|Year : 2021 | Volume
| Issue : 1 | Page : 30-34
Microanatomy of Left Internal Thoracic Artery and Left Anterior Descending Artery: A Comparative Study
TK Kumari1, Lola Das2, K Renuka3, R Rajeev4, Nithin K Raju4
1 Professor, Department of Anatomy, Government Medical College, Thiruvananthapuram, Kerala, India
2 Professor, Department of Anatomy, Government Medical College, Thrissur, Kerala, India
3 HOD and Professor, Department of Anatomy, Government Medical College, Thiruvananthapuram, Kerala, India
4 Junior Resident (MD Anatomy), Government Medical College, Thiruvananthapuram, Kerala, India
|Date of Submission||08-Sep-2020|
|Date of Decision||27-Oct-2020|
|Date of Acceptance||24-Dec-2020|
|Date of Web Publication||27-Jan-2021|
REKHA, Indira Nagar, Ayyanthole, Thrissur - 680 003, Kerala
Source of Support: None, Conflict of Interest: None
Introduction: The knowledge of the microscopic structure of the left internal thoracic artery (LITA) and left anterior descending (LAD) artery can be correlated to the effectiveness of coronary artery bypass grafting using LITA graft. The objectives of the study were to study the microanatomy of the LITA and LAD artery, to compare the thickness of the wall of both arteries, and to report the elastic and muscular nature of these arteries. Materials and Method: Postmortem specimens of both the arteries from 50 samples of males were collected from the mortuary of Forensic Medicine, Government Medical College, Thiruvananthapuram. The tissues were subjected to hematoxylin and eosin and special staining. A photo micrometer was used for measuring the thickness of the wall, diameter of the lumen, and number of smooth muscles. Results: The mean thickness of tunica intima (TI) of LITA and LAD was 18.8 μ and 129.8 μ, respectively. The lumen diameter of LITA and LAD was 945.25 μm2 and 1653.45 μm2, respectively. The thickness of tunica media (TM) and the diameter of the lumen of LITA were increased with age. The number of smooth muscles in LITA was decreased with age. The thickness of TI, media, adventitia, and the luminal diameter of LAD were increased with age. The number of smooth muscles in LAD was decreased with age. Conclusion: Proximal segments of LITA had more elastic fibers arranged in concentric lamellae, especially in the outer zone. The TM of LAD has a greater number of smooth muscles and a smaller number of elastic fibers.
Keywords: Anterior interventricular artery, coronary revascularization, histology, left internal mammary artery, photomicroscope
|How to cite this article:|
Kumari T K, Das L, Renuka K, Rajeev R, Raju NK. Microanatomy of Left Internal Thoracic Artery and Left Anterior Descending Artery: A Comparative Study. Natl J Clin Anat 2021;10:30-4
|How to cite this URL:|
Kumari T K, Das L, Renuka K, Rajeev R, Raju NK. Microanatomy of Left Internal Thoracic Artery and Left Anterior Descending Artery: A Comparative Study. Natl J Clin Anat [serial online] 2021 [cited 2021 Feb 25];10:30-4. Available from: http://www.njca.info/text.asp?2021/10/1/30/308116
| Introduction|| |
Internal thoracic artery (ITA) also known as internal mammary artery arises from the first part of the subclavian artery enters the thorax, passes vertically downward and at the sixth intercostal space, the artery ends by dividing into musculophrenic and superior epigastric branches. ITA is commonly used for coronary artery bypass grafting (CABG). Restenosis is minimum when using ITA compared to other arteries and saphenous vein (SV) in CABG.
Left anterior descending (LAD) artery (LAD, also called anterior interventricular artery) is the continuation of the left coronary artery (LCA) that passes through the anterior interventricular groove and ends on the inferior surface of the heart. The LAD is one of the common sites of coronary occlusion.
The ITA and LAD are medium-sized arteries with three distinct layers in their walls. The innermost layer is tunica intima (TI) formed of the endothelium, subendothelial connective tissue, and internal elastic lamina (IEL). The subendothelial connective tissue consists of delicate elastic and collagenous fibers, few fibroblasts, and occasional smooth muscles. IEL is a thick fenestrated band formed of closely interwoven elastic fibers. Tunica media (TM) is the middle layer consisting of circularly disposed smooth muscle fibers. Between the layers of muscle, there are small amounts of connective tissue composed of elastic fibers and fibroblasts. At the junction between TM and adventitia, there is a fenestrated elastic membrane called external elastic membrane. Tunica adventitia (TA) is the outermost layer composed of connective tissue containing collagenous and elastic fibers, nerve endings, lymphatics, and blood vessels.
