Cardiomyocyte contributions to the formation, growth and sculpting of mature cardiac structures. Cardiomyocyte contributions to the formation, growth and sculpting of mature cardiac structures. (A) Formation of trabecular muscle in mice begins at approximately E10.5, when inductive signals from the endocardium (not shown) stimulate cell division in the myocardium (orange cells). Newly divided cardiomyocytes (pink cells) form muscular ridges known as trabeculae that are thought to strengthen contraction and improve myocardial oxygenation. (B) By E13-E14, the ventricular wall thickens substantially as cardiomyocytes proliferate (pink cells) in the outer layers of the myocardium to form compact muscle. (C) In zebrafish, the ventricular wall does not thicken until 6-8 weeks post-fertilization (wpf), when rare trabecular muscle cardiomyocytes (pink cells) in the chamber lumen emerge through the primordial wall (orange) and proliferate to cover the ventricle in a cortical muscle layer. Matthew J. Foglia, and Kenneth D. Poss Development 2016;143:729-740 © 2016. Published by The Company of Biologists Ltd