Previous ascorbic acid-induced CM differentiation from ESCs, we sought to determine the role of local microenvironments created by co-culture with neonatal CMs (NCMs) in the EB development and CM differentiation that focuses on homogenous differentiation and long-term functional maintenance of the ESCMs.reporter-based fluorescence-activated cell sorting (FASC) (Figure 1D, E). Under differentiation conditions, ESCs consistently aggregated and formed EBs. Figure 2 show EBs photographed from 5 to 10 days after initiation of cellular aggregation of the ESCs. Initially, EBs were formed by hanging drop culture and largely composed of densely packed ESCs. After suspension culture for 4 days, the EBs adhered to plates and the center of the bodies became cavitated. The rhythmically contracting areas consisted of 10 to 200 CMs began to appear in EBs, suggesting the occurrence of CM differentiation of ESCs. Beating EBs were first observed approximately at day 7 of differentiation. Starting on day 10 of differentiation, areas of rhythmically contracting cells in solid aggregates became evident, with more similar morphology to native CMs in NCMs co-culture (Figure 2).Results The CM Differentiation of ESCs in the Indirect Co-culture ModelUndifferentiated ESCs were cultured on gelatin-coated dishes without feeder layer in the mentioned ESC medium (Figure 1A). In the indirect co-culture model, the co-culture cells were seeded on 6- or 12- well hanging cell culture inserts to prevent direct contact with the subnatant EBs (Figure 1B). EKs, obtained from the skin of newborn (2?-day old) mice, were used as negative coculture cells to better assess the differentiating potential of NCMs (Figure 1C). To ensure the purity of isolated NCMs population, we generated aMHC promoter driven eGFP-Rex-Neomycin transgenic mice (AKT inhibitor 2 site aMHC-GFP), in which only mature cardiomyocytes but not other cell types expressed the green fluorescent protein (GFP). The isolated NCMs were quantitatively purified throughCo-culture with NCMs Improve the Differentiation EfficiencyDuring time course of ESC differentiation, the percentage of EBs 
with contracting areas in NCMs co-culture was significantly higher than that without co-culture or in EKs co-culture. NCMs co-culture did influence the CM differentiation rate of ESCs in intermediate-stage and late-stage (Figure 3A). To verify the promoting effect of NCMs co-culture on CM differentiation of ESCs, the expression of cardiac marker genes were analyzed by semi-quantitative and real-time PCR. GATAbinding protein 4 (GATA-4) and NK2 transcription factor related locus 5 (Nkx2.5) were included as markers for cardiac mesoderm. a-myosin heavy chain (a-MHC), atrial natriuretic factor (ANF) and myosin light chain 2 atrial and ventricular transcripts (MLC2a, MLC2v) were included as markers for cardiomyocytes. Semi-quantitative RT-PCR analysis on 20- and 28-day-old EBsFigure 1. The indirect co-culture model. A, Morphology of undifferentiated ESC colonies cultured without feeder layer. B, Scheme of the indirect co-culture system: a facile cell expansion and stem cell differentiation system with continuous medium conditioning while preventing mixing of stem cells and co-culture cells by hanging culture inserts. Two cell populations are co-cultured in different LED 209 site compartments (insert and well) but can communicate via paracrine signaling through the pores of the membrane. C, The morphology of EKs. D, Reporter-based fluorescence-activated cell sorting (FACS) f.Previous ascorbic acid-induced CM differentiation from ESCs, we sought to determine the role of local microenvironments created by co-culture with neonatal CMs (NCMs) in the EB development and CM differentiation that focuses on homogenous differentiation and long-term functional maintenance of the ESCMs.reporter-based fluorescence-activated cell sorting (FASC) (Figure 1D, E). Under differentiation conditions, ESCs consistently aggregated and formed EBs. Figure 2 show EBs photographed from 5 to 10 days after initiation of cellular aggregation of the ESCs. Initially, EBs were formed by hanging drop culture and largely composed of densely packed ESCs. After suspension culture for 4 days, the EBs adhered to plates and the center of the bodies became cavitated. The rhythmically contracting areas consisted of 10 to 200 CMs began to 
appear in EBs, suggesting the occurrence of CM differentiation of ESCs. Beating EBs were first observed approximately at day 7 of differentiation. Starting on day 10 of differentiation, areas of rhythmically contracting cells in solid aggregates became evident, with more similar morphology to native CMs in NCMs co-culture (Figure 2).Results The CM Differentiation of ESCs in the Indirect Co-culture ModelUndifferentiated ESCs were cultured on gelatin-coated dishes without feeder layer in the mentioned ESC medium (Figure 1A). In the indirect co-culture model, the co-culture cells were seeded on 6- or 12- well hanging cell culture inserts to prevent direct contact with the subnatant EBs (Figure 1B). EKs, obtained from the skin of newborn (2?-day old) mice, were used as negative coculture cells to better assess the differentiating potential of NCMs (Figure 1C). To ensure the purity of isolated NCMs population, we generated aMHC promoter driven eGFP-Rex-Neomycin transgenic mice (aMHC-GFP), in which only mature cardiomyocytes but not other cell types expressed the green fluorescent protein (GFP). The isolated NCMs were quantitatively purified throughCo-culture with NCMs Improve the Differentiation EfficiencyDuring time course of ESC differentiation, the percentage of EBs with contracting areas in NCMs co-culture was significantly higher than that without co-culture or in EKs co-culture. NCMs co-culture did influence the CM differentiation rate of ESCs in intermediate-stage and late-stage (Figure 3A). To verify the promoting effect of NCMs co-culture on CM differentiation of ESCs, the expression of cardiac marker genes were analyzed by semi-quantitative and real-time PCR. GATAbinding protein 4 (GATA-4) and NK2 transcription factor related locus 5 (Nkx2.5) were included as markers for cardiac mesoderm. a-myosin heavy chain (a-MHC), atrial natriuretic factor (ANF) and myosin light chain 2 atrial and ventricular transcripts (MLC2a, MLC2v) were included as markers for cardiomyocytes. Semi-quantitative RT-PCR analysis on 20- and 28-day-old EBsFigure 1. The indirect co-culture model. A, Morphology of undifferentiated ESC colonies cultured without feeder layer. B, Scheme of the indirect co-culture system: a facile cell expansion and stem cell differentiation system with continuous medium conditioning while preventing mixing of stem cells and co-culture cells by hanging culture inserts. Two cell populations are co-cultured in different compartments (insert and well) but can communicate via paracrine signaling through the pores of the membrane. C, The morphology of EKs. D, Reporter-based fluorescence-activated cell sorting (FACS) f.