Mechanisms for removal of developmentally abnormal cells: cell competition and morphogenetic apoptosis.

Various cell differentiation signals are tightly linked with apoptotic signals. For example, as a result of somatic mutations, cells within a developing field occasionally receive an altered level of morphogenetic signaling that gives rise to an abnormal cell type. However, these developmentally abnormal cells are frequently removed by activating apoptotic signals. Although such phenomena are crucial for assuring normal development and maintaining a healthy state of various organs, the molecular mechanisms that sense aberrant signals and activate the apoptotic pathway(s) have not fully been investigated. In this review, we discuss recent progress in this area. Cell competition and morphogenetic apoptosis are two kinds of cell death, both of which are mediated by abnormal signaling of Dpp, a member of the TGF-beta superfamily that functions in Drosophila as a morphogen, mitogen and survival factor. Cell competition results in autonomous apoptosis induced by reduced reception of the extracellular survival factor Dpp, while morphogenetic apoptosis is nonautonomous, and is induced by contact of cells receiving different levels of Dpp signaling.