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Features of atherosclerosis in the tunica adventitia of coronary and carotid arteries in a black Kenyan population.

1. Introduction

Tunica adventitia of arteries was previously believed to be involved only in physical and nutritive support of the vessel wall [1]. Currently, however, it is known to be a highly cellular and metabolically active component of the vessel wall, capable of controlling its structure, function, and disease processes from "outside-in" [2-4]. It plays a leading role in the initiation, progression, and complications of atherosclerosis [5, 6]. Adventitial thickness has been independently correlated with cardiovascular risk factor profile [7-9]. Consequently, it has become a target for therapeutic interventions in the treatment of atherosclerosis [10,11]. In spite of these developments, there are few histological reports of the changes which occur in the tunica adventitia of vulnerable arteries. Further, the adventitia role in atherosclerosis and the stage of involvement are not universally accepted, as some studies suggest it is not involved [12]. As atherosclerosis increases in Sub-Saharan Africa, there is need to document these adventitial features so as to improve understanding of the disease process as well as aid in early diagnosis and intervention. This study therefore examined changes in tunica adventitia of both left anterior descending (LAD) and common carotid arteries (CCA), in a black Kenyan population.

2. Materials and Methods

Materials for this study were obtained from proximal left anterior descending arteries and distal common carotid arteries of 108 black Kenyans [76 males, 32 females; of mean age 34.6 years; range 2-82 years], during autopsy. Samples were taken within 48 hours of death, to avoid overt postmortem damage to the tissues. Two millimeter long specimens were fixed by immersion in 10% formaldehyde solution for three days, then trimmed, and processed routinely for paraffin embedding. Seven micrometer sections were cut and stained with Haematoxylin/Eosin for demonstration of the general organization of the mural components and Mason's trichrome stain for demonstration of collagen and smooth muscle cells. The slides were then examined at various magnifications, using a light microscope and images taken using a high resolution digital camera. Results are presented in micrographs.

3. Results

Tunica adventitia was thickened in 16 (14.8%) and 12 (11.1%) of the LAD and CCA, respectively. In 7 (6.5%) cases of LAD, adventitial thickening was not associated with increased vasa vasora density and occurred with normal tunica intima (Figure 1(a)). In 8 (7.4%) cases, the thickening was associated with mild intimal hyperplasia and disintegration of internal elastic lamina (Figure 1(b)). In 9 (8.3%) cases, the tunica adventitial thickening was associated with marked increase in density of vasa vasora and intimal hyperplasia (Figure 1(c)). In 3 (2.8%) cases, the vasa vasora penetrated into the tunica media, splitting the outer layers of smooth muscle cells (Figure 1(d)).

In the CCA, similar features were observed. Only 3 (2.8%) cases of tunica adventitial thickening were associated with intimal thickening. In 8 (8.3%) of the cases, thickening of the tunica adventitia was associated with only slight intimal thickening. The thickening was associated with increased density of vasa vasora in 6 (5.5%) cases. Additionally, however, in 4 (3.7%) cases, marked thickening of the adventitia with proliferation of vasa vasora occurred without intimal hyperplasia (Figure 2(a)). Further, in 3 (2.8%) cases, proliferation of vasa vasora in the tunica adventitia was associated with medial degeneration whereby the tunica media appeared unstructured (Figure 2(b)).

4. Discussion

Observations of the current study reveal that marked thickening of tunica adventitia occurred in over 10% of the cases. Adventitial thickening is known to increase during atherosclerosis [13, 14]. This increase in the thickness is thought to be due to activation of adventitial fibroblasts by atherogenic stimuli, leading to production of more extracellular matrix. The activated fibroblasts also upregulate production of chemokines and cytokines that lead to recruitment of inflammatory cells [2, 15]. The findings of the present study imply that a significant proportion of asymptomatic individuals in the Kenyan population display features of atherosclerosis. Accordingly, screening for atherosclerosis in individuals at risk of the disease should also include the tunica adventitia.

A remarkable finding of the present study was that, in a substantial proportion, increased thickness of tunica adventitia occurred in the absence of intimal hyperplasia, which is well known to herald atherosclerosis [16,17]. This appears at variance with the generally accepted view that adventitial thickening occurs in advanced atherosclerosis because of inflammation and increased neovascularization by vasa vasora [17, 18]. This early isolated adventitial thickening is consistent with the outside-in mechanism of atherogenesis in which the inflammation is initiated in the adventitia as the first responder early in the disease process and progresses inwards towards the intima [3, 19, 20]. Accordingly, in evaluating vulnerable arteries, due attention should be paid to both the tunica adventitia and intima for features of early atherosclerosis.

Adventitial vasa vasora proliferation, usually triggered by atherogenic stimuli such as hypertension, dyslipidemia, and hypoxia, occurs in the pathogenesis, distribution, progression, and development of complications of atherosclerosis [11, 21, 22]. Their density increases with growth of atherosclerotic plaque in asymptomatic patients and is correlated with plaque vulnerability and haemorrhage [11, 23, 24]. The findings of the current study that, in 8.3% of LAD and 5.5% of CCA, increased tunica adventitial thickening occurred in the wake of high vasa vasora density suggest that, in this proportion of asymptomatic patients, atherosclerosis was present and in some cases had attained complicated stages and cardiovascular events were imminent.

Proliferation of adventitial vasa vasora and their invasion into the tunica media and intima, called mural neovascularization, as observed in the present study, facilitate pathogenesis of atherosclerosis by providing a considerable endothelial exchange surface for harmful circulating substances and cells to the vessel wall [25-27]. Further, they create a conduit for transport of inflammatory cells and mediators into the arterial wall to promote chronic inflammation and plaque neovascularization [21, 28, 29]. The increased density of vasa vasora within the thickened tunica adventitia observed in the present study suggests that, in patients at risk of atherosclerosis, there is need for early evaluation of tunica adventitial integrity and commencement of appropriate measures to avert complications.

5. Conclusion

Findings of the present study reveal that features of atherosclerosis, namely, increased adventitial thickness and vasa vasora density, occur in the tunica adventitia of coronary and carotid arteries in over 10% of the black Kenyans studied. These features often precede intimal and medial changes. Tunica adventitia should therefore be prioritized in evaluation of vulnerable arteries for atherosclerosis, in individuals at risk. This may enhance early detection and intervention.

http://dx.doi.org/ 10.1155/2014/456741

Conflict of Interests

The authors declare that they have no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are grateful to Acleus Murunga, Judith Machira, and Esther Mburu for technical assistance and Antonina Odock-Opiko for typing the paper.

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Julius Ogeng'o, Kevin Ongeti, Moses Obimbo, Beda Olabu, and Philip Mwachaka

Department of Human Anatomy, University of Nairobi, P.O. Box 30197, Nairobi 00100, Kenya

Correspondence should be addressed to Philip Mwachaka; pmaseghe@gmail.com

Received 14 October 2013; Revised 27 January 2014; Accepted 14 February 2014; Published 17 March 2014

Academic Editor: Ayhan Comert
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Title Annotation:Research Article
Author:Ogengo, Julius; Ongeti, Kevin; Obimbo, Moses; Olabu, Beda; Mwachaka, Philip
Publication:Anatomy Research International
Article Type:Report
Geographic Code:4EUAU
Date:Jan 1, 2014
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