In the most basic anatomical sense, the atrioventricular valves are made up of three main components:

The leaflets of the atrioventricular valves can be thought as forming a skirt that hangs from the annulus; leaflets are divided into a series of sections that constitute the distinct leaflets of each valve. Due to the extent of variations between individuals with regard to leaflet morphologies, there has been much debate relative to nomenclature on the number of leaflets of both the mitral and tricuspid valves [8–10]. Traditionally, the division of the leaflets has been determined by the presence of commissures which can be described as the peripheral attachment of a break in the skirt [1].

The leaflets themselves are attached to the ventricles via the sub-valvar apparatus of each valve. In general, each apparatus consists of the tendinous cords and the papillary muscle complexes of each valve. The tendinous cords are usually categorized by (1) those that support the free edges of the valves, (2) those that support the rough zones (the region between the free edge and each annulus), and/or (3) those that attach to the leaflets near to the annulus. Typically, the cords supporting the free edges of the leaflets are known as fan cords due to the presence of multiple fenestrations. Those that attach to the rough zone of the leaflets are distinguished by their larger size and are commonly defined as strut cords . Finally, those that attach near the annulus are known as basal cords . The strut cords are of specific importance, as they bear the highest mechanical loads during systole [11]. Furthermore, the number and distribution of the tendinous cords across a given valve are critical to its function; it is well documented that dysfunction of these structures can lead to prolapse of the valves [12–14]. In general, the cords attach to the heads of the papillary muscles, which themselves play an important role in the function of each valve by contracting during systole to cushion the valve closure and help prevent the valve from prolapsing into the atrium.

A healthy semilunar valve is composed of three valve leaflets, each attached to its respective sinus. These valves lie between the ventricular outflow tracts and the arterial trunks, the main arteries carrying blood away from the heart. This elegant structure is much simpler than that of the atrioventricular valves described previously, in that the semilunar valve leaflets do not require a tension apparatus to maintain competency. When closed, the three leaflets of each valve coapt along zones of apposition, or commissures, which are fibrous zones some distance from the free edge of the leaflets. At the center of the valve where all three leaflets coapt, a distinct fibrous nodule can be found. The valve leaflet margins are attached to the arterial wall in the shape of a half-moon, hence the semilunar moniker. Normally, the regions of the valves where the commissures meet the arterial wall are considerably higher than the seats of the leaflets, thereby giving the valve a crown-like shape. These three points, particularly in the aortic valve, are used to define the sinotubular junctions (Fig. 7.6). Although we have discussed the positioning of the valves in the heart by referring to their respective annuluses, many anatomists contest the idea that there are single defined annuluses for both the pulmonary and aortic valves [19]. Interestingly, there is a defined annulus where the respective arteries are attached to the ventricular outflow tract; however, due to the crown-like structure of the valve, the hemodynamic junction of the valves spans this annulus. This structural shape results in part of the arterial wall being considered a ventricular structure (in a hemodynamic sense) and, in turn, part of the ventricular wall an arterial structure. Just distal to the valves are the arterial sinuses that are represented by dilations of the artery positioned above each leaflet and additionally house the coronary artery ostium. The sinus also provides a recess for the valve leaflets to retract into, allowing for unrestricted flow from the ventricle to the artery. Finally, the virtual ring, upon which many annular measurements are based and which defines the basal plane of aortic valve, is defined by the three anatomical anchors at the nadir of each aortic leaflet [20]. These features are illustrated by the diagram in Fig. 7.6. The position and definition of the valve annulus is often contested by different medical specialists, and a recent questionnaire highlighted the current lack of consensus between physicians regarding the optimal means of describing the semilunar valve anatomy [21]. As such, it is important to be precise in the definition of exactly what is being measured when documenting the size and shape of the semilunar valve annuluses.

Interestingly, the atrioventricular valves share very similar leaflet histology. The atrial sides of the leaflets consist of spongy tissue (lamina spongiosa) comprised of fibrocytes, histiocytes, and collagen fibers [22]. It is these collagen fibers that are considered to supply the mechanical strength required of the atrioventricular valves. The ventricular sides consist of fibrous tissue (lamina fibrosa), and both these layers are surrounded by endothelial cells. Additionally, the valve leaflets have been shown to incorporate both primary sensory and autonomic innervation. In general, it is considered that the anterior leaflet of the mitral valve has twice the innervation of the posterior leaflet [23]. These nerves are typically situated in the lamina spongiosa and extend over the proximal and medial portions of the leaflet [22]. Fibroblasts [24], smooth muscle cells [25, 26], and myocardial cells [27] are also commonly located within the leaflet tissue.

