS
lesions in hypercholesterolemic monkeys appear to progress and show no signs of regression (Figure 3). In addition, endothelial regeneration appears to be defective in the chronic hypercholesterolemic monkeys. Thus the sequence of events noted in the response-to-injury hypothesis can be shown to occur in animals that are injured experimentally with various forms of intraarterial mechanical devices. A similar sequence of events can be shown to occur as a result of chemi cally induced injury. Such injury can occur in baboons with chronic homo cysteinemia (32, 33), as a result of chronic hypercholesterolemia (21), or through immune injury that results from circulating antigen-antibody com plexes (34, 35). In both chronic homocysteinemia and chronic hypercholes-
Figure 2
Twenty-four hours after endothelial removal, platelets adhere to the denuded
surface and are seen in this electron micrograph to be closely apposed to the microfibrils
(arrows) of the elastic fibers as well as
to the basement membrane-like material. Fibrin is seen ·
on the luminal side of the vessel between the platelets.
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
THE ARTERIAL WALL AND ATHEROSCLEROSIS
7
terolemia, nonhuman primates can be shown te have decreased platelet survival. After six days of chronic homocysteinemia baboons have as much as a 50% decrease in platelet survival. Autopsy revealed that as much as 10% of the endothelium of the thoracic and abdominal aorta was missing. In fact, Harker and his colleagues (32, 33) showed a linear correlation between the level of homocysteinemia and the amount of missing endo-
Figure 3
This low power electron micrograph represents a small segment of a three-month
lesion in a pigtail monkey that was continuously hyperlipemic for the entire duration of the experiment. The deeper portion of the lesion contains numerous fat-filled smooth muscle cells. The extensive lipid deposits fill the cells and severely extend them. Many of these lipid deposits have been partially extracted and appear as central clear areas. In the more superficial portion of the lesion, numerous small deposits of extracellular lipid can be seen in the connective tissue matrix
(arrows).
Annu. Rev. Med. 1979.30:1-15. Downloaded from www.annualreviews.org Access provided by University of Texas Southwestern Medical Center on 01/27/15. For personal use only.
8
ROSS
thelium in the aorta. There is also a correlation between the amount of missing endothelium and the decrease in platelet survival observed in these animals (Figure 4). If the animals were kept homocysteinemic for 90 days instead of 6 days, not only did they continue to manifest a decrease in platelet survival, but at the end of the 90 days there was a marked intimal smooth muscle proliferation at the sites of missing endothelium. The smooth muscle cells that had proliferated in these lesions were surrounded by large amounts of newly formed connective tissue. In addition, some of the lesions contained accumulations of intracellular lipid deep within the lesions, a characteristic commonly associated with an early fibrous plaque, even though the animals were not hypercholesterolemic (Figure 5). Interestingly, the decrease in platelet survival was clearly correlated with atherosclerotic lesion formation, as demonstrated with pharmacologic agents inhibitory to platelet function. One of these, dipyridamole, returned platelet survival to normal levels in a group of chronically homocysteinemic baboons. Inhibition of platelet function with dipyridamole did not affect the amount of endothelium that was missing, yet it was specifically able to inhibit intimal smooth muscle proliferation. These studies were first to demonstrate that decreased platelet survival was associated with intimal 30
20
\:.
\.
B
\
6 '.\6 66 6 6 •
:\
001L---��--�---� 4 2 6
2
4
6
Plalelet survival (days)
Figure 4
The relationships of platelet survival and plasma homocystine level and endothe
lial cell loss. (A) Platelet survival time correlated with the logarithm of the plasma homo cystine concentration
(dots) with r
=
0.965, P < 0.001,
and regression line represented by
y = 0.453(-O·368x). (0) Measurements of platelet survival time also correlated with the logarithm of endothelial cell loss by y
=
(dots) with r
=
0.958, P < 0.001,
and a regression line represented
44.O
