Abstract
ABSTRACTBiology 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.
Highlights
The importance of time is a tacit assumption in many accounts of developmental mechanisms
Cell division and differentiation is necessary for the growth and assembly of functional, well-proportioned tissues
Abnormalities in timing can lead to defects in the assembly of tissues, resulting in dysfunction that can be incompatible with survival
Summary
The importance of time is a tacit assumption in many accounts of developmental mechanisms. The order in which the different types of neurons are generated in vitro are the same as in vivo (Fig. 1A), and the timing of transitions between cell types is remarkably similar (Eiraku et al, 2008; Gaspard et al, 2008) This does not rule out a role for external signals in modifying the tempo and order of neurogenesis, it emphasises that cell-autonomous mechanisms play a central role in timing developmental progression. Consistent with this, the loss of Ring1B, a component of the polycomb group (PcG) complex, in cortical progenitors results in the prolonged generation of early neuronal subtypes, implicating epigenetic mechanisms in tempo control (Morimoto-Suzki et al, 2014) How this encodes time and what additional mechanisms are involved are poorly understood. To see the full collection as it grows, go to: https:// dev.biologists.org/content/advocating-developmental-biology
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