Journal of Vascular Surgery
Volume 17, Issue 2 , Pages 371-381, February 1993

Adventitial elastolysis is a primary event in aneurysm formation☆

Presented at the Fortieth Scientific Meeting of the International Society for Cardiovascular Surgery, North American Chapter, Chicago, Ill., June 9-10, 1992.

Frederick A. Reichle Surgical Research Laboratory and Department of Anatomy (Dr. Phillips), Temple University Medical School, Philadelphia. Philadelphia, Pa.

Received 12 June 1992; accepted 28 September 1992.

Article Outline

Abstract 

Purpose: Adventitial clastin degradation is a hallmark of abdominal aortic aneurysm (AAA) formation in human beings, although the quantitative relationship between elastin loss and AAA formation and growth is unknown. This study was undertaken to quantitate the reduction of adventitial elastin for small AAA, to determine whether the loss of this structural component parallels aneurysm growth, and to examine the ultrastructure of the remaining elastin elements. Methods: Longitudinal strips of anterior aneurysm wall were taken from 12 patients having elective repair of small (diameter < 5 cm, n = 4), moderate (diameter < 5 to 7 cm, n = 4), or large (diameter > 7 cm, n = 4) AAA and from six normal control subjects at autopsy. Specimens were prepared with elastin and collagen stains for histologic examination or formic acid for scanning electron microscopic evaluation of elastin architecture. Adventitial elastin content of aneurysmal and control aortas was quantitated with video microscopy and compared by aneurysm diameter. Results: The inner portion of adventitia of normal aortic wall was composed of densely compacted alternating lamellae of elastin and collagen, which were grossly disrupted in all aneurysms. The remaining elastin fibers were disorganized and tortuous. There was an 81.6% ± 2.1% reduction in elastin lamellae and an 85.7% ± 4.2% reduction in fibers per lamellae compared with the number in control aortas (p < 0.001). Size of the aneurysm made no difference in adventitial elastin content. Conclusion: These data strongly suggest that elastolysis is a primary event in AAA formation that occurs before overt loss of adventitial structural integrity and the development of small aneurysms. (J VASC SURG 1993;17:371-81.)

 

An increasing body of data suggest that infrarenal abdominal aortic aneurysms (AAA) result from elastin degradation of the aortic wall.1, 2, 3, 4 The pattern of this elastin loss, however, remains undetermined. Clinical and experimental observations of Halsted,5 Holman,6 and Zatina et al.7 have suggested that disruption of medial elastin results in aneurysm formation. These observations are supported by several studies that demonstrate elevated elastase levels associated with diffuse elastolysis within the aneurysm wall.8, 9, 10, 11 Adventitial elastolysis, a characteristic of aneurysm histologic changes, is presumed to occur as a result of stretching and thinning of the weakened aortic wall after aneurysm formation.12 If this concept is correct, then adventitial elastin loss should be proportional to aneurysm diameter and may represent a manifestation of aneurysm growth.

The purpose of this study was to quantitate adventitial elastin depletion in small aneurysms and to correlate adventitial elastin disappearance with aneurysm size in moderate and large aneurysms. The elastin elements of the adventitia of normal and aneurysmal aortas were carefully examined to determine whether they were morphologically altered in the discased vessels. It was expected that if stretching and fracturing of adventitial elastin is the major mechanism of destruction then these fibers should have the characteristic morphologic signs of excessive unidirectional loading. If, however, adventitial elastin disruption results from the activity of elastase or other proteases, then a less consistent pattern of residual elastin elements might be expected. Determination of the pattern of adventitial elastolysis should provide insight into the mechanisms of aneurysm formation and growth.

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Material and methods 

Study population 

The aneurysm walls of 12 patients, eight men and four women (mean age 66 years, range 60 to 74 years), who were undergoing elective repair of asymptomatic infrarenal AAA were studied. All patients represented index cases without evidence of familial clustering or genetic predisposition by history. No patient had collagen vascular disease or previous arterial system infection. All aneurysms were fusiform. Measurements of aneurysm diameter were made with calipers on multiple images from computerized tomography in 11 patients and ultrasonography in one patient. Maximal transverse diameter was recorded. An infrarenal aortic aneurysm is defined as an outer aortic diameter at least 0.5 cm greater than the diameter of the segment of aorta between the origins of the superior mesenteric artery and the left renal artery or when the infrarenal aortic diameter is greater than 4 cm in maximal diameter. For the purpose of this evaluation, small aneurysms (n = 4) were defined as those less than 5 cm in diameter, moderate aneurysms (n = 4) were between 5 and 7 cm, and large aneurysms (n = 4) exceeded 7 cm in diameter. Studies were performed on longitudinal strips of full-thickness aneurysm wall taken from the anterior midportion of the aneurysm sac in the operating room before repair.

