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Cardiovascular Research 1997 34(3):557-567; doi:10.1016/S0008-6363(97)00056-4
© 1997 by European Society of Cardiology
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Copyright © 1997, European Society of Cardiology

Contrasting structure of the saphenous vein and internal mammary artery used as coronary bypass vessels

Peter B. Canhama,*, Helen M. Finlaya and Derek R. Boughnera,b

aDepartment of Medical Biophysics, University of Western Ontario, and John P. Robarts Research Institute, London, Ont. N6A 5C1, Canada
bDepartment of Medicine, University of Western Ontario, and John P. Robarts Research Institute, London, Ont. N6A 5C1, Canada

* Corresponding author. Tel.: +1 (519) 661-3053; fax: +1 (519) 661-2123; e-mail: medbpa@uwoadmin.uwo.ca

Received 1 August 1996; accepted 3 February 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objectives: To report quantitatively on the three-dimensional layered organization of the collagen and smooth muscle component of the two most successful vessels for coronary bypass—the internal mammary artery (IMA) and the long saphenous vein (SV). Our aim was to provide an explanation for the differential structural stiffness of these two vessels (both functioning at arterial pressures in their new environment), and how they might be susceptible to endothelial thickening. Methods: Eleven human saphenous veins and 23 internal mammary arteries were fixed at arterial distending pressure of 110 mmHg, and were sectioned in cross-section at 7 µm thickness. A subset of these was also sectioned tangentially. Measurements of the three-dimensional alignment of collagen and smooth muscle fibers within the vessel wall were made using polarized light microscopy and the universal stage attachment. Data were plotted and analysed using circular statistics. Results: The IMA, structured like an elastic artery, is dominated by a media with discrete lamellae of wavy collagen and smooth muscle, aligned nearly circumferentially, with a low variability of alignment (mean circular SD 12°). The SV is more variable in its size and structure, characteristically with a narrow circumferential media comprised mostly of collagen which is straightened and highly aligned at arterial pressures (mean circular SD 9°). Circumferential collagen in the vein was often adjacent to longitudinal bundles of smooth muscle and collagen. Conclusions: The strikingly aligned structure of the SV complements the known high mechanical stiffness of this vessel when at arterial distending pressure. The high fraction of longitudinal muscle, in addition to the circumferential muscle cells in the SV make it vulnerable to any pre-implant surgical preparation, and to the cyclical luminal pressures and longitudinal strains characteristic for epicardial arteries.

KEYWORDS Human, saphenous vein; Human, arteries; Coronary artery bypass grafting; Collagen, orientation; Polarized light microscopy; Stereology


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
The clinical need for coronary artery bypass vessels is being met, in large measure, by two very different graft vessels—the internal mammary artery (IMA) and the long saphenous vein (SV). While both have reasonable long-term patency, the IMA has proven to be the better graft vessel, with an 85–95% patency record 7–10 years after surgery [1–4]. Differences in the success rates of the IMA and SV have been attributed to a number of causes, many of which are associated with the intrinsic differences between veins and arteries. Our premise, supported by the work of others [5–7], was that the success of vessels used for coronary artery bypass is due, in a major way, to the physical structure of the replacement vessel. The measurements that we especially wanted to emphasize were of the orientation, and the dispersion of orientation, of the collagen and smooth muscle fibers of the long saphenous vein when distended at arterial pressure.

The internal mammary artery and the long saphenous vein have strikingly different structural features characteristic of their specific classes of blood vessels and yet they have been called upon to function similarly in the treatment of ischemic heart disease. The IMA is a transition artery, having microscopical features both of the elastic type, as in the aorta and pulmonary artery, and of the muscular type such as the arteries of the heart or brain [8]. The SV is a long large-caliber peripheral vein, highly variable in structure, featuring distinct longitudinal bundles of smooth muscle cells in the adventitia and the inner media [9].

There is a well-established parallel between microscopic structure and mechanical behavior of vascular tissues [10–12]. For the vessels commonly used for bypass, pre-implantation dimensions and proportions of the principal layers have been examined [8, 9]. However, little attention has been accorded the structural organization of the collagen and muscle fibers in the graft vessel. In addition, few of the studies have added the extra step in tissue preparation of fixing vessel segments at arterial pressure. To redress this, we pressure-fixed both the IMAs and the SVs at arterial distending pressures, which establishes the physical dimensions at which the transplanted vessels operate after bypass surgery. In the case of the SV this served to emphasize the striking structural accommodation of this vessel under its unaccustomed pressure loading.

The underlying hypothesis is that principal factors leading to occlusive disease of the saphenous vein are the high and pulsatile intraluminal pressures to which it is subjected as a result of transplantation. Under these pressures the structural components of the vessel wall become mechanically strained, promoting hyperplasia and longer-term occlusion. The reason for undertaking the research was to obtain definitive data about IMA and SV structure that shows clearly the directional organization of the principal fibers of collagen and smooth muscle, in order to provide explanations of the stiffness of the vessels and how they might be vulnerable to endothelial thickening.

