Before revision:  i. e., the instability spontaneously appears when the cell density in the sheet plane increases and/or when the number of neighboring cellsdecreases, as observed in vivo.
After revision:  i. e., the forces act to break the symmetry of the monolayer, leading to cell extrusion that is directed to the apical or basal side of the monolayer by the balance of contractile and adhesive forces on the sheet plane increases and/or when the number of neighboring cellsdecreases, as observed in vivo.