a Immunostaining to detect TRPM4 (red), a Purkinje cell marker, and cTnT (green) in HOs (left panels, n = 14, two independent experiments) and in E9.5 and E10.5 hearts (right panels). Scale bar: 100 μm. b Purkinje-specific myocytes in the HOs (over n = 3, over two experiments). A wide Z band, abundant intermediate filaments, and/or sparse myofibrils were observed (left panels). Both Purkinje cells (M bands (arrows) and wide Z bands) and working (ordinary) myocytes in the HOs were observed together in cell-to-cell contact, showing the protrusion of Purkinje cells (white dotted circle) into working myocytes (right panels). Scale bar: 400 nm. c RYR2 immunostaining in the HOs (left and middle panel, n = 13, three biologically independent experiments) and an E12.5 heart (right). Inset indicates an entire section of the HO. Scale bar: 100 μm.