Factors before, during and after the transplant procedure may contribute to the trauma of the vessel wall. Techniques used for vessel removal and preparation sometimes involve very high pressures [13–16]. Damage to the endothelium or media can be sustained as a direct result of the stretched vessel wall, either from high pressures or as a result of pulsation [17]. Experiments have shown that cyclic stretching at physiologic frequencies promotes proliferation of smooth muscle cells [18], as well as changes in orientation [19]. The potential for intimal proliferation has also been identified as a result of hemodynamic factors owing to vessel caliber or to geometry at the anastomosis [20, 21], the mismatch of diameter and mechanical properties [22, 23], and the condition of the replacement vessel prior to transplant [9, 24].

The numerous factors contributing to the success of bypass vessels are associated with vessel organizational structure, information obtainable by polarized light microscopy. Measurements of three-dimensional organization of the fibers of collagen and smooth muscle, both of which are birefringent, can be made with the universal stage, an attachment to the polarizing microscope. The calibrated rotational movement, in three dimensions, of the inner stage of the universal stage permits the measurement of directional alignment of individual fibers, relative to the reference plane of the section [25]. Quantitative results help to explain the paradoxical finding that the saphenous vein at arterial pressure is mechanically less distensible than the internal mammary artery at the same transmural pressure.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
Blood vessels were obtained from autopsy and from surgery. Eleven long saphenous veins were provided as unneeded lengths of vessel from bypass grafts (males, ages 37–67), and 23 segments of internal mammary artery were obtained from 20 autopsies, either right or left artery, and in 3 cases both the right and left (male and female, ages 26–83). Segments of each vessel were ligated and cannulated, and fixed at a distending pressure of 14.5 kPa (110 mmHg) in 10% neutral-buffered formalin. The segments were embedded in paraffin wax, and sectioned at 7 µm thickness at right angles to the vessel axis. Tangential sections were cut at 4 or 5 µm thickness in the direction parallel to the vessel axis (Fig. 1). Slides were stained for birefringence enhancement with picrosirius red F3BA [26], which causes collagen to appear bright orange-yellow when viewed with polarized light. In addition, some were stained with James' silver impregnation stain, revealing collagen under polarizing microscopy to have a blue-pink appearance, and smooth muscle a gold color. Other slides were stained for bright field microscopy, with either fuchsin or Verhoeff's stain to identify elastin, or with Gomori's one-step trichrome to assist in identifying areas of collagen and smooth muscle. These sections were used to define layer boundaries and areas of subendothelial thickening, and to reveal wall thickness and lumen diameters. For these measurements a projection microscope was used, and tracings made of the cross-sections of each vessel. Because positional data are not incorporated in the measurements of three-dimensional orientation, we took low-power light micrographs of sections chosen for detailed measurements.


Figure 1
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  		Fig. 1 Schemata to illustrate the two sectioning planes. Tangential sections belong to the much larger set of longitudinally-cut sections, but only the first few sections, up to the ones grazing the lumen, would be included (as well as the matching set on the opposite side of the vessel wall).

 
The principal instrument for the microscopical measurements was the universal stage, an attachment to the rotating stage of our Zeiss polarizing microscope; it is a long-established optical instrument for crystallographic studies in geology [27]. The universal stage has 4 calibrated axes that allow the specimen slide to be rotated in and out of the plane of the stage. The section is positioned at the center of rotation between two glass hemispheres, with the top and bottom of the slide coated with glycerol where they contact the upper and lower glass spherical segments. This minimizes both external and internal light reflection as the stage is tilted. Orientation measurements are obtained by rotation of the section in three dimensions in such a way that optical extinction is achieved when the birefringent fiber is aligned either parallel or perpendicular to the polarizing filter. The fundamental principle underlying the universal stage is that the birefringent fiber lies within a transparent section of known thickness. Thus the angle of tilt (elevation) of the fiber relative to the section plane can be measured as well as its azimuth angle within the plane. These two angles uniquely define, in three dimensions, the orientation of the birefringent fibers at that point relative to the microscope slide. Individual measurements may be made from zones of approximately 4 µm width; however, due to interference from zones of different orientations within the 7 µm thickness when the section is tilted, the practical lower limit of size of the zone of coherently organized tissue is 8–10 µm. In the circumstances that the fiber may have waviness along its axis, we have made measurements to focus on the general fiber orientation, and deliberately avoided the local alignment variations due to the waviness alone.

We restricted our readings to a narrow arc of the wall, using sets of orientation measurements within each tunic so that we could compare regional differences across the vessel wall. For all the cross-sectioned vessels, readings were taken across the complete tunica media, and from the subendothelium and tunica adventitia where they were sufficiently thick. To complement these results, we also took orientation measurements from 3 of the SVs and 3 of the IMAs using tangential sections from adjacent segments of the same vessels. These tangential sections have two advantages: first, all fibers that are either circumferential or longitudinal in orientation will lie close to the section plane, increasing the accuracy of measurements, and secondly, since the section through the wall is very oblique, the width of each layer on the slide is greater (Fig. 2B). Thus layers become more clearly defined, particularly when thin (4 or 5 µm) sections are used to minimize overlap of regions of different alignments [28]. For these tangential measurements we used sections that were cut just through to the lumen so that the complete thickness of the vessel wall was revealed, with two sets of readings available from each vessel, one from each side of the lumen center axis (Fig. 2B).


