| Summary: | Cell size homeostasis can be achieved by size checkpoints that couple cell size to cell-cycle progression or by alternative mechanisms such as constant extension. In mammalian cells, the existence of strict size checkpoints remains controversial due to the technical limitations in determining cell size directly and accurately. We developed a microfabricated channel system that linearizes mammalian cell growth and facilitates cell size measurements. By tracking cell length, while directly visualizing cell-cycle progression in rat basophilic leukemia cells and RAW 264.7 macrophages, we examined the mechanisms of size homeostasis and the existence of a size checkpoint at the G1/S transition. Our analysis revealed a two-tier size homeostasis mechanism where a G1 “sizer” or “adder” could operate, depending on the birth size of the cells.
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