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 degrees C induced apoptosis in blastodermal cells (14%). A modest amount of apoptosis (10%) was also induced at the heat shock temperature of 48 degrees C, but not at 45 degrees 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.