Cell death in the endocardial cushions of the developing heart

The incidence of apoptotic cells in the hearts of chick embryos between days 4 and 8 of development was examined using an in situ technique for the detection of DNA fragmentation. Using this method it was possible to demonstrate foci of apoptotic cells primarily in two locations: the outflow tract cushions and the atrioventricular cushions. Both occurred only during narrow time windows: between embryonic days 4.5 and 6.5 in the outflow tract, and between embryonic days 5.5 and 7.5 in the atrioventricular canal. This is a much more restricted distribution of dying cells than previously thought, with reproducible cell death notably absent from the atrial and ventricular walls. Dying cells were also unexpectedly absent from the fusion seam of apposed cushions. In a complementary study, cell proliferation in these tissues was examined over the same time period using the expression of proliferating cell nuclear antigen as a marker for dividing cells. Cell proliferation occurred throughout the region of the cushions at these stages, including the myocardium and the fusion points of the apposed cushions. It is concluded that cells undergoing programmed cell death at this time in the developing chick heart are abundant in, and largely restricted to, the cushion tissue, and that cushion morphogenesis is regulated by the co-ordination of cell transformation, cell proliferation and, during a narrow time window, cell death.