Imaging the Microstructure of the Human Fetal Heart

Cardiomyocyte maturation: advances in knowledge and implications for regenerative medicine

Our knowledge of pluripotent stem cell (PSC) biology has advanced to the point where we now can generate most cells of the human body in the laboratory. PSC-derived cardiomyocytes can be generated routinely with high yield and purity for disease research and drug development, and these cells are now gradually entering the clinical research phase for the testing of heart regeneration therapies. However, a major hurdle for their applications is the immature state of these cardiomyocytes. In this Review, we describe the structural and functional properties of cardiomyocytes and present the current approaches to mature PSC-derived cardiomyocytes. To date, the greatest success in maturation of PSC-derived cardiomyocytes has been with transplantation into the heart in animal models and the engineering of 3D heart tissues with electromechanical conditioning. In conventional 2D cell culture, biophysical stimuli such as mechanical loading, electrical stimulation and nanotopology cues all induce substantial maturation, particularly of the contractile cytoskeleton. Metabolism has emerged as a potent means to control maturation with unexpected effects on electrical and mechanical function. Different interventions induce distinct facets of maturation, suggesting that activating multiple signalling networks might lead to increased maturation. Despite considerable progress, we are still far from being able to generate PSC-derived cardiomyocytes with adult-like phenotypes in vitro. Future progress will come from identifying the developmental drivers of maturation and leveraging them to create more mature cardiomyocytes for research and regenerative medicine.

Learn from Your Elders: Developmental Biology Lessons to Guide Maturation of Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cells (hPSCs) offer a multifaceted platform to study cardiac developmental biology, understand disease mechanisms, and develop novel therapies. Remarkable progress over the last two decades has led to methods to obtain highly pure hPSC-derived cardiomyocytes (hPSC-CMs) with reasonable ease and scalability. Nevertheless, a major bottleneck for the translational application of hPSC-CMs is their immature phenotype, resembling that of early fetal cardiomyocytes. Overall, bona fide maturation of hPSC-CMs represents one of the most significant goals facing the field today. Developmental biology studies have been pivotal in understanding the mechanisms to differentiate hPSC-CMs. Similarly, evaluation of developmental cues such as electrical and mechanical activities or neurohormonal and metabolic stimulations revealed the importance of these pathways in cardiomyocyte physiological maturation. Those signals cooperate and dictate the size and the performance of the developing heart. Likewise, this orchestra of stimuli is important in promoting hPSC-CM maturation, as demonstrated by current in vitro maturation approaches. Different shades of adult-like phenotype are achieved by prolonging the time in culture, electromechanical stimulation, patterned substrates, microRNA manipulation, neurohormonal or metabolic stimulation, and generation of human-engineered heart tissue (hEHT). However, mirroring this extremely dynamic environment is challenging, and reproducibility and scalability of these approaches represent the major obstacles for an efficient production of mature hPSC-CMs. For this reason, understanding the pattern behind the mechanisms elicited during the late gestational and early postnatal stages not only will provide new insights into postnatal development but also potentially offer new scalable and efficient approaches to mature hPSC-CMs.