Development of the papillary muscles of the mitral valve: morphogenetic background of parachute-like asymmetric mitral valves and other mitral valve anomalies.

Objectives: To understand papillary muscle malformations, such as in parachute mitral valves or parachute-like asymmetric mitral valves, we studied the development of papillary muscles.

Methods: Normal human hearts at between 5 and 19 weeks of development were studied with immunohistochemistry, three-dimensional reconstructions, and gross inspection. Scanning electron microscopy was used to study human and rat hearts.

Results: In embryonic hearts a prominent horseshoe-shaped myocardial ridge runs from the anterior wall through the apex to the posterior wall of the left ventricle. In the atrioventricular region this ridge is continuous with atrial myocardium and covered with cushion tissue. The anterior and posterior parts of the trabecular ridge enlarge and loosen their connections with the atrial myocardium. Their lateral sides gradually delaminate from the left ventricular wall, and the continuity between the two parts is incorporated in the apical trabecular network. In this way the anterior and posterior parts of the ridge transform into the anterolateral and the posteromedial papillary muscles, respectively. Simultaneously, the cushions remodel into valve leaflets and chordae. Only the chordal part of the cushions remains attached to the developing papillary muscles.

Conclusions: Disturbed delamination of the anterior or posterior part of the trabecular ridge from the ventricular wall, combined with underdevelopment of chordae, seems to be the cause of asymmetric mitral valves. Parachute valves, however, develop when the connection between the posterior and anterior part of the ridge condenses to form one single papillary muscle. Thus parachute valves and parachute-like asymmetric mitral valves originate in different ways.