Regulation of cell number in Drosophila.

During animal development, different parts grow independently (such as the left and right hands) but they stop growing when they reach the correct size. In most insects, growth of the epidermis is so controlled that, at each moult, there is a precise and proportionate increase in cell number. The mechanisms responsible for this size regulation are not known, but rigid programming of the number of cell divisions is not a requirement as even sister cells in an epithelial sheet divide variably. In the abdomen of dipterans, such as Drosophila, the opportunity for regulation is limited, because mitoses occur only in the embryo and during metamorphosis and not during larval growth. Here we used embryos with a reduced number of cells in the abdominal primordia to determine whether they can regulate towards the normal during subsequent growth. In contrast to expectations, we find no evidence for regulation of cell number.