Figure 2
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  		Fig. 2 Schemata to show the measurement angles, {alpha} and β: (A) for sections cut in the cross-section plane, and (B) the tangential plane. Note that in the tangential plane there are zones of measurement on each side of the center line, and that measurements of circularly aligned fibers further away from the center line will have an additional elevation component which requires a correction factor.

 
2.1 Analysis of results
The analysis and graphical methods are well established [25, 29], and have been used by us in other related studies [28, 30–32]. The 3 reference axes for a cylindrical vessel (as shown in Fig. 2) are (i) the longitudinal axis (L) running parallel to the vessel axis, (ii) the circumferential axis (C), and (iii) the radial axis (R). For vessels cut in cross-section (Fig. 2A) the azimuth angle ({alpha}) is the angle between the projection of the fiber in the plane of the section and the tangent to the vessel wall at that point, and is equivalent to a spiral orientation of the fibers within the vessel wall, while the elevation angle (β) represents a helical orientation, with the highest β (90°) for longitudinal fibers. When measurements are made from tangential sections, the azimuth angle ({alpha}) becomes a measure of the helical variation, and the elevation angle (β) indicates the radial component (Fig. 2B). At each point of measurement, an additional reading for alignment reference was taken of the angle of the tangent to the vessel wall for the cross-sectioned vessels, or of the vessel axis for the tangential sections.

Three-dimensional data were plotted on a Lambert equal-area projection (Fig. 3). To illustrate this projection, each orientation may be pictured as a line passing through the center of a hemisphere and intersecting its surface at a unique point. The hemisphere is then mapped onto a two-dimensional plane as in a map of the world. Each three-dimensional measurement is thus represented by a single point on the graph, and regions of concentration of data show general alignments of the birefringent fabric. The projection may be rotated about any axis so that the region of interest is shown in the central area of the plot (Fig. 3B), and contours of relative concentration density may be drawn (Fig. 3C).


Figure 3
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  		Fig. 3 Lambert projections illustrating rotation about the East–West axis so that the groupings of orientation measurements are presented more meaningfully. For these projections, data from several lamellae of the media of an internal mammary artery, cut in cross-section, have been combined. (A) The primary data superimposed on a computer drawn graph with the grid lines at 2° intervals. Positions on the projection of fibers with exactly circumferential, longitudinal, or radial orientations are indicated, respectively, as C, L or R (positions which rotate when the projection is rotated as in panels B and C). (B) The same data rotated by 90° around the East–West axis, so that the scatter of alignments can be seen. Points a and b are shown in their new rotated positions. While point a moves upward 90° along its upward trajectory, point b moves upward approximately 30°, and then completes its 60° rotation moving up from the position on the rim diametrically opposite. (C) The same rotated data, contoured at 1, 5, 10, and 15 times the uniform concentration.

 
For most of the measurements the data from any one region have a central tendency of orientation, with a symmetrical scatter about that mean orientation. Three-dimensional dispersions of this kind are known as ‘Fisher distributions’. Thus the statistical analysis for obtaining means and variations of alignment was done by the Fisher method for spherical data [29, 33]. Parameters include the {alpha}95 value, which is a measure of confidence in the mean orientation, similar to the standard error of the mean in ordinary statistics, and is the solid angle about the mean direction within which there is a 95% probability of finding the true mean. The CSD, or circular standard deviation, is the solid angle about the mean enclosing 63% of the data, and is a measure of the scatter of directions about the mean.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Comparison of structural differences
Tracings made of the vessels using a projection microscope, in conjunction with examination of the sections using different stains, revealed obvious gross differences between saphenous veins and internal mammary arteries. There was a marked variability in the lumen diameter of the SVs ranging from 3.1 to 8.5 mm. Since the cross-sectional area is proportional to the square of the diameter, this results in a variation in the SV luminal areas of up to 7.5 to 1. The IMA diameters are much more consistent, from 1.9 to 2.6 mm, giving a maximum difference in cross-sectional area of less than 2 to 1. Similarly the relative widths of the walls of the 11 SVs varied widely, ranging from 180 to 650 µm, compared with 180 to 430 µm in the 23 arteries. Although the SV is often described as having little or no subendothelium, our examination revealed it to be present in 6 of the 11 vessels, being substantially thickened in regions of two of them. Eleven of the 23 IMAs had a measurable subendothelium, but only 3 had a thickness greater than the width of the media. The demarcation of the 3 main vessel wall tunics of the IMA was always distinct, but in the SV they were not easily identified. As well as the circumferential fibers of the SV media there was commonly an inner region, sometimes of greater width than the circumferential band, containing bundles of longitudinally oriented smooth muscle cells. This was bounded on the inside by the subendothelium, in which elastin was present, as well as collagen and some smooth muscle. On the outer side of the media there were frequently more bundles of longitudinal smooth muscle, appearing in some cases to be a part of an outer media, but more often being located within the adventitia.

