Congratulations to mimi on a very nice piece of work.
It is always great when you can recycle existing data. In this case, Mimi analyzed some beautiful data from the Greco lab Mesa et al 2018 link here where they revisit and measure the same epithelial cells on a mouse epithelium for a week. Fantastic data!
tl;dr synopsis: cells exhibit much better size control in vivo than in vitro.
Recent work from many labs has explored size control in animal cells in vitro and concluded that they exhibit an adder, where a constant amount of volume is added in each cell division cycle (Ginzberg, et al., Elife (2018), Cadart, et al., Nat Comm (2018)). However, to date, there are no comparable studies of mammalian cells in vivo so the physiological relevance of adder mechanisms is unclear.
Mimi performed a single cell analysis of cell size control in epidermal stem cells growing and dividing in a living mouse under normal tissue turnover. She quantified the 3D volume growth and cell cycle progression of epidermal stem cells and found that cell growth is coupled to division through a sizer mechanism operating largely in the G1 phase. I.e., regardless of their size at birth, cells transition through G1/S at similar sizes. Thus, while in vitro tissue culture studies identified adder mechanisms, our work demonstrates that sizer mechanisms are important in vivo. Indeed, cells in vivo exhibit a tighter size control than tissue culture cells in vitro.