Ultrastructural and immunocytochemical study of the leptomeres in the mouse cardiac muscle fibre.

The three-dimensional configuration and chemical composition of leptomeres in the mouse ventricular cardiac muscle fibre were electron microscopically and immunocytochemically investigated using semithin sections and specific antibodies against actin, the intermediate filament proteins desmin and vimentin, and the actin-binding proteins alpha-actinin, filamin and vinculin. The leptomeres appeared columnar in shape and periodically segmented by electron-dense disc-like septa. The electron-lucent areas between these septa were composed of fine interlinked filaments running obliquely to the major axis of the leptomere. Actin was localized in the electron-dense lines of the leptomeres but not in the fine filaments. No reaction was, however, detected for desmin, vimentin, vinculin, filamin and alpha-actinin. The present results suggest, therefore, that the leptomeres may not have a contractile function.