Elastin and collagen fibre microstructure of the human aorta in ageing and disease: a review

Abstract

Aortic disease is a significant cause of death in developed countries. The most common forms of aortic disease are aneurysm, dissection, atherosclerotic occlusion and ageing-induced stiffening. The microstructure of the aortic tissue has been studied with great interest, because alteration of the quantity and/or architecture of the connective fibres (elastin and collagen) within the aortic wall, which directly imparts elasticity and strength, can lead to the mechanical and functional changes associated with these conditions. This review article summarizes the state of the art with respect to characterization of connective fibre microstructure in the wall of the human aorta in ageing and disease, with emphasis on the ascending thoracic aorta and abdominal aorta where the most common forms of aortic disease tend to occur.

Figure 1.

Idealized architecture of a healthy human artery. Arteries possess a three layer structure consisting of an inner layer (intima (I)), middle layer (media (M)) and outer layer (adventitia (A)). The intima is composed mainly of a single layer of endothelial cells, a thin basal membrane and a subendothelial layer of collagen fibrils. The media is composed of smooth muscle cells, a network of elastic and collagen fibrils, and elastic laminae which separate M into a number of transversely isotropic fibre-reinforced units. The adventitia is the outermost layer surrounded by loose connective tissue. The primary constituents of the adventitia are thick bundles of collagen fibrils arranged in helical structures. (Adapted from Gasser et al. [33].)

Figure 2.

(a) Illustration of the different regions of the human aorta. (i) ascending thoracic, (ii) descending thoracic, (iii) supraceliac abdominal, (iv) suprarenal abdominal and (v) mid-infrarenal abdominal aorta. (Adapted from Halloran et al. [31].) (b) Schematic of ascending thoracic aorta and arch. R, root; P, proximal ascending thoracic aorta; D, distal ascending thoracic aorta; and A, arch. (Online version in colour.)

Figure 3.

DeBakey classification (types I, II, III) and Stanford classification (types A, B) of aortic dissection. Type I refers to dissections that propagate from the ascending aorta, extend to the aortic arch, and commonly, beyond the arch distally. Type II refers to dissections that are confined to the ascending portion of the aorta. Type III dissections are limited to the descending aorta. Type III also includes the dissections that start in the descending aorta that extend proximally to the arch and ascending aorta. Stanford type A includes dissections that involve the ascending aorta, arch and descending thoracic aorta. Stanford type B refers to dissections that involve only the descending aorta. Rarely, dissections that start in the descending aorta can also extend proximally into the aortic arch and the ascending aorta. This is a special case of type B dissection, and should be called retro-A dissection. (Adapted from Tran & Khoynezhad [123].)

Figure 4.

Diagrammatic representation of the sample sites in the thoracic aorta. (a) Band position with respect to anatomical features: B1, B2, B3 and B4, bands 1–4. (b) Transverse section of the vessel wall indicating orientation of the sample sites with respect to each other; arrow denotes blood flow into the page. S1 samples were taken from the outer curvature of the aortic wall. (c) Relative sample positions illustrating sites within bands. Design allows for longitudinal and transverse comparisons to be made between sites. (Adapted from Cattell et al. [49].)

Figure 5.

(a,b) Three-dimensional reconstructions (confocal microscopy) of collagen networks in the normal media and adventitial layer. (c,d) Aneurysms in patients with MFS. (e) Aortic abdominal aneurysm. (Adapted from Lindeman et al. [48].)