Aneurysm specimens were compared with non-aneurysmal (control) infrarenal aortas (n = 6). Longitudinal strips of the anterior wall of the infrarenal aorta 2 cm distal to the origin of the left renal artery were harvested at autopsy within 24 hours of death from six patients (mean age 65 years, range 56 to 76 years). Causes of death were sepsis (two deaths), pneumonia (one), lung carcinoma (one), gastrointestinal hemorrhage (one), and cardiac arrhythmia (one). None had symptomatic vascular disease, collagen vascular disease, or a familial history of aneurysmal disease. This information was supported by the absence of significant vascular pathologic changes noted on autopsy.

Specimen preparation 

Specimens were marked with sutures to indicate the direction of blood flow (DBF) and were consistently oriented throughout specimen preparation and analysis. Specimens were rinsed with saline solution and immersed in formalin immediately after harvest. Rectangular paraffin tissue blocks were prepared. Histologic sections 10 μm thick were cut in the longitudinal and transverse planes and stained with Weigert—van Gieson elastin stain, hematoxylin-eosin, or Mallory's trichrome stain. Tissue section 6 μm thick were cut in the frontal plane and prepared with Weigert—van Gieson elastin stain or Hart's elastin stain counterstained with aniline blue to distinguish collagen. All sections were slide-mounted and analyzed in reference to the DBF. Slides were examined with an inverted Zeiss light microscope (Carl Zeiss Inc., Thornwood, N.Y.), and photographic prints with various magnifications were made of areas of greatest elastin staining within the adventitia. These light microscopy prints served as road maps for scanning electron microscopy (SEM).

Because formic acid dissolves all nonelastin aortic wall elements, selected specimens from each aneurysm group and the control group were examined with SEM after formic acid digestion. Mature elastin is relatively resistant to formic acid degradation. Therefore, to obtain adventitial elastin skeletons of aneurysmal and control aortas, the full-thickness formalin-fixed tissue blocks used for histologic preparation were placed in 88% formic acid and incubated at 45°C for 96 hours in a gently agitating water bath. After formic acid digestion, the remaining elastin skeleton was washed several times with 0.002N HCl and dehydrated in graded concentrations of ethanol. Special care was necessary to avoid fracturing the tissue, which becomes brittle as the elastin dehydrates and becomes rigidly fixed. The delicate elastin preparations were placed in hexamethyldisilozane for 24 hours to complete the dehydration process and then allowed to air dry. The tissue was then attached to 14 mm aluminum SEM stubs with double-sided acetate sticky tape with the luminal surface of the vessel facing down so that the adventitial surface could be examined. The tissue preparations were sputter-coated with gold (Desk-1; Denton Vacuum, Inc., Cherry Hill, N.J.) and studied with an ETEC Autoscan SEM (Baxter Healthcare Corp., Scientific Div., McGaw Park, Ill.) at 20 kV. The entire adventitial surface was examined to visualize the three-dimensional architecture of elastin in aneurysms and normal aortas. Photographs were taken of representative areas.

Elastic fiber quantitation 

A calibrated video system was used to quantitate the number of elastin fibers per unit area in each aneurysm and nonaneurysmal aorta. Longitudinal histologic sections that were stained for elastin and mounted on slides were viewed through a Zeiss light microscope. The magnified images were projected onto a color video monitor by an integrated high-resolution color video camera. A calibrated overlay grid delineating an area of 1.6 × 104 μm2 defined the reference area used for the quantitation of elastin within the projected image on the video monitor. The number of darkly stained elastic fibers crossing the reference area was recorded from each site examined. The same method for quantitation of elastic fibers present within the adventitia on frontal histologic sections was used, except the reference area of the overlay grid was 1.5 × 103 μm2. Ten separate equivalent fields of greatest elastin stain uptake on five longitudinal and five frontal histologic sections (100 measurements) were evaluated for each specimen.

