When cultured as aggregates in suspension, embryonic stem cells form three-dimensional, multicellular complexes called embryoid bodies. These spherical structures resemble the egg cylinders of embryos 6 to 8 days after fertilization.
After about 4 to 8 days, these embryoid bodies typically contain skeletal structures between the inner mesothelial cells and the outer epithelial cells that begin to contract spontaneously.
These skeletal structures express key cardiac proteins and exhibit contractile properties, action potentials, and chronotropic responses similar to those described for embryoid bodies.
Early embryonic stem cells derived from embryonic stem cells that form in epithelial cells resemble pacemaker cells and primary cardiomyocytes.
Later, atrial, ventricular, ganglion cells, histaminergic cells, and Purkinje cells develop.
Cardiac transcription factors Nkx2.5, GATA-4, and MEF2C are expressed early during differentiation, while more mature stem cells express atrial natriuretic factor (ANF), sodium-calcium exchanger, and phospholamban as well as structural proteins such as myosin light chain (MLC)-2v, myosin heavy chain α and β, titin (Z and M domains), α-actinin, cardiac troponin T, α-actin, and M protein.