In revascularization of LAD using SV grafts, recurrence of angina, myocardial infarction, and necessity of reoperation were reported., Postoperative complications are less with ITA graft; thus, the revascularization of LAD using ITA was considered to be a gold standard. The structural adaptation and the high flow rate of the ITA constitutes a major positive prognostic factor in patients undergoing CABG and has been standard for more than two decades. The coronary revascularization of the LAD using the ITA has a superior long-term patency when compared to venous grafting. The ITA is essentially an elastic artery, able to change the blood flow as per the demand of the myocardium due to the presence of multiple concentric elastic laminae in the TM. The structural integrity of the IEL of ITA supports the concept of lower incidence of atherosclerosis. This notion is further supported by the presence of more number of elastic fibers in its TM. Atherosclerosis is less common in ITA graft. The objectives of the study were to study the microanatomy of the left internal thoracic artery (LITA) and LAD, to compare the thickness of the wall of both arteries, and to report the elastic and muscular nature of these arteries.
| Methodology|| |
This cross-sectional study was carried out in the Department of Anatomy and Mortuary of Forensic Medicine, Government Medical College, Thiruvananthapuram. The institutional ethics committee approved the study before the beginning of the study (approval letter number 09/01/2018/MCT dated 20/7/2018). The study duration was 1 year. Vascular segments were collected from LITA and LAD of 50 postmortem bodies of males of 18–40 years of age. The bodies with crush injury of the chest were excluded from the study. With a population variance of 12%, 95% confidence interval, and 10% margin of error, a sample size of 41 was arrived at. Therefore, this study was done with vascular segmental analysis from 50 postmortem cases.
During the autopsy, 2 cm of tissue was taken from the middle of LITA and LAD and immediately transferred to a bottle containing 10% formalin. A 2 cm LITA segment was taken at the level of the 2nd intercostal space/3rd rib in all the bodies. Similarly, a 2 cm LAD segment was taken 4 cm distal to the origin from the left coronary artery in the interventricular groove. In cases where the origin was obscure, as in the subepicardial position of LAD, segment was taken in the interventricular groove midway between the apex of the heart and atrioventricular groove. The specimens were allowed to get fixed in formalin for 24 h. The fixed tissues were subjected to routine histological processing, and the sections were stained with hematoxylin and eosin (H and E,). Special stains such as Verhoeff's and Van Gieson's were used to demonstrate elastic fibers and smooth muscles, Masson's trichrome for collagen fibers and smooth muscles. Mounted sections were observed under low power (×100) and high power (×400) of a binocular microscope. Using a photo micrometer, the wall thickness, diameter of the lumen, and smooth muscles in TM were counted and recorded. The number of smooth muscles in the TM was counted from 100 μm2 area of five different sites and the average was taken to reduce the error. In case of noncircular vascular sections, the luminal area is measured using the area tool of the photo micrometer, and diameter is computed with the formula, luminal diameter
All continuous variables were expressed as mean with standard deviation. Statistical differences between the parameters of LITA and LAD were analyzed with Student's t-test. Correlation of the number of smooth muscles, luminal diameter, and wall component (TI, TM, and TA) with age is analyzed with linear regression. p < 0.05 was considered statistically significant. The analysis of data was done using version 16 of the Statistical Package for the Social Sciences (SPSS software, IBM Inc, New York, USA).
| Results|| |
Tissue segments from fifty postmortem bodies were analyzed. The mean age was 28.8 (±5.8) years. The samples were divided into five subgroups, 18–22, 23–27, 28–32, 33–37, and 38–40 years.
Microscopic analysis of left internal thoracic artery and left anterior descending artery
Both the arteries were having the similar layers, namely inner TI, middle TM, and outer TA. In the case of LITA with H and E staining the TI was thin compared to LAD and was lined by endothelium. The subendothelial connective tissue is formed of collagen and elastic fibers, connective tissue cells, and occasional smooth muscles. A well-defined IEL was seen outer to the TI. Significantly more elastic fibers were noted in TM in LITA with Verhoeff's staining [Figure 1]a arranged in the form of concentric elastic laminae and minimum number of smooth muscles (Van Gieson's stain). These concentric elastic laminae were located mostly in the outer zone of TM. TA was formed of collagen and elastic fibers, blood vessels, connective tissue cells, nerve endings, and lymphatics. The thickness of TM increased with age and was significant [Figure 2].
|Figure 1: (a) Concentric elastic lamina in the outer zone of tunica media (green arrows) of the left internal thoracic artery after staining by Verhoeff's method (magnification × 100). (b) Fragmentation of the internal elastic lamina (blue arrows) in the section of the left anterior descending artery stained by Verhoeff's method (magnification × 100)|
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|Figure 2: Graph showing tunica media thickness of the left internal thoracic artery in various ages|
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The thickness of TA was not increased with age significantly [Table 1]. The lumen diameter of LITA increased with age but was not statistically significant [Figure 3]a. The number of smooth muscles decreased with age [Table 1].
|Figure 3: Graph showing the luminal diameter of (a) left internal thoracic artery, (b) left anterior descending artery in various ages|
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|Table 1: Histological parameters of the left internal thoracic artery and left anterior descending artery|
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In H and E staining of LAD, the TI formed of innermost endothelium and outer to that the subendothelial connective tissue consisted of collagen fibers and elastic fibers, connective tissue cells, and an IEL. The thickness of TI, TM, and TA at 18 years was 49.2 μ, 82.2 μ, and 62 μ and at 40 years was 598.1 μ, 247.7 μ, and 169 μ, respectively. A statistically significant increase in lumen diameter of LAD with age [Figure 3]b was noted. The mean thickness of LITA was 19.90 μ and significantly lesser than the thickness of LAD (P = 0.093). Wall thickness overall significantly increased with age (P = 0.01) [Figure 4]. The number of smooth muscles in TM significantly decreased with age [Figure 5]. All these findings were statistically significant [Table 1]. Fragmentation of the IEL at various sites was noticed [Figure 1]b. In the TM of LAD, both smooth muscles and elastic fibers were noted. The quantity of smooth muscle was significantly more than that of elastic fibers [Figure 1]b. The luminal diameter at 18 years was 622.3 μ and at 40 years was 1593.2 μ.