Because elastin distribution is not uniform in the adventitia of AAA, quantitative analysis was confined to the areas of greatest elastin staining within the adventitia on each histologic section rather than to a defined anatomic area. Though this method likely overestimates the amount of elastin in the adventitia of aneurysms, it is necessitated by the lack of consistent anatomic markers within the adventitia of AAA.

Statistical analysis 

Results of elastin fiber quantitation are expressed as mean plus or minus the standard deviation. Statistical analysis of the number of fibers compared in all groups was accomplished with a Wilcoxon rank sum test for unpaired samples. The values were generated using a CLINFO II (version 1.5) statistical program (BBN Software Products Corp., Cambridge, Mass.). Differences were considered significant if the p value was less than 0.05.

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Results 

Normal aortic adventitia 

Light microscopy revealed that though grossly normal, all control aortas demonstrated some subintimal thickening. Aortic wall architecture appeared unaltered (Fig. 1).

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  • Fig. 1. 

    Histologic appearance of elastin-stained normal infrarenal aorta (longitudinal section, original magnification × 40, DBF left to right). Media demonstrates multiple elastin lamellae. Inner layer of adventitia, just beyond border of media, demonstrates dense elastin staining with multiple, closely applied layers of elastin.

There was a clearly evident internal elastic membrane and media, which appeared to be layers of smooth muscle alternating with elastin-stained lamellae. The adventitia was relatively acellular. The inner layer of the adventitia, just beyond the outer border of the media, as indicated by the absence of regularly spaced smooth muscle cells, demonstrated a significant amount of elastin. The elastin stain diminished in intensity toward the outer layers of adventitia so that the outermost portion demonstrated only a slight uptake of elastin stain. Examination of the inner layer of the adventitia under higher magnification demonstrated alternating layers of elastin and collagen (Fig. 2).
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  • Fig. 2. 

    Histologic appearance of inner portion of normal adventitia (longitudinal section, original magnification × 500, DBF left to right). Inner layer of normal human aortic adventitia is composed of densely compacted alternating lamellae of elastin (black) and collagen (pink). Direction of elastin fibers is parallel to DBF, but direction of collagen fibers is perpendicular to DBF.

These layers were densely compacted, with collagen and elastin closely applied. On longitudinal sections, the elastic fibers appeared to run parallel to the DBF. These alternated with collagen fibers, which coursed in a more oblique, circumferential direction. Frontal sections demonstrated that the elastic fibers were juxtaposed to form sheets. These sheets of elastin ran contiguously throughout the entire field without evidence of anatomic boundaries. Histologic reconstruction of transverse, longitudinal, and frontal sections demonstrated that the inner layer of adventitia was constructed of sheets of elastin composed of longitudinally oriented thick elastic fibers closely applied. These elastin lamellae alternated with sheets of collagen composed of fibers that appeared to run on an oblique axis, repeatedly surrounding the adventitia of the aortic wall in a wrap of fibers aligned as a circumferentially directed spiral.

The light microscopic appearance of the adventitia was confirmed by SEM. SEM of the adventitial elastin skeleton demonstrated closely applied thick elastic fibers that separated only to form fenestrae for the vasa vasorum (Fig. 3).

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  • Fig. 3. 

    Scanning electron micrograph of adventitial elastin fibers in formic acid—digested normal aortic wall (DBF left to right). Inner layers of adventitia contained lamellae of elastin composed of thick fibers, with axis of orientation parallel to DBF. These fibers appeared to be held together by thinner circumferential elastin fibers.

These closely applied thick fibers were interlinked with thin fibers that appeared to run circumferentially. A few of the SEM preparations demonstrated significant interdigitation of the fine fibers and thick fibers, which suggested that the fine fibers originated from the thick fibers to run circumferentially across the elastin layer (Fig. 4).
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  • Fig. 4. 

    High-resolution scanning electron micrograph of thin circumferential elastin fibers (DBF left to right). Thinner circumferential fibers appear to arise from thick fibers and course circumferentially to bind thick fibers together.

AAA adventitia 

The aortic wall architecture was significantly altered in all aneurysm specimens (Fig. 5).

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  • Fig. 5. 

