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 p arachute mitral valves or parachute-like asymmetric mitral valves, we studied the development of papillary muscles, Methods: Normal human he arts at between 5 and 19 weeks of development were studied with immuno histochemistry, three-dimensional reconstructions, and gross inspectio n. 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 w all of the left ventricle. In the atrioventricular region this ridge i s continuous with atrial myocardium and covered with cushion tissue. T he anterior and posterior parts of the trabecular ridge enlarge and lo osen their connections with the aerial myocardium. Their lateral sides gradually delaminate from the left ventricular wall, and the continui ty between the two pasts is incorporated in the apical trabecular netw ork, In this way the anterior and posterior pasts of the ridge transfo rm into the anterolateral and the posteromedial papillary muscles, res pectively, Simultaneously, the cushions remodel irate valve leaflets a nd 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 ven tricular wall, combined with underdevelopment of chordae, seems to be the cause of asymmetric mitral valves. Parachute valves, however, deve lop when the connection between the posterior and anterior part of the ridge condenses to form one single papillary muscle. Thus parachute v alves and parachute-like asymmetric mitral valves originate in differe nt ways.