With two of the IMAs we were able to evaluate, independently, proximal and distal segments separated by approximately 6 cm. The proximal and distal lumen diameters were 2.6 and 2.1 mm in the first vessel, and 2.5 and 2.1 mm in the second. Widths of the tunica media were 250 and 90 µm in the first and 190 and 180 µm in the second.

Micrographs of internal mammary artery and saphenous vein are shown in Fig. 4, illustrating both cross and tangentially cut sections. Fig. 4A,B shows cross-sections of SV with bright-field microscopy and with circularly polarized light. Both inner and outer regions of longitudinal smooth muscle are visible. With the polarized light these appear dark owing to the relatively high elevation angle of the optical axis of the fibers. The micrograph of the cross-sectioned IMA of Fig. 4C shows the very distinct layering within the media, a feature also apparent under polarized light in Fig. 4D. Tangentially cut sections of Fig. 4E,F show the thin, highly aligned collagen within the media of the SV as opposed to the discrete, variously oriented layers within the media of the IMA.


Figure 4
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  		Fig. 4 Color micrographs of saphenous veins and internal mammary arteries. (A) Trichrome-stained section of saphenous vein (SV 2), cut in cross-section. Longitudinal bundles of smooth muscle are seen as red granular clusters in the inner media and in the adventitia (up arrows). Note the straightened circumferential fibers of the media seen as horizontal threads (down arrows). Muscle is stained red and collagen blue or blue-green. (B) Circularly polarized light micrograph of SV 5 showing straightened collagen of the tunica media (up arrows; red-yellow interference colors), with longitudinal muscle bundles appearing dark (down arrows; muscle birefringence not enhanced by the picrosirius red stain). (C) Internal mammary artery (IMA 4), trichrome with Verhoeff's elastin stain. The elastin layers appear black, muscle is red and collagen blue. The obliquely oriented fibers between the elastin layers of the tunica media can be seen (arrows). There is no subendothelium. (D) The same artery (IMA 4), stained with picrosirius red and viewed with circularly polarized light. Birefringent collagen appears red. The distinctly layered nature of the media, and the fine waviness of the collagen fibers are visible (arrows). (E) Tangential section of IMA 3, trichrome stain. The section is cut almost to the lumen and the vessel axis is running North–South. Red smooth muscle cells of the media are seen in lamellae (arrows), interspersed with blue collagen. Collagen of the adventitia is seen on the right side. (F) Tangentially sectioned saphenous vein (SV 4), stained with picrosirius red (circularly polarized light). The straightened circumferential collagen of the media is seen as yellow/orange interference colors (up arrows), and the wavy red collagen of the adventitia is at the left and right edges (down arrows). A longitudinal bundle of smooth muscle is visible at the top left (arrowhead). Bars in panels A–D are 100 µm, in panels E and F are 200 µm.

 
3.2 Measurements of alignment
3.2.1 Cross-sectioned vessels
Detailed orientation measurements were made from 5 internal mammary arteries and 5 long saphenous veins, based on a series of readings from a band across the vessel wall. These measurements were from sections stained with picrosirius red for birefringent enhancement of the collagen. Depending on the morphology of each tissue section, the size and number of regions examined were varied from vessel to vessel in order to allow us to assess regional differences wherever possible. In all the vessels readings were made from the media and, where there was sufficient thickness, from the subendothelium and adventitia. The 5 IMAs examined had 7, 9, 10, 10 and 11 lamellae respectively within the media, each separated by a layer of elastin. In contrast, the media of the SV did not show distinct sublayering, and were subdivided into regions for measurements mainly on the basis of textural and compositional variations revealed under polarized light. Measurements of the adventitia were made from two arteries and 3 veins, and of the subendothelium from two arteries and 4 veins.

Results from the media of one IMA, and from one SV are given in Table 1. The values for the spiral and helical angles for each sublayer are a measure of the mean alignment, and the circular standard deviation (CSD) reflects the scatter of data about that mean; the {alpha}95 indicates the confidence in the mean. It can be seen from the low helical and spiral angles that the collagen fibers of the vein were highly aligned circumferentially, whereas the alignment of the IMA collagen within each lamella was varied from layer to layer, especially in the helical (longitudinal) direction. When the data from all the layers of each vessel were pooled (combined media), the mean fiber orientations of both the IMA and the SV media were close to the circumferential direction, but the larger CSD for the artery reflected the variation of the average alignments of the individual layers. Table 2 presents the results of the pooled media readings of the circumferential groups of fibers from the 5 arteries and 5 veins, all stained with picrosirius red. The longitudinally oriented smooth muscle groups from the inner media were not included mathematically in these combined results since they are clearly distinct from the circumferential fibers.