Cells within the leaflets have been shown to elicit two types of contractile activity : (1) a brief contraction or twitch at the beginning of each heartbeat (reflecting contraction of myocytes in the leaflet in communication with, and excited by, atrial muscle) which has relaxed by mid-systole and whose contractile activity is eliminated with β-receptor blockade, and (2) sustained tonic contractions (or tone) during isovolumic relaxation, which has been shown to be insensitive to β-blockade, but doubled by stimulation of the neurally rich region of aortic-mitral continuity [28]. These contractile activities within the leaflets are hypothesized to aid in the maintenance of anterior leaflet shape. This, in turn, could help prevent mechanical shock to the leaflets upon valve closure and also aid in optimizing the leaflet shape for funneling blood into the left ventricular outflow tract [28].

The tendinous cords are composed of a collagen core, surrounded by elastin fibers interwoven in layers of loose collagen. Similar to the valve leaflets, they also have an outer layer of endothelial cells, but it is the collagen cores that support the greatest degree of mechanical load during systole and allow for the wavy configuration during diastole. The elastin fibers are normally arranged in parallel fashion relative to the collagen fibers, and as the cords are stretched during systole, the elastin fibers are also stretched, straightening the collagen. It is hypothesized that it is this composite configuration of elastin and collagen that provides a smooth mechanism for the transmission of cordal forces from the leaflets to the papillary muscles. Additionally, during diastole, the stretched elastin fibers likely help to restore the wavy configuration of the primary collagen cores. The relative amount of collagen and elastin within the given chordae varies according to their relative types, as does the relative amount of contained DNA and their degree of vascularization. Normally the vascularization of the tendinous cords is located between their collagen cores and the elastin fibers and is further considered to supply nutrients to the leaflets. It has been reported that a higher DNA content within both the anterior and posterior marginal chordae relates to inherently higher rates of collagen syntheses in order to prevent mechanical deterioration compared with other types of chordae [13].

The papillary muscles can be considered part of the ventricular myocardium and hence are composed of aggregated myocytes. The cells exhibit complex junctions, called intercalated discs , allowing multiple cells to form long cellular networks. Within the papillary muscles, these muscle fibers run parallel to each other along the length of the muscle to increase contractile force and efficiency. The papillary muscles are extensively innervated and have complex vascular systems in order to maintain coordinated contractions with the continuum of the ventricular myocardium [29].

It was Gross who first drew attention to the specific histological structures of the arterial valves, his account then being endorsed by others such as Misfeld and colleagues [22, 30]. Each leaflet of the semilunar valve was described to have a fibrous core, or fibrosa , with an endothelial lining containing delicate sheets of elastin on its arterial and ventricular aspects. This so-called fibrous “backbone” is represented by a dense collagenous layer, which gives way to a much looser structure, or spongiosa , toward the ventricular aspects of the leaflet cusps. The zone of apposition of the leaflets consists of an abrupt thickening of the fibrous layer made up of closely packed vertically directed fibers and builds at the central portion of the free edge, creating a node termed the nodulus Arantii [22, 30]. Figure 7.9 displays a cross section of an aortic valve leaflet displaying the varying tissue types [31].

The left atrioventricular valve, or mitral valve, named by Andreas Vesalius due to its structural resemblance to the cardinal’s mitre, is situated in the left atrioventricular junction and modulates the flow of blood between the left atrium and ventricle. Commonly, the valve consists of an annulus, two leaflets, two papillary muscle complexes, and two sets of tendinous cords, as seen in Fig. 7.10.

