Cytoneme-mediated cell-to-cell signaling during development.

Cell-to-cell communication is vital for animal tissues and organs to develop and function as organized units. Throughout development, intercellular communication is crucial for the generation of structural diversity, mainly by the regulation of differentiation and growth. During these processes, several signaling molecules function as messengers between cells and are transported from producing to receptor cells. Thus, a tight spatial and temporal regulation of signaling transport is likely to be critical during morphogenesis. Despite much experimental and theoretical work, the question as to how these signals move between cells remains. Cell-to-cell contact is probably the most precise spatial and temporal mechanism for the transference of signaling molecules from the producing to the receiving cells. However, most of these molecules can also function at a distance between cells that are not juxtaposed. Recent research has shown the way in which cells may achieve direct physical contact and communication through actin-based filopodia. In addition, increasing evidence is revealing the role of such filopodia in regulating spatial patterning during development; in this context, the filopodia are referred to as cytonemes. In this review, we highlight recent work concerning the roles of these filopodia in cell signaling during development. The processes that initiate and regulate the formation, orientation and dynamics of cytonemes are poorly understood but are potentially extremely important areas for our knowledge of intercellular communication.