Cardiomyocytes derived from human embryonic and induced pluripotent stem cells: comparative ultrastructure

Abstract

Induced pluripotent stem cells (iPSC) are generated from fully differentiated somatic cells that were reprogrammed into a pluripotent state. Human iPSC which can be obtained from various types of somatic cells such as fibroblasts or keratinocytes can differentiate into cardiomyocytes (iPSC-CM), which exhibit cardiac-like transmembrane action potentials, intracellular Ca2+ transients and contractions. While major features of the excitation-contraction coupling of iPSC-CM have been well-described, very little is known on the ultrastructure of these cardiomyocytes. The ultrastructural features of 31-day-old (post-plating) iPSC-CM generated from human hair follicle keratinocytes (HFKT-iPSC-CM) were analysed by electron microscopy, and compared with those of human embryonic stem-cell-derived cardiomyocytes (hESC-CM). The comparison showed that cardiomyocytes from the two sources share similar proprieties. Specifically, HFKT-iPSC-CM and hESC-CM, displayed ultrastructural features of early and immature phenotype: myofibrils with sarcomeric pattern, large glycogen deposits, lipid droplets, long and slender mitochondria, free ribosomes, rough endoplasmic reticulum, sarcoplasmic reticulum and caveolae. Noteworthy, the SR is less developed in HFKT-iPSC-CM. We also found in both cell types: (1) ‘Ca2+-release units’, which connect the peripheral sarcoplasmic reticulum with plasmalemma; and (2) intercellular junctions, which mimic intercalated disks (desmosomes and fascia adherens). In conclusion, iPSC and hESC differentiate into cardiomyocytes of comparable ultrastructure, thus supporting the notion that iPSC offer a viable option for an autologous cell source for cardiac regenerative therapy.

Physiological and ultrastructural features of human induced pluripotent and embryonic stem cell-derived skeletal myocytes in vitro

“…Although peripheral couplings represent a landmark of cardiac muscle structure, signs of myocardiogenesis such as the presence of adhering structures ("primitive" intercalated disks) were not encountered in the present survey. Furthermore, in contrast to iPS-derived skeletal myocytes, cardiomyocytes display very few internal T-SR junctions (39) and no M-bands even after 4 mo in culture (40).…”

Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes

“…It has been reported that early stage iPSC-CMs are small morphologically and exhibit an immature ultrastructure closely resembling that of fetal CMs (i.e., absence of T-tubules and underdeveloped contractile machinery). 42-44 However, upon prolonged culture, there were significant improvements in myofibril alignment, density and morphology showing adult-like appearance of Z disks, A-bands, I-bands and H-zones, although no clear M-bands or T-tubules were observed. 43, 44 Similarly, by combining bioengineering approaches with electrical stimulation, Nunes et al 45 developed a platform called ‘biowire’ that enables the generation of iPSC-CMs with ultrastructural properties similar to those seen in the native CMs.…”

“…59 Indeed, iPSC-CMs exhibit a poorly developed SR and absence of T- tubules that likely affect their Ca 2+ handling properties. 42, 43 …”

Induced pluripotent stem cell–derived cardiomyocytes in studies of inherited arrhythmias

“…The latter feature is more accentuated in iPS-derived myocytes. Remarkably, iPS-derived myocytes lack gap junctions, intercalated disks, and T tubules (despite the presence of caveolae, considered precursors to T tubules) (28). However, studies in cardiomyocytes derived from human ESCs have demonstrated the presence of gap junctions (29).…”