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  		Table 1 Data from the tunica media of IMA 3 and SV 2

 

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  		Table 2 Tunica media, summary of data for 5 IMAs and 5 SVs

 
Readings from the subendothelium and tunica adventitia are presented in Table 3. In some cases the tunica was divided into an inner and outer region, and in others, areas of collagen and bundles of smooth muscle were measured separately. It can be seen that in these tunicae the helical component is very much higher than it is in the media, and there is often a high circular standard deviation. Results from the subendothelium of the IMA are separated into inner (adjacent to the lumen) and outer regions and also include readings taken from both the picrosirius-red- and the silver-stained sections. Lambert projections illustrating orientation data from representative regions of media, subendothelium and adventitia are shown in Fig. 5 for cross-sectioned vessels, and Fig. 6 for the same vessels sectioned tangentially. By the coherence of data on some Lambert projections, versus the dispersion of orientations on other projections, the quantitative differences between the two bypass vessels are immediately apparent.


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  		Table 3 Results from adventitia and subendothelium

 

Figure 5
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  		Fig. 5 Tracings of cross-sectioned vein and artery (SV 3 and IMA 3). Lambert projections of representative regions are shown. Rotations of the projections are such that circumferential fibers, for the plots of media and inner adventitia (#7), appear at the center of the graph. Projections #5, 6, and 8, which are comprised of mainly longitudinal fibers, have been rotated so that the longitudinal orientation is at the center. Note the coherence of alignment in the media of the saphenous vein, and the greater dispersion of the data in the media of the artery. The scatter of data within the individual lamellae of the IMA media can be compared with the projection of Fig. 3B in which the data from all the lamellae of an IMA have been pooled, revealing the greater dispersion of alignments. Examples of the statistics relating to some of these regions (#1–8) are indicated in Table 1 and Table 3.

 

Figure 6
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  		Fig. 6 Tracings of tangential sections of the same vein and artery as Fig. 5. The vessel axes are oriented North–South on the figure, and the sections were cut so that they are almost through to the lumen. Lambert projections of some of the measured regions are shown. The projections of the media present circumferential fibers at the center, and those of the adventitia and subendothelium present longitudinal fibers at the center. The strong circumferential co-alignment of the fibers in the SV media is clearly seen. Representative statistical data for some of these regions are:

#1: helical angle = 2°; spiral angle = 2°; CSD = 5°; {alpha}95 = 3° (SV media).

#2: helical angle = 83°; spiral angle = 1°; CSD = 19°; {alpha}95 = 9° (SV outer adventitia).

#3: helical angle = 81°; spiral angle = 2°; CSD = 28°; {alpha}95 = 13° (IMA adventitia).

#4: helical angle = 37°; spiral angle = 27°; CSD = 13°; {alpha}95 = 9° (IMA media).

 
The media of the SV is comprised mostly of collagen, with a small fraction of scattered smooth muscle cells. In the media of the IMA, however, the smooth muscle predominates. Since the picrosirius red stain is specific to collagen, we made some additional sets of measurements of 3 of the cross-sectioned arteries using adjacent slides stained with silver impregnation stain, which enhances the birefringence of both collagen and smooth muscle. In this way we were able to compare readings of the collagen component with those of the total tissue content of the tunica media. In each vessel the mean value of the CSD, averaged across all the layers, was lower for the silver-stained sections by 1 or 2° than for the picrosirius-stained collagen. For these 3 matched pairs of adjacent sections there was a strong correspondence between separate layers when comparing the mean values and dispersion of orientation.

3.2.2 Tangential sections
Measurements were made from 3 saphenous veins and from 3 mammary arteries, from regions of the vessels approximately 5–10 mm from the cross-sections of the same vessels. In this way, a general comparison of the results from the two methods is possible. The lamellar appearance of the media was very evident for the artery sections, but less so in the saphenous veins. From each vessel, two sets of readings were obtained, one from each side of the midline. For measurements that are at a distance from the center axis, especially where the wall is relatively thick compared to the diameter, there is an error introduced due to the vessel curvature (Fig. 2B). Corrections were made for this effect based on the vessel diameter measured from the cross-sections, and on the radial distance of the measurement from the center axis as recorded on the micrograph [28]. The number of lamellae in the tunica media of the 3 IMA sections was 7, 7, and 9, although in some regions there were different numbers on each side of the center line. The mean CSD of individual lamellae in the IMAs was 9.3°, compared to a mean CSD in the SV media of 6.2°. Measurements were made from the adventitia of all 3 SVs and two of the IMAs, and from the subendothelium of two SVs and one IMA. The longitudinal alignment of these layers is clearly apparent in tangential sections, with mean helical angles for the subendothelium being 76° for the vein, and 71° for the artery (90° would be longitudinal). The average CSD for the sublayers of the subendothelium of the veins was 14°, and for the one artery subendothelium the CSD was 45°, indicating widely dispersed tissue alignment. Values for the adventitia of the IMAs were close to longitudinal and gave a mean helical angle of 86°, with a CSD of 28°, and for the SVs the mean helical angle was 81°, and the average CSD was 21°.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
The epicardial physical environment is new for the two commonly used bypass vessels, but for the vein the difference from its normal environment is substantial. A saphenous vein segment chosen to function as a graft vessel is often exposed to considerable physical trauma during preparation for implantation, including transmural pressures of 300 mmHg or even higher being applied in order to reduce the risk of spasm [16, 17, 34]. These pressures would translate into exceptional stretching forces with high strain (perhaps exceeding 100% circumferentially, and 40% longitudinally [7]). Post-implantation, the SV segment is exposed to a new physical environment that includes the much higher arterial transmural pressure, a cyclically changing pressure, and also longitudinal strains that affect both arterial and venous transplants, imposed by the reversible expansion of the heart with the cardiac cycle. These strains may be as high as 30%, as measured by phase-contrast magnetic resonance imaging [35]. Work from other laboratories on smooth muscle cells cultured on cyclically stretching membranes has demonstrated that pulsatile stretching at 60 cycles/min stimulates smooth muscle proliferation in the SV but not in the IMA [36], results that are important because of the longitudinal and circumferential cyclic strains imposed on the transplanted vessels.