    Histologic appearance of human AAA wall (longitudinal section, original magnification × 40, DBF left to right). All aneurysm specimens demonstrated extensive loss of media. Elastin within inner portion of adventitia was also significantly reduced.

The intima and nearly all of the media were obliterated in all of the specimens, whether from small, moderate, or large aneurysms. There were few discernible aortic wall anatomic landmarks. The scant rim of outermost media permitted recognition of the origin and inner layer of the adventitia. The adventitia of AAA was somewhat more cellular than that of control aortas, with clustering of mononuclear inflammatory cells in the outer layers of the adventitia. These cells were not specifically associated with elastic fibers. There was an easily recognizable deficiency of adventitial elastin in all AAA. The inner layer of the adventitia, which demonstrated such abundant elastin in control aortas, contained only sporadic, isolated patches of elastin. On longitudinal sections, these patches of elastin were irregular in shape and not clearly oriented to the DBF. The fibers were thick but discontinuous. On higher magnification, these fibers were not organized in a lamellar pattern but appeared as short ribbons of elastin. There was no clear relationship to the surrounding collagen. Rather than having an alternating network of elastin and collagen within the inner layer of adventitia, the patches of elastin were surrounded by collagen fibers, many of a finer order than those in control aortas (Fig. 6).
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  • Fig. 6. 

    Histologic appearance of inner portion of adventitia in aneurysm wall (longitudinal section, original magnification ×500, DBF left to right). Few remaining elastin elements were surrounded by collagen fibers, which were of finer order than those in normal aortas.

On frontal sections, residual elastin appeared as small aggregates rather than sheets. Discrete borders of these patches were clearly seen, which indicated that the circumferential lamellar pattern did not exist. At higher magnification, the elastic fibers were discontinuous and disorganized (Fig. 7).
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  • Fig. 7. 

    Histologic appearance of residual elastin in adventitia of aneurysm (frontal section, original magnification × 500, DBF left to right). Fibers in remaining lamellae were coiled and without organization.

Individual fibers were short, were tortuous or coiled, and were not closely aligned. There was no clear-cut interlinking of the elastic elements.

The formic acid elastin skeleton preparations grossly demonstrated that the outermost adventitial elastin layer was present. The inner adventitial layers of elastin were difficult to evaluate because of the diminution of elastin within the inner portion of the adventitia. On SEM, the thick elastic fibers within the midportion of the adventitia were without a discrete axis of orientation (Fig. 8).

Each element manifested a somewhat serpiginous configuration. No thin, circumferentially directed fibers were seen on any specimen. There were no contiguous sheets of elastin present on any specimen.

Elastic fiber quantitation 

As expected by the histologic appearances, all AAA contained significantly less elastin within the adventitia than did control aortas. Overall, on evaluation of longitudinal sections, there was an 81.6% ± 2.1% reduction in elastic fibers per unit area of AAA adventitia compared with the number in controls (2.1 ± 0.6 fibers per reference area of AAA vs 11.4 ± 1.5 fibers per reference area of control; Fig. 9, A).

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  • Fig. 9. 

    Quantitation of adventitial elastolysis in aneurysm. A, On longitudinal sections there was 81.6% ± 2.1% reduction in elastin content compared with that in controls (p < 0.001), which shows significant decrease in elastin lamellae in aneurysmal adventitia (reference area = 1.6 × 104 μm2). B, On frontal sections there was 85.7% ± 4.2% reduction in elastin content compared with that in controls (p < 0.001), which shows significant decrease in number of fibers per lamella in aneurysmal adventitia (reference area = 1.5 × 103 μm2).

This difference was statistically significant with p < 0.001. A similar diminution of adventitial elastic fibers was found on frontal section analysis. An 85.7% ± 4.2% reduction in adventitial elastic fibers was found in AAA compared with the number in control aortas (4.2 ± 1.0 fibers per reference area of AAA vs 29.4 ± 3.2 fibers per reference area of control; Fig. 9, B). This was also statistically significant, with p < 0.001. These results indicate a diminution in total elastin content demonstrated by decreases in both the number of adventitial elastin lamellae (longitudinal sections) and the number of elastic fibers within the remaining lamellae (frontal sections).

