Measuring dimensions: the regulation of size and shape

Over many Sears evidence has accumulated that plants and animals can regula te growth with reference to overall size rather than cell number. Thus, org ans and organisms grow until they reach their characteristic size and shape and then they stop - they can even compensate for experimental manipulatio ns that change, over several fold, cell number or average cell size. If the cell size is altered, the organism responds with a change in cell number a nd vice versa. We look at the Drosophila wing in more detail: here, both ex tracellular and intracellular regulators have been identified that link cel l growth, division and cell survival to final organ size. We discuss a hypo thesis that the local steepness of a morphogen gradient is a measure of len gth in one axis, a measure that is used to determine whether there will be net growth or not.