Fig. 9

The schematic of our hypothesis on YY1a-mediated PSR engulfing dead cell system affects normal embryonic development and organogenesis. Development begins at the completion of the first zygotic cell cycle (0–0.5 h; YY1a and PSR genes also were expressed), proceeds to the cleavage stage (0.75 h) and then to cell cycles 2–7, which occur rapidly and synchronously. During the gastrula stage, three germ layers are formed (5.25 hpf). At this point, the cell mobility is needed for epiblast, hypoblast, and embryonic axis formation through to the end of epiboly. Apoptosis can occur during the early shield-stage gastrula, but cell corpses are removed promptly through PS receptor-mediated engulfment about starting at 5.4 hpf, which may be regulated by transcription factor zfYY1a. If not removed, the cell corpses accumulate gradually and progressively impede cell movement necessary for triggering downstream developmental events. Cells in the three germ layers (ectoderm, mesoderm, and endoderm) participate in segmentation (10 h), leading to development of somites, pharyngeal structures, primordia, and neuromeres, which is necessary for primary organogenesis and proper appearance of the tailbud. A disturbance at this early stage influences organogenesis at later stages (i.e., the pharyngula [24 h], hatching [48 h], and early larval [72 h] stages). YY1a knockdown induced morphant defects that are also corrected by extra YY1a mRNA and PSR mRNA injection at one cell or two cell stage