There was no significant difference in adventitial elastic fiber content among small, moderate, and large AAA. The amount of elastin, especially within the inner layer of the adventitia, did not decrease as the diameter of the aneurysm increased. The reduction in adventitial elastin was clearly evident on small-diameter aneurysms and did not appreciably worsen as the aneurysm diameter increased (Fig. 10).

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  • Fig. 10. 

    Quantitation of adventitial elastolysis in small, moderate, and large aneurysms. There was diffuse adventitial elastolysis in aneurysms of all diameters. There was no statistically significant difference in extent of elastolysis in adventitia of small, moderate, or large aneurysms (reference area = 1.6 × 104 μm2).

Mean number of fibers per reference area was 2.55 ± 0.68 for small, 2.2 ± 0.74 for moderate, and 2.3 ± 0.65 for large aneurysms. None of the differences approached statistical significance. These results indicate that all AAA had similar reductions in the number of elastin lamellae within the adventitia and the number of fibers within each remaining elastin deposit.

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Discussion 

Elastin is the major structural component of the aortic wall and is responsible for its viscoelastic properties.13, 14 This structural element provides energy-independent recoil in response to deforming stresses.13 Loss of aortic wall elastin is a hallmark of aneurysm formation.15 Numerous studies have demonstrated that aneurysm walls have decreased elastin,8, 16 that aneurysmal aortic tissue contains elevated levels of elastase,2, 9 and that elastase-induced degradation of arterial elastin results in altered arterial wall biomechanics, weakening, and dilation.17, 18 The actual site and sequence of critical elastin loss that permits initial aortic dilation and aneurysm growth, however, have not been identified.

Our study carefully examined the inner portion of the adventitia. Though the media has been the focus of study for aneurysm formation, it is unlikely that this layer plays a singular role in the maintenance of aortic wall strength, prevention of aneurysm formation, or aneurysmal growth.19 Recent anatomic studies of infrarenal aortic adventitia demonstrated that the inner portion of this layer contains densely compacted, alternating layers of elastin and collagen.20 Biomechanical studies by Butcher21 and by Sumner et al.22 demonstrated that, after removal of the media, aortic adventitia is capable of maintaining normal maximal aortic diameter throughout the range of expected pressures. These results are supported by the clinical observation that aortic endarterectomy does not cause aneurysm formation. Additionally, because the media is significantly diminished in even small aneurysms, it is unlikely that the residual elements of this layer exert a great effect on the maintenance of residual aortic wall strength. Thus aneurysm formation and growth must require loss of the structural integrity of the adventitia.

The current study demonstrates several striking anatomic features of normal and aneurysmal adventitia that permit an understanding of aortic wall function. The inner layer of normal adventitia is composed of alternating lamellae of elastin and collagen. The appearance of this densely compacted band adds additional support to previous anatomic and biomechanical studies that have documented that the adventitia is capable of tolerating both longitudinal and radial stresses. The longitudinal orientation of thick elastin fibers within each lamellae is particularly interesting. Energy-independent recoil of these fibers would provide longitudinal traction and prevent lengthening or tortuosity of the aorta. This anatomic arrangement helps to explain the findings of Dobrin et al.,23 who noted that arterial tortuosity results from elastin failure.

Aortic wall architecture of all aneurysms was grossly distorted. Histologically, there was significant loss of the media, a reduction of adventitial elastin, and complete disorganization of the few remaining adventitial elastin elements. SEM demonstrated significant abnormalities in the aneurysm adventitial elastin skeleton. Formic acid digestion of the aneurysm wall did not permit high-resolution examination of the residual small, isolated elastin deposits within the inner layers of the adventitia. Elastin in these areas was scant and surrounded by large amounts of collagen. Digestion of these areas permitted fallout of the supported elastin. Therefore formic acid digestion of the aneurysm wall permitted examination of only the intact, thin outer layers of adventitial elastin. Even here, the fibers were disorganized, curled, and unlinked by the thin circumferential bands seen in normal aortic adventitia.

The reduction of adventitial elastin content resulted from decreases in both the number of elastin lamellae and fibers per lamella. The 81.6% ± 2.1% reduction in elastin lamellae and the 85.7% ± 4.2% diminution in fibers per lamella determined in this study compare favorably with the findings of others. Campa et al.8 documented a 77.8% decrease in the elastin content of aneurysms as determined by dry weight, and Rizzo et al.16 found a 90% reduction in the concentration of delipidized, decalcified elastin within aneurysm wall.

