CD34+ fibrocytes in normal mitral valves and myxomatous mitral valve degeneration.

We investigated a total of 15 mitral valves with myxomatous degeneration and compared these with normal mitral valves. In normal mitral valves, stromal cells located in the fibrosa and spongiosa showed small bipolar cytoplasmic processes and were found to be positive for CD34, suggesting a close relationship to CD34+ fibrocytes. In cases of myxomatous degeneration, stromal cells showed an altered morphology in that they exhibited multipolar cytoplasmic processes, appeared to be hyperplastic, and were increased in number. This study is the first to report on CD34+ fibrocytes making up the majority of mitral valve stromal cells. Major factors in the development of myxomatous valve degeneration are MMP-9, as well as collagen I and III, which have been reported to be secreted by CD34+ fibrocytes. Therefore, it is likely that CD34+ fibrocytes are involved in the pathogenesis of myxomatous mitral valve degeneration.