Origin and migration of cushion tissue in the developing heart.

The origin of cushion tissue mesenchyme in the developing chick heart was investigated by three basic methods: use of an inert metabolic marker; time lapse recording of organ-cultured heart regions; and scanning stereo-microscopy. All three approaches support the hypothesis that the endocardium is the progenitor of cushion tissue mesenchyme. Additional observations show a cell:matrix interaction by the endocardium prior to the formation of mesenchyme. It is postulated that this activity in some manner alters the underlying matrix and helps to initiate and maintain migratory activity of the mesenchyme.