In 1976 Carpentier described the mitral valve as consisting of two apposing leaflets—a posterior leaflet with three scallops and an anterior leaflet with one scallop. Each region of the leaflets is designated an alphanumerical label to distinguish it from the rest of the valve (Fig. 7.11) [32]. However, when one considers these structures relative to the landmarks of the body (i.e., in an attitudinally correct nomenclature), the leaflets are located in posteroinferior and anterosuperior positions. Confusion regarding positional nomenclature can be avoided when adopting the more traditional approach suggested by Vesalius for distinguishing between the leaflets and recognizing that they are aortic and mural in their locations [33]; in this chapter, we will use such nomenclature. The junctions of the two leaflets are commonly referred to as the anterolateral and the posteromedial commissures ; however, they are more accurately described as superior and inferior. The line of apposition of the leaflets during valve closure is known as the fibrous ridge . The simplicity and practicality of Carpentier’s anatomic description of the mitral leaflets led to its widespread use after being introduced in 1976 [32]; yet, while this description depicts a majority of mitral valve anatomies, there can be wide variability in the number of scallops within each leaflet and their relative positions [34].

In general, the aortic leaflet is found to be attached to approximately one-third of the annulus circumference and is supported by the aorto-mitral fibrous continuity, which terminates in the left and right fibrous trigones (Fig. 7.12). The mural leaflet is attached to the remaining two-thirds of the annulus and also to the fibrous extensions that continue from the trigones around the mitral valve. However, the lengths of these extensions can be highly variable. Furthermore, a fibrous-fatty tissue surrounds the valve in areas where the cardiac skeleton is not present. The mitral annulus is a highly dynamic feature of the heart, changing dramatically in shape and size throughout the cardiac cycle. It is often described as being saddle-shaped with the highest point of the saddle, the saddlehorn, being found at the midpoint of the area of aortic-to-mitral valvar continuity [35] (Figs. 7.4 and 7.13). Both Delgado and Veronisi and their colleagues reported a series of annular dimensions that were recorded using echocardiography in healthy patients; these data are summarized in Table 7.1 [36–38].

In general, the sub-valvar apparatus of the mitral valve consists of two adjacent papillary muscle complexes—the superoposterior (anterior or APM) and the inferoanterior (posterior or PPM)—with their attached tendinous cords which, in turn, insert onto the ventricular surfaces of each of the two valve leaflets [33]. In other words, the superoposterior papillary muscle complex is not solely associated with the aortic leaflet, but rather both the leaflets; likewise, the inferoanterior papillary muscle complex is not solely associated with the mural leaflet. It is important to note that the morphologies of the papillary muscle complexes are highly variable [36]. Some have proposed a complicated alphanumeric classification to account for the number of heads within each muscle and the number of attachments with the ventricular walls [39]. Even this complex code can be deemed as an oversimplification, as both papillary complexes can exhibit enormous anatomic variation [40]. For example, Fig. 7.14 displays images of the sub-valvar apparatus of the mitral valve taken from human hearts in the Visible Heart^® library [40].

The tendinous cords are typically classified by their number and length and quantified by one of two measurement techniques—tethering length and insertion length. Tethering length is defined as the distance from the papillary head to the saddlehorn of the mitral annulus. Insertion length is defined as the length of the cords from their origin at the papillary head to their insertion into the leaflet tissue. Anatomical dimensions obtained from patients with no reported mitral regurgitation or other valvar pathologies were reported by Sonne et al. and Lam et al. and are summarized in Table 7.2 [41, 42]. Yet, it should be emphasized that, as with other anatomical studies, these data do not account for all anatomical variations. For example, it has also been reported that the cordal attachments to the mural leaflet may extend simply from the ventricular myocardium to the leaflet without a papillary muscle attachment. Furthermore, it is well known that the tendinous cords themselves may elicit highly variably anatomies, and various subpopulations of chordae have been classified by both function [43] and type [44]. Figure 7.15 shows examples of these identified variations in the types of chordae, including posterior marginal chordae, commissural chordae, anterior strut chordae, anterior marginal chordae, basal posterior chordae, and posterior intermediate chordae.

The right atrioventricular valve, or tricuspid valve , is situated within the right atrioventricular junction and modulates the flow of blood between the right atrium and right ventricle. This valve is typically defined by three leaflets suspended from the muscular atrioventricular junction and connected to the ventricular wall via three distinct papillary muscle complexes (as seen in Fig. 7.10). When defined using attitudinally correct nomenclature, these leaflets are located in septal, anterosuperior (traditionally anterior), and inferior (traditionally posterior) positions (Figs. 7.16 and 7.17) [45].

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