|Figure 4: Graph showing (a) tunica intima thickness, (b) tunica media thickness, (c) tunica adventitia thickness of the left anterior descending artery in various ages|
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|Figure 5: Graph showing the number of smooth muscles in the left anterior descending artery in various ages|
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The special stains such as Verhoeff's, Van Gieson, and Masson's Trichrome were done. In Verhoeff's the elastic fibers were seen as black and other structures red in color. In Van Gieson, the smooth muscles were seen as yellow, the connective tissue parts were red in color. In Masson's Trichrome, the collagen fibers were seen as green, the smooth muscles and other connective tissue parts were seen as red. The findings in H and E staining were confirmed using the special stains.
A well-defined IEL and external elastic lamina (EEL) were noticed in both the arteries [Figure 1]. It was found that the IEL of LAD fragmented at various sites [Figure 1]b and the TA has a loose consistency. In the present study, no splitting of the elastic membrane was noticed in the LAD. The TM consisted of more smooth muscle fibers and less elastic fibers. These elastic fibers were arranged in between the muscles.
| Discussion|| |
Arteries have three layers in their walls, namely TI, TM, and TA. Using H and E and special stains, we have demonstrated elastic fibers, collagen fibers, connective tissue cells, and smooth muscles in TM of both LITA and LAD. A concentrically arranged elastic laminar architecture was demonstrated in this study in the LITA proximal part. Similar concentric elastic fibers' arrangement was reported by Märkl et al. and Acar et al. As per the present work, the LITA was found to be an elastic artery as the tissue segments were taken from the middle portion of the vessel supported the findings of Märkl et al. study in that the LITA at its site of origin and middle portion showed features of the elastic artery. The concentric elastic laminar pattern of TM of LITA will definitely help in the maintenance of luminal patency favored the findings such as bypass grafting of LAD using LITA provided positive prognostic factor., The TM of LAD consisted of more smooth muscles (Van Gieson) and minimum elastic fibers (Verhoeff's) in between the smooth muscles [Figure 6]. These findings were favorable to the study of Gross et al. Our findings regarding IEL and EEL were consistent with reports of Waller, and Svendsen, where fragmentations with gaps were reported.
|Figure 6: Photomicrograph of the left anterior descending artery showing a greater number of smooth muscles in the tunica media (yellow) stained by Van Gieson's method (magnification × 100)|
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In the present study, we have compared the microanatomy of LITA and LAD in terms of wall thickness, TM composition, and luminal diameters. No quotable references were there in the case of wall thickness, lumen diameter, and number of smooth muscles in the TM of both the arteries. This information of comparative evaluation of LITA and LAD is of immense significance for critical appraisal of choice of graft during CABG. The coronary arteries were affected by intimal thickening early and more severely compared to the other arteries of the same size. As noted in the present study, LITA and LAD internal diameters are almost comparable with each other. The use of LITA for the restoration of blood flow in obstructed LCA or its branches (LAD) must reasonably establish effective blood flow to the ischemic areas. As the LITA proximal segment has more elastic fibers and the number of muscle fibers decreases more distally, post anastomosis of LITA with LAD may lead to near normal restoration of functions.
In the present study, the cause of death is not noted. Although vascular segments from male dead bodies were considered, the pathogenesis of atherosclerosis starts early and may influence the histoarchitecture of the coronary arteries. The presence of smooth muscles, thickness of the intima, and other cells and products in the TI are not unreported. The present study has not considered the microanatomy in the light of these early yet significant changes in the vessel wall. During the processing of the tissues, the changes in the TI and change in the luminal diameter are well-known. Although enough care is emphasized to select a near-normal vessel cut section from each vascular segment, we cannot rule out completely the effects of tissue processing on these parameters considered in the study.
During CABG, a distal portion of LITA is anastomosed with LAD distal to the obstructed part. In the current study, the parameters considered are from proximal segments of LITA (2nd intercostal space) and LAD (2 cm from the origin of LAD) and may not reflect a true clinically significant outcome.
| Conclusion|| |
LAD luminal diameter increases with age. The number of smooth muscles in LAD significantly decreases with age. LAD had significantly more smooth muscle than elastic fibers. TI of LITA was significantly thinner than the LAD. Significantly more elastic fibers were noted in TM in LITA with Verhoeff's staining arranged in the form of concentric elastic laminae and minimum number of smooth muscles. These concentric elastic laminae were located mostly in the outer zone of TM. The lumen diameter of LITA increased with age but was not statistically significant. The number of smooth muscles in LITA decreased with age.
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