ABSTRACT
Over many years evidence has accumulated that plants and animals can regulate growth with reference to overall size rather than cell number. Thus, organs and organisms grow until they reach their characteristic size and shape and then they stop – they can even compensate for experimental manipulations 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 and vice versa. We look at the Drosophila wing in more detail: here, both extracellular and intracellular regulators have been identified that link cell growth, division and cell survival to final organ size. We discuss a hypothesis that the local steepness of a morphogen gradient is a measure of length in one axis, a measure that is used to determine whether there will be net growth or not.
A morphogen is a molecule that usually spreads from a localised source; it forms a graded distribution, and the concentration (the scalar of the gradient) at a point or points some distance from the source determines the local differentiation of the cells. Morphogens may act directly on responding cells, and they may also initiate the production of secondary morphogens (see, for example, Lawrence and Struhl, 1996).
Compartments are defined regions of the adult which were first identified in insects, but are also found in vertebrates (Lumsden, 1990). They are founded by small groups of cells, whose descendants form the whole compartment but do not contribute to neighbouring ones. The development of each compartment is specified by a unique set of ‘selector genes’. Compartments are fundamental units of pattern formation and design in the fly (Garcia-Bellido et al., 1979; Lawrence, 1992; Lawrence and Struhl, 1996).