Our focus has been on the differences revealed when the respective wall structures of the SV and IMA are brought under arterial pressure load. The SV has both a larger diameter and a relatively thin wall, with considerable variability of structural components and dimensions. Although far from rupture, the vein bears a much higher internal circumferential wall stress ({sigma}C) than does the smaller IMA (i.e., {sigma}C=R{Delta}P/t) [37]. When compared at the same transmural pressure, {Delta}P, the average wall stress is proportional to R/t (vessel radius/wall thickness) which had values of 6.6 for the veins, and 3.7 for the IMA. Thus, in addition to the new pulsatile strains discussed above, the much higher transmural pressures for the transplanted vein adds to the physical circumstances that lead to proliferation.

The tunica media is the predominant layer morphologically in both the SV and the IMA. Results from the three-dimensional analysis indicate that it also makes the greatest contribution to the stiffness of the wall at higher pressures. The media of the IMA was comprised of distinct lamellae of elastin that were often straight, but sometimes wavy and fragmented. Within the lamellae, the collagen was wavy, and the smooth muscle was oriented at a somewhat oblique angle although co-aligned in each lamella. Both these alignments permit further extension of the artery wall. In the case of the SV, the media contained a layer, mostly of collagen with some smooth muscle. that was highly aligned in the circumferential direction. This had the appearance of being stretched taut. The alignment of smooth muscle is important because its passive extension, or over-extension, is implicated as the triggering event for hyperplasia [36, 38–40]. In most of the SVs in our study there were also bundles of smooth muscle in the inner region of the media, that were straight and co-aligned close to the long axis of the vein. The micrograph of Fig. 4A shows a saphenous vein with a strong presence of longitudinal smooth muscle throughout the wall. It is this longitudinal tissue component that would react to strains from heart wall movement.

The subendothelium of the IMA was generally very thin. Where sets of measurements could be made, a narrow band adjacent to the lumen was found to be circumferential in orientation, but for the main part the subendothelium was helically aligned, and had a high degree of disorganization. The orientation of the subendothelium of the SV was generally longitudinal, also with a high CSD indicating a large scatter of alignments. Collagen, smooth muscle and elastin were present, sometimes with bundles of longitudinal smooth muscle adjacent to the media. In the case of collagen orientation, its variability from fiber to fiber, and variety of helical alignments in both the SV and IMA would allow more give, or elastic softness, for that region of the wall.

The collagen of the adventitia of the IMA was oriented in the longitudinal or high helical direction (near to longitudinal) with a wide scatter of alignment, and for a few arteries there was also a thin outer layer having a predominantly circumferential orientation. In the SV, the adventitia was generally comprised of longitudinally aligned wavy collagen, interspersed with longitudinal straight smooth muscle, often arranged in large bundles. The scatter of alignment and the waviness of the collagen of the adventitia provides it with the capability for further distension with pressure. In addition, there was occasionally an inner, more circumferential collagen band.

Experimental results from the work of others show that veins have high distensibility up to the pressure range of 30–50 mmHg, after which they become extremely stiff [7, 41], whereas the arteries are less distensible than the veins at low pressures but remain relatively distensible even up to 200 mmHg [7]. Our results on structural alignment, specifically the high coherence of alignment of the collagen in the SV compared to the IMA, both at arterial pressure, nicely complement these mechanical assessments of high venous stiffness.

It is a continuing theme that wall structure is important for the function of vessels used for bypass. The internal mammary artery is successful in an environment that is prone to intimal proliferation, and in which many other replacement vessels fail. Native coronary arteries, which are muscular arteries, are themselves prone to endothelial hyperplasia [30, 42, 43]. The process of subintimal hyperplasia and atherosclerosis continues to plague the major arteries of the heart and brain as well as the aorta. Because of the particular success of the IMA as a bypass vessel its study may lead to insight into the larger problem of atherosclerosis in general. The new morphometric information from this study will serve as a basis for progress in understanding the long-term patency of these vessels.

