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Cell death during embryogenesis

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Cell death during embryogenesis Reproduction in multicellular organisms functions to pass the cell's genome on to the next generation. The genotype survives and continues through time as the phenotype of the organism itself dies. The organism can be considered the vehicle that the genome uses to get into the next generation. After the fertilization of two gametes, cell proliferation through mitosis must occur to increase the population of cells in the developing organism. However, cell death is also necessary for embryological development, differentiation and morphogenesis to occur. The health of all animals depends not only upon the production of new cells, but also on the orderly death and removal of superfluous cells when they are no longer necessary for the functioning of the whole organism. There are two different types of death that must be considered here. Normally people think of cell death as a traumatic, injurious, often accidental event - this is necrosis. There is another type of cell death that is essential to the normal growth and development of a multicellular organism. Apoptosis, or programmed cell death, is genetically programmed into cells and is activated only under very special circumstances. ...read more.


The first dying cells are detected in the dorsal region of the head approximately 7 hours after egg laying, which corresponds to stage 11. As development proceeds, apoptic cell death becomes more prominent throughout the embryo, and corpses are engulfed by circulating macrophages. Time-lapse photography of these embryos shows that while cell death occurs in a consistent pattern, the precise spatial and temporal aspects of this pattern are somewhat variable, indicating that here is a certain degree of plasticity in embryonic cell death (Abrams et al., 1993). Regulators of cell death were identified as three genes, reaper (rpr), grim, and head involution defective (hid). Their gene products are involved in the initiation of all embryonic cell death in Drosophila (White et al., 1994). Over expression of any of these genes in cultured fruit fly cells results in the rapid transduction of cell death (Chen et al., 1996). Drosophila development begins in the fertilized egg with a series of 13 syncytial divisions, followed by three rounds of cellular divisions for most cells. Cells are allocated and positioned along dorsal-ventral and anterior-posterior axes according to maternally established morphogen gradients. ...read more.


Conversely, if the compression of a developmental domain drops the cell number below a certain threshold, structural problems occur or particular organs fail to develop (Namba et al., 1997). Drosophila provides a powerful model system to investigate both the mechanisms and roles of apoptosis during animal development. Genetic manipulation allows the study of apoptotic regulators and effectors in the context of the whole animal. This provides the opportunity to understand how different cellular processes are coordinately regulated to form the developed organism. Apoptosis is necessary to establish and maintain the correct cell densities required for efficient and precise patterning mechanisms. Coupled with cell proliferation, cell death ensures a plasticity in developmental processes that allows an embryo to develop dynamically according to circumstances. The investigation of how cell division and apoptosis are coupled is likely to reveal fundamental mechanisms of organogenesis. Our understanding of apoptosis during animal development is still in its infancy. It is apparent that cell death occurs through development and rudimentary maps detailing the spatial and temporal occurrence of cell death allow us to examine its consequences. The identification of many of the central genes required for apoptosis provide tools for the exploration of how this process is integrate with signaling pathways that specify proliferation or differentiation during development. ...read more.

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