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Apoptosis is a mechanism employed by multicellular organisms throughout development as a means of eliminating damaged or otherwise unwanted cells. From oogenesis through fertilization and gastrulation, organisms use an array of cell- and tissue-specific mechanisms to regulate the apoptotic program in response to stress or developmental cues. Since cell death regulation is tightly interwoven with cell cycle and checkpoint controls, and embryos of the fly, fish and frog exhibit unique embryonic cell cycle regulation, it is of great interest to understand how early embryos coordinate these cellular functions.
Fig. 1.
Mos/MAPK pathway as a specific regulator of germ cell death. The Mos/MAPK pathway regulates meiosis completion. MAPK family signaling in different cellular contexts has been shown to promote survival or apoptosis. In oocytes of Drosophila, starfish, and Xenopus, the Mos/MAPK pathway seems to prevent apoptosis, whereas in starfish, p38 MAPK may act to promote cell death. MAPK signaling is one of the regulatory controls that may give germ cell apoptosis some of its unique characteristics.
Fig. 2.
Acquisition of the ability to undergo stress-induced apoptosis is developmentally regulated. Early embryos of Drosophila and Xenopus are refractory to induced apoptosis. In Drosophila, several pathways could be involved in early apoptosis since the onset and the regulation of apoptosis varies with the type of stress used to trigger apoptosis. In Xenopus, all stress-induced apoptosis initiates at the early gastrulation transition (EGT), following the activation of zygotic transcription, the remodeling of the cell cycle and the acquisition of damage checkpoints. MBT: Midblastula transition.
Fig. 3.
Coordination of cell cycle arrest and apoptosis. Following DNA damage, cells or embryos activate a coordinated response leading to cell cycle arrest or apoptosis. p53 is at the branching point of this decision in mammalian cells and plays an important regulatory role in both cell death and apoptosis. In contrast, p53 in invertebrates regulates only cell death and lies downstream of the decision between cell cycle arrest and cell death.