Time for primary review 19 days.


   Acknowledgements
 
The authors would like to thank Jan Dixon for her histological assistance, and Eric Talman who contributed as a summer research student. The work was supported by the Heart and Stroke Foundation of Ontario, and by the Atkinson Foundation.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

  1. Lytle B.W., Loop F.D., Cosgrove D.M., Ratliff N.B., Easley K., Taylor P.C. Long-term (5 to 12 years) serial studies of internal mammary artery and saphenous vein coronary bypass grafts. J Thorac Cardiovasc Surg (1985) 89:248–258.[Abstract]
  2. Barner H.B., Swartz M.T., Mudd J.G., Lyras D.H. Late patency of the internal mammary artery as a coronary bypass conduit. Ann Thorac Surg (1982) 34:408–412.[Abstract]
  3. Tector A.J., Schmahl T.M., Janson B., Kallies J.R., Johnson G. The internal mammary artery graft; its longevity after coronary bypass. J Am Med Assoc (1981) 246:2181–2183.[Abstract/Free Full Text]
  4. Grondin C.M., Campeau L., Lesperance J., Enjalbert M., Bourassa M.G. Comparison of late changes in internal mammary artery and saphenous vein grafts in two consecutive series of patients 10 years after operation. Circulation (1984) 70(suppl I):I208–I212.[Medline]
  5. van Son J.A.M., Smedts F., Vincent J.G., van Lier H.J.J., Kubat K. Comparative anatomic studies of various arterial conduits for myocardial revascularization. J Thorac Cardiovasc Surg (1990) 99:703–707.[Abstract]
  6. van Son J.A.M., Smedts F., Korving J., Guyt A., de Kok L.B. Intercostal artery: histomorphometric study to assess its suitability as a coronary bypass graft. Ann Thorac Surg (1993) 56:1078–1081.[Abstract]
  7. Dobrin P., Canfield T., Moran J., Sullivan H., Pifarré R. Coronary artery bypass: The physiological basis for differences in flow with internal mammary artery and saphenous vein grafts. J Thorac Cardiovasc Surg (1977) 74:445–454.[Web of Science][Medline]
  8. van Son J.A.M., Smedts F., de Wilde P.C.M., et al. Histological study of the internal mammary artery with emphasis on its suitability as a coronary artery bypass graft. Ann Thorac Surg (1993) 55:106–113.[Abstract]
  9. Milroy C.M., Scott D.J.A., Beard J.D., Horrocks M., Bradfield J.W.B. Histological appearances of the long saphenous vein. J Pathol (1989) 159:311–316.[CrossRef][Web of Science][Medline]
  10. Burton A.C. Relation of structure to function of the tissues of the wall of blood vessels. Physiol Rev (1954) 34:619–642.[Free Full Text]
  11. Wolinsky H., Glagov S. Structural basis for the static mechanical properties of the aortic media. Circ Res (1964) 14:400–413.[Abstract/Free Full Text]
  12. Cox R.H. Passive mechanics and connective tissue composition of canine arteries. Am J Physiol (1978) 234:H533–H541.[Web of Science][Medline]
  13. Ramos J.R., Berger K., Mansfield P.B., Sauvage L.R. Histologic fate and endothelial changes of distended and nondistended vein grafts. Ann Surg (1976) 183:205–228.[Web of Science][Medline]
  14. Angelini O.D., Breckenridge I.M., Williams H.M., Newby A.C. A surgical preparative technique for coronary bypass grafts of human saphenous vein which preserves medial and endothelial functional integrity. J Thorac Cardiovasc Surg (1987) 94:393–398.[Abstract]
  15. Cox J.L., Chiasson D.A., Gotlieb A.I. Stranger in a strange land: The pathogenesis of saphenous vein graft stenosis with emphasis on structural and functional differences between veins and arteries. Prog Vasc Dis (1991) 34:45–68.
  16. Soyombo A.A., Angelini G.D., Bryan A.J., Newby A.C. Surgical preparation induces injury and promotes smooth muscle cell proliferation in a culture of human saphenous vein. Cardiovasc Res (1993) 27:1961–1967.[Abstract/Free Full Text]
  17. Angelini G.D., Passani S.L., Breckenridge I.M., Newby A.C. Nature and pressure dependence of damage induced by distension of human saphenous vein coronary artery bypass grafts. Cardiovasc Res (1987) 21:902–907.[Abstract/Free Full Text]
  18. Grande J.P., Glagov S., Bates S.R., Horwitz A.L., Mathews M.B. Effect of normolipemic and hyperlipemic serum on biosynthetic response to cyclic stretching of aortic smooth muscle cells. Arteriosclerosis (1989) 9:446–452.[Abstract/Free Full Text]
  19. Dartsch P.C., Hämmerle H. Orientation response of arterial smooth muscle cells to mechanical stimulation. Eur J Cell Biol (1986) 41:339–346.[Web of Science][Medline]
