Characterization of Cobblestone mitral valve interstitial cells.

Mitral valve interstitial cells (MVICs) are important in the structure and function of the valve. In a cell culture system that used explants to harvest MVICs, we found that after several passages, some of the cultures at confluence formed a monolayer of cobblestone-shaped cells (CB type). The rest of the cultures were, however, different, consisting of elongated cells (EL type) that formed overlapping orthogonal patterns. When compared with the MVICs-EL, cells from cultures of MVICs-CB were less adherent to a plastic substratum initially than were MVICs-EL and were more sensitive to trypsinization following culture for 3 to 7 days. Ultrastructurally, the cultures of MVICs-CB showed prominent Golgi's complexes and endoplasmic reticulum and fewer microfilaments when compared with those of MVICs-EL. Most strikingly, the CB cells contained much less alpha-smooth-muscle cell actin compared with that of EL cells. The MVICs-CB were not endothelial cells as they did not show the characteristic dense peripheral band that was present in endothelial cells in a confluent monolayer and showed only trace amounts of fluorescence when incubated with 1,1-dioctadecyl 1-3,3,3,3 tetramethyl-indocarbocyanine-percholate (Dil-)--acetylated low-density lipoprotein compared with large amounts of fluorescence with endothelial cells. Cells that were morphologically similar to MVICs-CB were present in the in vivo valve as well and also formed small islands of cells even in the primary cultures that grew out of the explant. The findings supported the hypothesis that MVICs-CB represent a distinct phenotype of the MVICs that is different from that of the MVICs-EL and is not that of either endothelial or medial smooth-muscle cells.