Cell death in the avian blastoderm: resistance to stress-induced apoptosis and expression of anti-apoptotic genes

We investigated the expression of an apoptotic cell death program in blastodermal cells prior to gastrulation and the susceptibility of these cells to stress-induced cell death. A low frequency (3.1%) of apoptotic blastodermal cells was observed in Hoechst 33342-vitally stained cytological preparations of complete blastoderms from unincubated eggs. These cells showed the stereotypic features of apoptosis including a progression of nuclear changes, cell shrinkage and blebbing, and the formation of apoptotic bodies. Prolonged storage of eggs at 12°C induced apoptosis in blastodermal cells (14%). A modest amount of apoptosis (10%) was also induced at the heat shock temperature of 48°C, but not at 45°C. Etoposide and other potent cytotoxic drugs failed to induce apoptosis in the blastodermal cells after 4 h of exposure. Progressively more apoptosis was induced at 8 and 24 h, but it did not exceed 35% of the cells. We detected transcripts for the anti-apoptotic genes bcl-2, bcl-xL, and hsp70. The developmental expression of these genes, especially hsp70, correlated with the delayed and limited stress-induction of apoptosis. These studies reveal the capacity of pre-streak blastodermal cells to engage in apoptosis and their relative resistance to stress conditions. This may be due to the prominent expression of hsp70 and/or multiple cell death genes which primarily antagonize cell death.