Figure 1.

Atrioventricular valve development across species. (A) Valve development begins as cardiac jelly accumulates between the myocardial (red) and endocardial cell layers. (B) Endocardial cushions (ECs, in green) are created as endocardial cells invade the cardiac jelly following epithelial to mesenchymal transition (mouse, chick) or via a local invagination of the endocardium (zebrafish). (C) ECs mature as the mesenchymal cells differentiate into valve interstitial cells (VICs, in yellow) as the cushions begin to function to occlude retrograde flow. (D) Valve leaflets form from remodeled ECs, as VICs proliferate, stratify, and differentially modify the extracellular matrix (ECM). Gradient represents three layers of stratified ECM. A, atrium; V, ventricle; OFT, outflow tract. Figure created using BioRender.