A method for producing cardiomyocyte cells including implanting a substrate within a heart such that a first portion of the substrate is in physical contact with an endocardium and a second portion of the substrate is not in contact with the endocardium, maintaining the first portion of the substrate in contact with the endocardium for a time at least sufficient to form trabecular fibers extending between the endocardium and the second portion of the substrate, cutting away the trabecular fibers from the endocardium, cutting away the trabecular fibers from the substrate, and removing the trabecular fibers from the heart, wherein the trabecular fibers include cardiomyocyte cells.
