The generation and modification of cell polarity.

The generation of polarity in the cells of eukaryotes in most cases requires an external asymmetrical input. This signal may originate from a random event such as the point of penetration of an egg by a sperm or from asymmetries present in the maternal environment as, for example, in megasporogenesis in flowering plants, or from an asymmetry in the physical environment usually involving either light or gravity. The latter affords the best opportunities for experimental manipulation and single cell systems in algae, mosses and ferns allow both cell biological and genetical investigation of the mechanisms establishing polarity. These simple systems have already shown that the generation of polarity involves two distinguishable processes, axis alignment and axis orientation, but a detailed understanding of the mechanisms by which the environmental inputs are transduced to set up a polar axis is still lacking. Non-motile organisms must also be able to modify cell polarity in response to changed environmental inputs, and here too, lower plant systems afford the best prospect for understanding the mechanisms responsible.