149
viable human cardiomyocytes. Human embryonic stem cells have been used to differentiate stem cells into functional human cardiomyocytes. hESC-CMs have welldocumented electrophysiological properties, contract rhythmically, and are
responsive to cardiac compounds. Similar to NRVCMs, hESC-CMs can be genetically manipulated but have the advantage of being maintained in vitro for longer
periods. These cells can also be engrafted into the human myocardium [98]. While
these cells are advantageous for studying human cardiomyocytes, there are ethical
and regulatory concerns related to the accessibility to embryos as well the immune
response that arises with transplantation [74]. Additionally, hESCs have been shown
to have genetic instability, and specifically, cardiomyocytes are immature in their
development. Finally, due to the pluripotent state of the stem cells, they have the
potential to form teratomas when transplanted in vivo. To rectify these problems,
recent work has established that purifying the culture and removing undifferentiated
stem cells should reduce the probability of uncontrolled cell proliferation.
Human-induced pluripotent stem cells (hiPSCs) have similar characteristics to
hESCs and have the advantage that they can be used autologously. The hiPSCderived cardiomyocytes (hiPSC-CMs) circumvent ethical concerns associated with
hESCs, but there is variability among cell lines derived from different patients.
While these cell types are capable of spontaneously contracting, their functional and
structural immaturity needs to be addressed. For example, hiPSC-CMs are polygonal in shape and mononucleated (while the adult myocytes are larger, rod-like, and
binucleated), and they exhibit fetal-like sarcomeric organization, ion channel
expression, force generation, and action potential shape. Sarcomeres in hiPSC-CMs
are shorter and more disorganized than in the adult myocyte [133]. The hiPSC-CMs
also have reduced contractile machinery, as compared to the adult myocyte, and
have not undergone isotype switch to a stiffer form of titin. Similar to neonatal rat
cardiomyocytes, these cells exhibit immature calcium-handling properties and no
T-tubule formation [123]. Finally, gap junction proteins are located around the cardiomyocytes, rather than located at the intercalated disk space in pluripotent stem
cells, which yields slower conduction velocities compared to the native human adult
heart [133]. In spite of these limitations, hiPSC-derived cardiomyocytes (hiPSCCMs) are a suitable cell source for drug screens and disease modeling, based on
their electrophysiological characteristics.
6.3.2 Endothelial Cells
Several sources of endothelial cells have been used for vessel development. These
include human umbilical vein endothelial cells (HUVECs), endothelial colonyforming cells (ECFCs) (also termed blood outgrowth endothelial cells (BOECs) or
endothelial progenitor cells (EPCs)), human embryonic stem cell-derived endothelial cells (hESC-ECs), and human-induced pluripotent stem cell-derived endothelial
cells (hiPSC-ECs). HUVECs display a mature phenotype and have been shown to
6 Strategies for Tissue Engineering Vascularized Cardiac Patches to Treat Myocardial…
viable human cardiomyocytes. Human embryonic stem cells have been used to differentiate stem cells into functional human cardiomyocytes. hESC-CMs have welldocumented electrophysiological properties, contract rhythmically, and are
responsive to cardiac compounds. Similar to NRVCMs, hESC-CMs can be genetically manipulated but have the advantage of being maintained in vitro for longer
periods. These cells can also be engrafted into the human myocardium [98]. While
these cells are advantageous for studying human cardiomyocytes, there are ethical
and regulatory concerns related to the accessibility to embryos as well the immune
response that arises with transplantation [74]. Additionally, hESCs have been shown
to have genetic instability, and specifically, cardiomyocytes are immature in their
development. Finally, due to the pluripotent state of the stem cells, they have the
potential to form teratomas when transplanted in vivo. To rectify these problems,
recent work has established that purifying the culture and removing undifferentiated
stem cells should reduce the probability of uncontrolled cell proliferation.
Human-induced pluripotent stem cells (hiPSCs) have similar characteristics to
hESCs and have the advantage that they can be used autologously. The hiPSCderived cardiomyocytes (hiPSC-CMs) circumvent ethical concerns associated with
hESCs, but there is variability among cell lines derived from different patients.
While these cell types are capable of spontaneously contracting, their functional and
structural immaturity needs to be addressed. For example, hiPSC-CMs are polygonal in shape and mononucleated (while the adult myocytes are larger, rod-like, and
binucleated), and they exhibit fetal-like sarcomeric organization, ion channel
expression, force generation, and action potential shape. Sarcomeres in hiPSC-CMs
are shorter and more disorganized than in the adult myocyte [133]. The hiPSC-CMs
also have reduced contractile machinery, as compared to the adult myocyte, and
have not undergone isotype switch to a stiffer form of titin. Similar to neonatal rat
cardiomyocytes, these cells exhibit immature calcium-handling properties and no
T-tubule formation [123]. Finally, gap junction proteins are located around the cardiomyocytes, rather than located at the intercalated disk space in pluripotent stem
cells, which yields slower conduction velocities compared to the native human adult
heart [133]. In spite of these limitations, hiPSC-derived cardiomyocytes (hiPSCCMs) are a suitable cell source for drug screens and disease modeling, based on
their electrophysiological characteristics.
6.3.2 Endothelial Cells
Several sources of endothelial cells have been used for vessel development. These
include human umbilical vein endothelial cells (HUVECs), endothelial colonyforming cells (ECFCs) (also termed blood outgrowth endothelial cells (BOECs) or
endothelial progenitor cells (EPCs)), human embryonic stem cell-derived endothelial cells (hESC-ECs), and human-induced pluripotent stem cell-derived endothelial
cells (hiPSC-ECs). HUVECs display a mature phenotype and have been shown to
6 Strategies for Tissue Engineering Vascularized Cardiac Patches to Treat Myocardial…