Heart cells, called myocytes, do obviously divide as we grow – “to make new cells during embryonic development, but after birth, as childhood development progresses, these cells have reduced abilities to divide,” said Lewis C. Cantley, a biophysical chemist who is director of the Cancer Center at Beth Israel Deaconess Medical Center. Certain cardiac tumors do occur in infants and in utero, and often resolve without any medical intervention as cellular signals that turn off cell division kick in.