The Control of Morphogenetic Cellular Death in the Avian Wing Bud \u3cem\u3eIn Vitro\u3c/em\u3e

Cellular degenerations occur abundantly and predictably during embryogenesis in vertebrates, particularly in regions of rapid morphogenetic change (Glucksmann, \u2751). The emergence of the definitive contours of the limb in the chick embryo, for example, is correlated spatio-temporally with waves of degeneration which sweep through the superficial mesoderm along the pre- and post-axial borders of the limb buds and later occur in the soft tissues between the digits (Saunders, Gasseling and Saunders, \u2762) . A prominent component of this death pattern is a zone of necrosis, first visible at stage 24 (Hamburger and Hamilton, \u2751) , which occurs at the post-axial junction of the wing bud and body wall. For convenience this area of cell death was called the Posterior Necrotic Zone or PNZ by Saunders et al., (op. cit. ) . They mapped the prospective PNZ areas for wing buds of stages 17 through 23 on the basis or extirpation and of carbon-marking experiments. It appears that the cells of the PNZ are programmed for death as early as stage 17, for prospective PNZ\u27s of stages 17 through 23 grafted to the somite region of the donor, or even to an embryo of an earlier or later developmental stage, are visibly necrotic at donor stage 24. But PNZ\u27s grafted to the dorsal surface of the wing bud prior to stage 22 remain without necrosis indefinitely; whereas PNZ grafts made after stage 21 die on schedule, thus showing that the commitment to death becomes irreversible only after stage 21