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Fig. 8. Depiction of the hypothesis that PSR plays a crucial role in engulfing
apoptotic cell corpses that affect normal embryonic development and
organogenesis. The zygotic embryo is featured as a newly-fertilized entity
through to the completion of the first zygotic cell cycle at zero hours, then
at the cleavage stage (0.75 hpf), and cell cycles 2-7, which occur rapidly and
synchronously. Embryos enter the blastrula at 2.25 hpf, at which time the
metasynchronous cell cycles rapidly give way to a lengthening at the 8-10
cell-cycle stage, when the asynchronous cell cycles at the midblastula
transition and epiboly commence. From 5.25 hpf, the gastrula begins
development of the three germ layers, when all cells require the facility of
cell movement in order to achieve their developmental goals, such as the
morphogenetic movements of involution, convergence, and extension from the
epiblast, hypoblast and embryonic axis through to the end of epiboly. During
the early gastrula stage (6 hpf), apoptotic death can occur during the
entrance to the shield stage, if the apoptotic cell corpses are not removed
promptly. Inhibited removal can result from an absence of a specific engulfing
receptor, such as the PSR. The accumulated corpses gradually and progressively
impede the cell movement and cell-cell interaction necessary for the
triggering of signaling to activate the downstream developmental events. At
the onset of organ development, cells in the embryo are associated with one of
three germ layers, the ectoderm, mesoderm and endoderm, from the time of
segmentation (10 hpf), when somites, pharyngeal, primordia and neuromeres
develop, for primary organogenesis, and for the proper appearance of the
tailbud. This influences the morphogenesis of organs at the pharyngula (24
hpf), hatching (48 hpf) and early larval (72 hpf) stages.
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