The fusion of dorsal and ventral endocardial cushions in the embryonic chick heart: A study in fine structure

Fusion of atrioventricular endocardial cushions begins during Hamburger‐Hamilton stage 27 and is completed during stage 28 (5 to 6 days incubation age). Connective tissue cells of these cushions are derived from the proliferating endocardial cell layer. The character of the organelles in cells of these cushions changes progressively as the cushions approach, meet and fuse: granular endoplasmic reticulum accumulates; Golgi complexes become extensive, and nuclei develop irregular shapes. These occur first in the connective tissue cells of the cardiac jelly and later in the endocardial cells. Fusion apparently is accomplished by liberation of the endocardial endothelial cells into the cardiac jelly, where, along with pre‐existing connective tissue cells, they become cells of the membranous interventricular septum. Gradually the cells appear to merge into one morphological population and thus become indistinguishable from each other. Both endocardial cells and connective tissue cells exhibit uptake of tritiated thymidine and undergo mitosis before, during and after fusion of the cushions. An outstanding characteristic of these cells is their continuing vitality throughout the entire process of fusion.