Because it evaluated only areas that displayed elastin stain uptake, the method of elastin quantification used in this study may result in an overestimation of the amount of residual elastin in aneurysms of increasing diameter. Despite this, diffuse elastin loss was recorded. The uniform reduction of adventitial elastin in all aneurysms, whether small, moderate, or large, indicates that diffuse adventitial elastolysis occurs early in aneurysm formation and may be the antecedent to the clinical appearance of aneurysms. These findings strongly suggest that adventitial elastolysis is the primary event in aneurysm formation.

The chaotic organization of the remaining elastin elements within the inner portion of the adventitia makes it highly unlikely that this structural component provides any significant support to the aortic wall of even small aneurysms. Collagen is virtually the sole structural element of the adventitia of an aneurysm, regardless of size. Therefore aneurysm growth does not result from further elastolysis but must occur through continued collagen deposition. This concept is supported by the findings of McGee et al.,24 who demonstrated accelerated collagen synthesis and deposition in the walls of unruptured aneurysms.

The results of this study strongly suggest the sequence of aortic wall structural alterations that lead to the formation and growth of infrarenal AAA. Elastin degradation of the aortic wall is manifest first in the media and heralds an alteration in aortic wall metabolism. Loss of the media, however, has little impact on the structural integrity of the aortic wall as manifested by the maximal outer diameter of the aorta. The altered aortic wall metabolism eventually results in adventitial elastolysis. Loss of the majority of elastin within the adventitia causes weakening of the aortic wall and the onset of aortic dilation. Aneurysm growth results from the process of adventitial collagen deposition and reorganization. This predominant remaining element of the aortic wall maintains structural integrity until synthesis is inadequate and rupture occurs.

Though further study is required to confirm this sequence of events, our results suggest that investigative and therapeutic efforts directed toward the early detection and inhibition of elastolysis may permit interruption of the process of infrarenal AAA formation.

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Discussion 

Dr. William H. Pearce (Wilmette, Ill.). The complex architecture of the infrarenal aorta is a reflection of the tangential and longitudinal forces applied to it by flowing blood. The number of lamellar units and the orientation of the structural proteins that have been so elegantly demonstrated here are related to these forces. The major structural proteins of the aorta include elastin, collagen types I and III, glycoproteins, and the recently described fibrillin. The concentration of these proteins is a balance between their synthesis and degradation, through a complex biologic system that includes smooth muscle cells, endothelial cells, and fibroblasts.

From a biologic point of view, the title of the article implies that the primary loss of elastin is in the adventitia. This is a bit misleading. From a biomechanical standpoint, I agree with the authors. The study focuses on the loss of elastin in the adventitia as a primary event. In this study, all of the aneurysms demonstrated a marked loss of elastin in the media, which suggests that this is a global event and not one that is specifically localized to the adventitia. And, indeed, I would agree that a small aneurysm is probably the end stage of a complex biochemical event.

In our own molecular and biochemical studies of aneurysms, we found a similar loss of elastin and low levels of elastin gene expression consistent with the finding that aortic elastin gene expression is present until age 1 year and from then there is no further synthesis of elastin.

I would like the authors to further elaborate on several points. Since they have obtained the longitudinal sections, was there any difference in the elastin distribution along the strip of tissue? In any specimen, was there a portion of the neck or distal iliac artery that was normal so that the changes in the adventitia remote from the pathologic process could be seen?

Finally, what was the elastin architecture in relation to atherosclerotic plaques? We heard at this meeting last year that there is a relationship between the regression of these plaques and the degradation of the elastin fibers.

In sum, I think the study supplies us with new information on the organization of the elastin fibers in the adventitia, and although I understand the emphasis on the adventitia as a primary source for dilatation, I believe that the primary events occur in the media remote from the adventitia and that these are late events.

Dr. John V. White. Generally we agree with the statement that elastolysis of the aortic wall leading to aneurysm formation is indeed global and affects the intima, media, and adventitia. It was our hope to determine which of the layers provided structurally critical elastin loss and subsequent dilation and aneurysm formation. We do agree that elastolysis is a global phenomenon and we believe that it is initially manifest in the media, but that it subsequently occurs in the adventitia.

