The Apoptotic and Nonapoptotic Nature of the Terminal Differentiation of Erythroid Cells

The morphology of erythroid cells changes dramatically during the course of their terminal differentiation. According to calculations made with cytospin preparations obtained from Syrian hamster yolk-sac-derived erythroid cells, the area of nuclei at day 10 of gestation ranges from 25 to 85 μm²and is reduced to 15–25 μm²on day 13 [K. Morioka and R. Minamikawa-Tachino,Dev. Growth Differ.35, 569–582, 1993]. The DNA and protein contents of each nucleus also decrease during this period. Nonspecific fragmentation of DNA was detected by agarose gel electrophoresis in all samples obtained from day 10 to day 13 of gestation, while distinct ladders of DNA fragments were not detected. DNA fragmentation was also detected by anin situDNA-end labeling (TUNEL) assay. As the terminal differentiation proceeded, gradual decreases in levels of both histone H1 and most nonhistone proteins were observed by SDS–polyacrylamide gel electrophoresis, while levels of core histones appeared to be constant. In particular, lamin B₂was almost completely lost from the nuclear matrix fraction on day 11. These results suggest that the terminal differentiation of erythroid cells and apoptosis might have common mechanisms. However, expansion of the cytoplasm during the terminal differentiation distinguishes these processes. In addition, in the erythroid terminal differentiation, nuclei never form lobules or become fragmented; no apoptotic bodies are formed, occurrence of the apoptosis-like cellular change is not sporadic but rather synchronous, and the process is slow, with at least several days being required for cell death. These characteristics are different from those of typical apoptosis. Thus, the terminal differentiation of nucleated embryonic erythroid cells exhibits both apoptotic and nonapoptotic features.