What does time mean in development?

Biology is dynamic. Timescales range from frenetic sub-second ion fluxes and enzymatic reactions to the glacial millions of years of evolutionary change. Falling somewhere in the middle of this range are the processes we usually study in development: cell division and differentiation, gene expression, cell-cell signalling, and morphogenesis. But what sets the tempo and manages the order of developmental events? Are the order and tempo different between species? How is the sequence of multiple events coordinated? Here, we discuss the importance of time for developing embryos, highlighting the necessity for global as well as cell-autonomous control. New reagents and tools in imaging and genomic engineering, combined with in vitro culture, are beginning to offer fresh perspectives and molecular insight into the origin and mechanisms of developmental time.

Taken together, studies from a range of systems are beginning to shed light on the molecular and genetic mechanisms that encode and interpret developmental time. Comparisons between species and results from different tissues are identifying the players and mechanisms that account for the order and tempo of developmental events. New reagents, particularly novel imaging reporters, and the development of new techniques, such as in vitro tissue culture methods, are offering a fresh perspective on this long-standing problem. This is sharpening the focus on the underlying principles and relevant questions, answers to which will provide insight into a fundamental aspect of embryo development.