Progenitor Cell Heart Therapy Figure 2. Structural and functional assessment of a NRVM monolayer with a central island comprised of differentiated CPCs. A GFP(+) CPC island (green) within the cardiac network (red) (a–b). Composite image and separate fluorescence channels showing the border between NRVMs and CPCs (c). Note that significant number of GFP positive cells are also a-Actinin positive (cardiomyocytes). Composite image and separate channels showing the Connexin 43 staining within the CPC island. While NRVMs were connected via relatively long Connexin 43(+) gap-junction plaques (data not shown), the gap-junctions in CPC differentiated cardiomyocytes appear small and irregular (d). Electrical propagation initiated inside the CPC island propagated into the surrounding cardiac network (e). Individual hexagonal frames denote 19.5 mm diameter recording area with membrane voltage snapshots shown at given times. Electrical stimulus (pulse sign) was applied in the center of the CPC island at time 0 ms. Membrane voltage is color coded from rest (blue) to peak (red). Circles denote 504 recordings sites. Dashed white line denotes the CPC island. Membrane potentials at selected sites within (1) and outside (2&3) the CPC island are also shown. Electrical stimulus (yellow triangle) yielded action potential propagation that was significantly slower within the island than in the surrounding cardiomyocytes. Isochrones of electrical propagation (white lines) are shown at the bottom right of. Note central isochrone crowding due to slow propagation within the CPC island (black dashed line). doi:10.1371/journal.pone.0011536.g002 PLoS ONE | www.plosone.org 4 July 2010 | Volume 5 | Issue 7 | e11536