578 Chapter 18 • The Genetic Basis of Development These further processes— fate refinement —can be of two types. Sometimes, through asymmetrical di-visions of one of the precursor types of cells, descen-dants are created that have received different regula-tory instructions and therefore become committed to different fates. This mechanism for partitioning fates can be thought of as cell lineage dependent . Alterna-tively, such fate decisions are made by “committee”; that is, the fate of a cell depends on input from neigh-boring cells and feed-back to them. This mechanism for partitioning fates can be thought of as neighborhood dependent . The cell-neighborhood-dependent mecha-nisms provide for a certain flexibility so that an organ-ism can compensate for unscripted events such as the accidental death of a cell. If some cells are lost by acci-dent, the cell-to-cell communication system is used to reprogram the surviving neighbors to divide and divert a subset of their descendants to adopt the fates of the deceased cells. Indeed, the complete regeneration of sev-ered limbs, as occurs in some animals such as starfish or amphibians, depends on neighborhood-dependent mechanisms. 18.2 Binary fate decisions:
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