We did have longitudinal strips of the anterior wall of the aneurysms in this study, and there was no discernible difference in the distribution of elastin degradation throughout the length of the aneurysm. We did not have any normal proximal or distal sections of aortic wall to study and to compare with the extent of elastin degradation within the aneurysm wall itself.

Concerning the question of elastin degradation and plaque relationship, it was striking to us that there was nearly complete replacement of the media by thrombus and plaque elements in even quite small aneurysms. However, we could not discern a distinct relationship between the presence of significant atherosclerotic plaque in any area of sampling and enhanced elastin degradation.

Dr. David Tilson (New York, N.Y.). Dr. White has presented experimental evidence for a notion that has intrigued me for some time, so naturally I am enthusiastic. A few years ago I wrote a short essay on why collagen must fail in aneurysm disease. The concept was simple. The tensile strength of collagen is four orders of magnitude greater than that of elastin, and enzymatic destruction of elastin alone does not induce dilatation of aneurysmal proportions, as shown by Dobrin and his coworkers.18 Also, after endarterectomy, in which most of the media is removed, only the outer media and adventitia remain. Most of the collagen is localized to the adventitia, which is not disturbed, and aneurysms are rare.

Dr. White has shown that there is substantial depletion of adventitial elastin even in small aneurysms and that further depletion does not occur as aneurysms enlarge.

My question for Dr. White is whether there are detectable changes in the collagen as dilatation progresses. It is possible that the collagen may never show biochemical depletion as it yields strength, because new collagen can be synthesized during remodeling. We and others have found that collagen content as a gross percent of dry solids remains relatively stable. This does not mean, however, that it retains its mechanical characteristics as it undergoes disorganization and attempted repair.

Dr. Makis Tsapogas (Stony Brook, N.Y.). I noticed that the authors focused their attention on the correlation of elastin degradation and the development and growth of AAA. Other investigators on the other hand have stressed a more complex mechanism and, among other factors, they have included the significance of collagenase/collagen imbalance, in parallel with that of elastase/elastin. Atherosclerosis, congenital syndromes, and hemodynamic disturbances have also been considered important factors.

May I ask the authors if, besides elastin, they have quantitatively measured collagen and if so what their findings were? How frequent was there history of predisposition for the development of aneurysmal disease in their series? Finally, what was the incidence of clinically manifested atherosclerosis at other sites, as well as coexistence of chronic obstructive pulmonary disease, hernia, and smoking?

Dr. White. Dr. Tilson, we agree with you that, ultimately, collagen must fail for aneurysm rupture to occur. As we suggested in our final hypothesis, we believe that by the time there is a clinically recognizable aneurysm present, elastin is virtually absent from the adventitia. The adventitial wall then is composed strictly of collagen, and it is the process of collagen deposition and degradation that ultimately determines aneurysm growth and subsequent rupture. So we agree that this does occur.

In a slide preparation from a study of canine infrarenal aorta we do not see any significant detailed morphologic changes within collagen, although there is an alteration in the aorta, showing elastin running longitudinally and collagen coming straight out at the viewer. We might depict then, in an artist's concept, that elastin maintains the corrugated configuration of collagen. Once elastin is degraded, collagen is allowed to stretch, and this process of collagen stretching and renewed collagen deposition is what occurs during aneurysm growth. Subsequently, degradation exceeds synthesis and rupture does occur.

Dr. Tsapogas, we agree that once a clinically apparent aneurysm has formed, the metabolism of the adventitia and aneurysmal wall relies on collagen/collagenase balances. We have not measured collagen deposition or collagen loss actively within the aneurysm wall once an aneurysm is present.

As we stated, none of our patients had a strong familial history or evidence of genetic predisposition to aneurysm formation, and although we do subscribe to the concept that aneurysm is probably a genetically based disease, we selected these patients to avoid, perhaps, some potential genetic imbalances that are not common to spontaneous aneurysm production.

The patient demographics, such as smoking and hypertension, were no different in this group than in any standard aneurysm report, and the presence of occlusive disease was not severe in any of these patients.

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☆ Reprint requests: John V. White, MD, Temple University Hospital, 3401 N. Broad St., Philadelphia, PA 19140.

PII: 0741-5214(93)90422-I

doi:10.1067/mva.1993.43023

Journal of Vascular Surgery
Volume 17, Issue 2 , Pages 371-381, February 1993

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