**4. Microscopic anatomy**

The aortic valve is composed of different structures, each one with its own histological pro‐ file. The histology of the aortic root is characterized by a gradual shift from the primarily elastic aorta to the muscular ventricle. [31]

The annulus is a dense collagenous meshwork, in which elastic and collagenous fibrils and also neuronal structures are present. At the commissure level originate the collagen fibres of the intermediate layer, which are orientated in a radial fashion. Here, they do not only infil‐ trate the intima layer of the aortic root, but they also radiate into the media layer where they are anchored. The endothelial cells are separated by the basal layer from the elastic fibres and collagenous fibrils. The endothelial cells show microvilli at their surface, which increase the overall surface area for an increased exchange of substances.

The interleaflet triangles are different in their microscopic structure. The triangle between the left-coronary and non-coronary sinus forms part of the aortic–mitral valvular curtain, is histologically fibrous and equivalent to the mitral valve leaflet structure. The triangle be‐ tween the non-coronary and the right-coronary aortic sinus is incorporated within the mem‐ branous part of the septum and is also made of fibrous tissue. In contrast, the triangle between the right-coronary and left-coronary sinus in the area of the subpulmonary infun‐ dibulum is supported by muscular tissue and only fibrous at its apex.

The sinuses are arranged with very different components, but the largest part of all the three sinuses is composed in a similar manner to the three layers of the aortic wall: tunica intima, tunica media and tunica externa (adventitia). The inner layer of the intima is composed of endothelial cells arranged in the direction of the vessel. The subendothelial connective tissue is arranged in the same manner as the endothelial cells. This layer is divided from the intima by the membrana elastica interna. The media is composed of circular arranged structures: smooth muscle cells, elastic fibres, collagen fibres type II and III and proteoglycans. The ad‐ ventitia is the external layer. It is separated from the intima by the membrana elastica exter‐ na. Similar to the intima, the elements of the adventitia are arranged in a longitudinal fashion and composed of collagen fibres of type I. The sinotubular junction shows the same principal arrangement of tissue elements compared with the sinuses and the ascending aor‐ ta, but the diameter of the wall is thicker. [34]

**Figure 7.** Histology of the aortic valvular complex [1]

angle between the non- and right coronary aortic sinuses. Having penetrated through the fi‐ brous plane providing atrioventricular insulation, the bundle then branches on the crest of the muscular ventricular septum, the left bundle branch fanning out on the smooth left ven‐ tricular side, while the cord-like right bundle branch penetrates back through the muscular septum, emerging on the septal surface in the environment of the medial papillary muscle

**Figure 6.** Aortic sinuses, coronary arteries and the the location of the atrioventricular conduction axis, as seen by look‐

The aortic valve is composed of different structures, each one with its own histological pro‐ file. The histology of the aortic root is characterized by a gradual shift from the primarily

The annulus is a dense collagenous meshwork, in which elastic and collagenous fibrils and also neuronal structures are present. At the commissure level originate the collagen fibres of the intermediate layer, which are orientated in a radial fashion. Here, they do not only infil‐ trate the intima layer of the aortic root, but they also radiate into the media layer where they are anchored. The endothelial cells are separated by the basal layer from the elastic fibres and collagenous fibrils. The endothelial cells show microvilli at their surface, which increase

The interleaflet triangles are different in their microscopic structure. The triangle between the left-coronary and non-coronary sinus forms part of the aortic–mitral valvular curtain, is histologically fibrous and equivalent to the mitral valve leaflet structure. The triangle be‐

ing down through the aortic root (schematic from [4])

elastic aorta to the muscular ventricle. [31]

the overall surface area for an increased exchange of substances.

**4. Microscopic anatomy**

(figure 6). [4]

42 Calcific Aortic Valve Disease

The aortic valve leaflet is a three-layered structure (lamina ventricularis, lamina spongiosa and lamina fibrosa) composed of differing amounts of collagen, elastin, and glycosamino‐ glycans, that form a well-defined honeycomb or spongelike structure, suggesting that elastin forms a matrix that surrounds and links the collagen fiber bundles. [35] The leaflets are cov‐ ered by a continuous layer of endothelial cells with a smooth surface on the ventricular side and numerous ridges on the arterial side. The arrangement of the endothelial cells is across, not in line with the direction of flow. [36]

from the facing commissure to the base of the sinus, it becomes the only part of the aort‐

Anatomy and Function of Normal Aortic Valvular Complex

http://dx.doi.org/10.5772/53403

45

The ability to record high-quality echocardiographic images and obtain accurate Doppler flow recordings are essential determinants of the overall value of the echocardiographic ex‐ amination. As such, echocardiography is highly operator dependent. It is difficult to over‐ emphasize the critical role of the person who performs the imaging. To obtain a comprehensive and accurate echocardiogram, the echocardiographer must understand the anatomy and physiology of the aortic valve and have a thorough knowledge of the ultra‐

Anatomic evaluation of the aortic valve is based on a combination of short- and long-axis images to identify the number of leaflets, and to describe leaflet mobility, thickness, and cal‐

Two-dimensional imaging of the normal aortic valve in the parasternal long axis view dem‐ onstrates two leaflets (right and noncoronary), while the parasternal short axis demonstrates a symmetrical structure with three uniformly thin leaflets that open equally, forming a cir‐ cular orifice during most of systole. During diastole, the normal leaflets form a three pointed star with a slight thickening or prominence at the central closing point formed by the aortic leaflet nodules, known as the nodules of Arantius. The three aortic valve leaflets may also be

The aortic valve leaflets appear thin and delicate and may be difficult to visualize. In the long-axis view, the leaflets open rapidly in systole and appear as linear parallel lines close to the walls of the aorta. With the onset of diastole, they come together and are recorded as a faint linear density within the plane of the aortic annulus. Because the velocity of valve mo‐ tion during opening and closing is high relative to the frame rate of most echocardiographic systems, the normal aortic valve is usually visualized either fully opened or closed but rare‐ ly in any intermediate position. In the basal short-axis view, the three aortic leaflets can be visualized within the annulus during diastole. The three lines of coaptation can be recorded, normally forming a Y (sometimes referred to as an inverted Mercedes-Benz sign). With the onset of systole, the leaflets open out of the imaging plane, providing a view of the aortic annulus. The short-axis perspective is most helpful to determine the number of leaflets and whether fusion of one or more commissures is present. In patients who are difficult to im‐ age, normal leaflets are so delicate that they are hard to visualize, generally an indication

ic valve that is not intimately related to another cardiac chamber. [37]

sound equipment to optimize the quality of the recording. [38]

**6. Ecocardiographic anatomy**

**6.1. Transthoracic Echocardiography (TTE)**

visualized in a subcostal view.

that they are morphologically normal. [38]

cification.
