Size control: cell proliferation does not equal growth

Abstract

Division subdivides mass without increasing it. So one should not expect that an increase in cell division would make an organism bigger. Both classic and recent experiments confirm this simple rationale: altering proliferation produces normally sized body structures with either especially small or exceptionally large cells.

Figure 1

Alterations in cell proliferation have little effect on organ growth. As described in the text, in two recent studies [1,2], compartment-specific expression of transgenes in wild-type or mutant Drosophila wing discs was used to modulate cell proliferation in the anterior and posterior compartments (labelled a and p, respectively). Differences in cell proliferation are seen here as differences in nuclear density when stained with a DNA dye. (a) A Cdk1^ts mutant disc, in which a wild-type Cdk1 transgene was expressed in the posterior compartment. The shift to the restrictive temperature stopped division in the anterior compartment, but not the posterior compartment [1]. Note the decrease in nuclear density in the anterior compartment in this disc. Nuclei enlarged in the anterior compartment as a result of endoreplication. (b) A wild-type disc shown for comparison; the nuclear density is similar in the anterior and posterior compartments. (c) A disc in which Drosophila E2F and Dp have been overexpressed in the cells of the posterior compartment. This expression increases cell number, visible as an increase in nuclear density, in the posterior compartment relative to the anterior compartment [2]. While the DNA stain detects nuclear size, which did not change in this case, cell size was reduced in proportion to the increased cell number (not visualized here). Despite the changes in cell proliferation in (a) and (c), the compartments and whole discs are of nearly normal size and morphology. (In part reproduced, with permission, from [1,2].)