**5. Conclusion**

*Visions of Cardiomyocyte - Fundamental Concepts of Heart Life and Disease*

apoptosis and lipid accumulation in ARVC hiPSC-CMs [106].

H-zone and I bands, and evidence of t-tubules and M-bands [112].

**4. Limitations and future prospective**

not free from limitation.

hiPSC-CM model and showed that increased levels of testosterone accelerate ARVC pathologies, while premenopausal female estradiol levels slow down exaggerated

The reprogramming of somatic cells into pluripotent stems cells and subsequent differentiation into specific cell types is a newly emerging technique and is certainly

One of the most questionable issues of hiPSC-CMs is their maturity. Despite expressing relevant ion channels [107] and structural genes [25, 26, 75, 76, 89, 108], hiPSC-CMs lack t-tubules and exhibit lower expression of Kir2.1 and weaker contractility; thus they do not fully resemble adult CMs. In order to improve the maturity of hiPSC-CMs and consequently upgrade the functionality of hiPSC-CMs, various techniques have been investigated in different groups. Three-dimensional construction of engineered heart tissue is a rapidly growing technique for structural and functional maturations of hiPSC-CMs [109], which resulted in higher Na+ current density and upstroke velocity [110], and enhances the metabolic maturation [111] comparable to adult CMs. Furthermore, Shadrin and co-workers introduced the "Cardiopatch" platform for three-dimensional culture and maturation of hiPSC-CMs; this platform produces robust electromechanical coupling, consistent

Another issue of hiPSC-CMs is the purity of differentiated CMs. The CMs differentiated from hiPSCs yield in heterogeneous population of CMs. There are at least three subtypes of CMs such as ventricular, atrial, and nodal CMs; among them the majority (~70%) of CMs are ventricular-like, and only a minority of CMs are atrial-like (~20%) and nodal-like (~10%) [40, 58, 93, 107]. Although many molecular and functional characteristics are shared among these CMs subtypes, they also exhibit their own unique features. For example, ventricular CMs have prominent plateau phase (phase 2) in action potential profile, atrial CMs exclusively exhibit IKur channels, and nodal CMs lack strong upstroke velocity [113]. Most of the published methods of differentiation protocol yield in a lower amount of atrial-like and nodal-like CMs [40, 58, 93, 107], but sufficient numbers of subtype-specific CMs are needed to understand the subtype-related disease mechanism and development of specific therapeutic approaches. Atrial fibrillation (AF) is one of the most common cardiac arrhythmias; however, current antiarrhythmic drugs for treatment of AF are not atrial-specific and could cause unacceptable ventricular events [114]. Thus, sufficient supply of atrial CMs is crucial for investigating the AF cellular mechanism. hiPSCs have been differentiated into high-purity atrial-specific CMs by using retinoic acid signaling at the mesoderm stage of development [115]. These patient-specific atrial CMs allow us to investigate in detail mechanisms of AF and to develop atrial-specific therapeutic drugs. Furthermore, sinoatrial node (SAN) dysfunction can manifest bradycardia and asystolic pauses, but its pathophysiology is not completely understood [116]. SAN pacemaker cells from hiPSCs would facilitate the study of the disease mechanism and provide a cell source for developing a biological pacemaker. Protze and co-workers had reported the transgene-independent method for the generation of pacemaker cells (nodal-like CMs) from human pluripotent stem cells by stagespecific manipulation of developmental signaling pathways [117]. Besides CMs, the heart also consists of many other cell types such as fibroblast, endothelial and vascular smooth muscle cells, and also extracellular matrix. Importantly, the origin of cardiac diseases may not always exclusively originate from CMs, but might

**72**

While animal models fail to recapitulate human cardiac disease phenotype properly, hiPSC-CMs have been successful in recapitulating crucial phenotypes of many genetic cardiac diseases in terms of morphology, contractility, Ca2+ handling, ion channel biophysics, cell signaling, and metabolism. Most strikingly, hiPSC-CMs provide the patient-specific platform to study the disease mechanism and drug response individually, which the traditional disease modeling technique would never offer. In addition, cardiac subtype-specific arrhythmias and drug screening could be performed with the help of unlimited supply of hiPSC-CMs; thus chamber-specific treatment modalities could be identified. Certainly, by improving the current weaknesses of hiPSC-CMs and incorporating with new gene editing techniques, complex cardiac disease mechanism could be deciphered, and novel effective treatment therapies could be identified to improve the life of cardiac patients.