  20. Einav S., Avidor J., Vidne B. Haemodynamics of coronary artery – saphenous vein bypass. J Biomed Eng (1985) 7:305–309.[CrossRef][Web of Science][Medline]
  21. Klimach O., Chapman B.L.W., Underwood C.J., Charlesworth D. An investigation into how the geometry of an end-to-side arterial anastomosis affects its function. Br J Surg (1984) 71:43–45.[CrossRef][Web of Science][Medline]
  22. Abbott W.M., Megerman J., Hasson J.E., L'Italien G., Warnock D.F. Effect of compliance mismatch on vascular graft patency. J Vasc Surg (1987) 5:376–382.[CrossRef][Web of Science][Medline]
  23. Duan B., Zamir M. Mechanics of wave reflections in a coronary bypass loop model: the possibility of partial flow cut-off. J Biomechs (1995) 28:567–574.[CrossRef]
  24. Loop F.D., Lytle B.W., Cosgrove D.M. New arteries for old. Circulation (1989) 79(suppl I):I40–I45.[Medline]
  25. Phillips WR. Mineral optics: principles and techniques. San Franciso: WH Freeman, 1971:171–190.
  26. Puchtler H., Waldrop F.S., Valentine L.S. Polarization microscopic studies of connective tissue stained with picro-sirius red FBA. Beitr Pathol (1973) 150:174–187.[Web of Science][Medline]
  27. Emmons R.C. The universal stage. Geol Soc A Mem (1943) 8:1–24.
  28. Finlay H.M., McCullough L., Canham P.B. Three-dimensional collagen organization of human brain arteries at different transmural pressures. J Vasc Res (1995) 32:301–312.[Web of Science][Medline]
  29. Tarling DH. Palaeomagnetism: principles and applications in geology, geophysics and archaeology. London: Chapman and Hall, 1983:108–144.
  30. Canham P.B., Finlay H.M., Dixon J.G., Boughner D.R., Chen A. Measurements from light and polarized light microscopy of human coronary arteries fixed at distending pressure. Cardiovasc Res (1989) 11:973–982.
  31. Canham P.B., Talman E.A., Finlay H.M., Dixon J.G. Medial collagen organization in human arteries of the heart and brain by polarized light microscopy. Connect Tiss Res (1991) 26:121–134.[Web of Science][Medline]
  32. Finlay H.M., Dixon J.G., Canham P.B. Fabric organization of the subendothelium of the human brain artery by polarized-light microscopy. Arterio Thrombo (1991) 11:681–690.
  33. Fisher NI, Lewis T, Embleton BJJ. Statistical analysis of spherical data. Cambridge: Cambridge University Press, 1987:101–151.
  34. Roubos N., Rosenfeldt F.L., Richards S.M., Conyers R.A.J., Davis B.B. Improved preservation of saphenous vein grafts by the use of glyceryl trinitrate–verapamil solution during harvesting. Circulation (1995) 92(suppl II):II31–I136.[Medline]
  35. Pelc N.J., Drangova M., Pelc L.R., et al. Tracking of cyclic motion with phase-contrast cine MR velocity data. JMRI (1995) 5:339–345.[CrossRef][Medline]
  36. Predel H.G., Yang Z., von Segesser L., Turina M., Bühler F.R., Lüscher T.F. Implications of pulsatile stretch on growth of saphenous vein and mammary artery smooth muscle. Lancet (1992) 340:878–879.[CrossRef][Web of Science][Medline]
  37. Burton AC. Physical principles of circulatory phenomena: the physical equilibria of the heart and blood vessels. In: Hamilton WF, editor. Handbook of physiology. Circulation, vol 1, sect 2. Baltimore: Waverly Press, 1962:85–106.
  38. Soyombo A.A., Angelini G.D., Newby A.C. Neointima formation is promoted by surgical preparation and inhibited by cyclic nucleotides in human saphenous vein organ cultures. J Thorac Cardiovasc Surg (1995) 109:2–12.[Abstract/Free Full Text]
  39. Cockx M.M., Wuyts F.L., Buyssens N., et al. Longitudinally oriented smooth muscle cells in rabbit arteries. Virchows Arch A Pathol Anat (1993) 422:293–299.[CrossRef]
  40. Pietilä K., Nikkari T. Role of the arterial smooth muscle cell in the pathogenesis of atherosclerosis. Med Biol (1983) 61:31–44.[Web of Science][Medline]
  41. Wesly R.L.R., Vaishnav R.N., Fuchs J.C.A., Patel D.J., Greenfield J.C. Jr. Static linear and nonlinear elastic properties of normal and arterialized venous tissue in dog and man. Circ Res (1975) 37:509–520.[Abstract/Free Full Text]
  42. Gross L., Epstein E.Z., Kugel M.A. Histology of the coronary arteries and their branches in the human heart. Am J Pathol (1934) 10:253–274.[Web of Science]
  43. Velican C., Velican D. Some particular aspects of the microarchitecture of human coronary arteries. Atherosclerosis (1979) 33:191–200.[CrossRef][Medline]

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