The MV apparatus is a complex anatomic structure consisting of MVLs, chordae tendineae, papillary muscles, and the MVa.1–3 This structure requires all of its components, together with adjacent atrial and ventricular musculature, to function properly.1,4,5 The MV has 2 mitral cusps (anterior [aortic] and posterior [mural]). This definition is equally applicable to humans and to other mammals.2 In humans, the distance from the base to the free edge of the AMVL is > 2 times that of the posterior cusp; however, because the PMVL is longer, the areas are nearly identical.1 The area of apposition of the 2 MVLs represents the MV commissure.6 However, there is not a consensus on this because authors in another study7 defined commissures as the end of the zone of apposition between MVLs, thus describing 2 commissures for the MV.
The MVa represents the fibrous support of the MVLs. In humans, the MVa is shaped more like the letter D than a true circle.4 There is marked variation in the structure of the MVa among clinically normal humans. In fact, it is uncommon to find a complete ring of connective tissue encircling the atrioventricular junction.8
The chordae tendineae and papillary muscles represent the tension apparatus of the MV. Chordae tendineae originate from the papillary muscles and insert into the free edge or onto the ventricular surface of the MVL. Several classifications have been proposed for the chordae tendineae in humans.4,9,10 Currently, the most accepted classification categorizes the chordae tendineae into commissural and MVL chordae tendineae.10 The commissural chordae tendineae have a fan-shape structure and support the commissural area. The MVL chordae tendineae are divided into zona rugosa and basal chordae tendineae. The zona rugosa chordae tendineae are those that provide the major support to the MVLs. They originate from both papillary muscles and distribute between the line of closure and the free margin of the MVL. The basal chordae tendineae originate from the ventricular free wall and are present in only the basal area of the PMVL. It has been suggested that the basal chordae tendineae reinforce the basal part of the PMVL during systole, although their function is not completely understood.9 Investigators in 1 study10 of 50 human hearts found basal chordae tendineae in only 31 specimens.
Histologically, the MVL has a layered aspect. The central structure, called fibrosa, is composed of collagenous fibers that run parallel to the surface of the MVL and extend into the chordae tendineae. On the atrial portion of the fibrosa, there is a variable amount of loose connective tissue (spongiosa). A thin fibroelastic layer covers approximately two-thirds of the length of the atrial and ventricular surfaces. Cardiac muscle and blood vessels can extend into the proximal and middle thirds of the atrial surface in dogs.2,3 Endothelium covers the fibroelastic layer on both sides. A dense nerve plexus with possible sensory or motor roles in valve function has been described in several species.11 In dogs, most of the nerve fibers are sympathetic and associated with the myocardium in the valve base.12
Few studies2,3 have been conducted to determine the anatomy of the MV in cardiologically normal dogs. This is surprising considering that chronic MMVD represents the most common acquired cardiovascular disease in dogs13 and that the disease shares many similarities with MV prolapse syndrome in humans.14 In dogs, diagnosis of the disease is usually made on the basis of detection of a systolic left apical murmur in a typically affected breed, and it must be confirmed echocardiographically.15,16 In humans, some studies17,18 have revealed a correlation between anatomic measurements of the MV apparatus and echocardiographic measurements. Moreover, there is evidence that some echocardiographic indices can be used to evaluate the amount of residual regurgitation through the MV after surgical repair.19 To our knowledge, no studies have been conducted to determine the correlation between echocardiographic measurements of MVs and anatomic measurements in dogs. Therefore, the purpose of the study reported here was to describe histologic characteristics of the MV in a group of cardiologically normal dogs of various sizes and breeds and to compare anatomic measurements with 2-D echocardiographic measurements.
Anterior mitral valve leaflet
Myxomatous mitral valve degeneration
Mitral valve annulus
Mitral valve leaflet
Posterior mitral valve leaflet
T-61, Intervet/Schering-Plough, Boxmeer, The Netherlands.
Image Proplus software, Media Cybernetics, Betheseda, Md.
R, version 2.9.0, R Foundation for Statistical Computing, Vienna, Austria. Available at: www.r-project.org/. Accessed Month, Day, Year.
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