Cardiac jelly dynamics reveal the touchdown/ECM bubble model for cardiac trabeculation

G del Monte-Nieto1, M Ramialison2, AV Cherian3, LM Bourke4, SK Harten4, E Tzahor5, B Zhou6, DY Stainier3 and RP Harvey1

  1. Developmental and Stem Cell Biology Division, Victor Chang Cardiac Research Institute, Sydney, NSW, 2010, Australia
  2. Australian Regenerative Medicine Institute, Monash University, Clayton, Victoria, 3800, Australia
  3. Department of Developmental Genetics, Max Planck Institute for Heart and Lung Research, 61231, Bad Nauheim, Germany
  4. Epigenetics Laboratory, QIMR Berghofer Medical Research Institute, Herston, Queensland, 4006, Australia
  5. Department of Biological Regulation, Weizmann Institute of Science, Rehovot, 7610001, Israel
  6. Departments of Genetics, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York, 10461, USA

The early embryonic heart first forms as a simple tubular structure composed of outer myocardial and inner endocardial layers, separated by a prominent extracellular matrix (ECM) called cardiac jelly. During heart looping, chambers are specified at the outer curvature of the heart and the process of trabeculation is initiated. Trabeculae are myocardial sponge-like protrusions at the luminal side of the myocardial wall critical in force generation in the early ventricle, and for septation, wall thickening, conduction and valve formation during foetal heart development. Defects in early trabecular development lead to embryonic lethality. Later, defects in trabecular compaction promote the Left Ventricular Non-Compaction (LVNC) condition in mouse and human. Despite its importance for early and late chamber development, the molecular mechanisms and cellular processes involved in trabeculation are largely unknown. Here, we propose a new model for cardiac chamber formation with a focus on trabeculation describing in detail the cellular behaviours of endocardium and myocardium and integrating for the first time the cardiac jelly dynamics. Implicit in the model is the orchestrated regulation of ECM synthesis from the myocardium and degradation from the endocardium, starting as early as heart fusion (E8.0). The fine regulation of ECM creates matrix-rich and matrix-poor areas, which control trabecular organization and growth. Detailed analysis of different mouse mutant lines showing trabecular defects has identified the complex cross-regulation between the Neuregulin1, Notch and Vascular Endothelial Growth Factor (VEGF) signaling pathways in the control of ECM dynamics. Furthermore, our model generated new insights for the interpretation of the LVNC condition based on the lack of trabecular termination associated to defective ECM degradation. Collectively, the model describes the cellular and molecular regulation of cardiac trabeculation integrating the cardiac jelly and applying new concepts from vascular biology never considered before.