ONSET OF GASTRULATION, MORPHOGENESIS AND SOMITOGENESIS IN MOUSE EMBRYOS DISPLAYING COMPENSATORY GROWTH

This is a study on the ability of mouse embryos to compensate for a lo ss of cells and to develop with body parts of normal size and normal p roportions during post-implantation development. Micro-manipulations w ere performed on 4-cell pre-implantation mouse embryos to reduce the n umber of cells by 25% (3/4 embryos) or 50% (2/4 embryos). Blastocysts developed from these embryos showed a preferential loss of inner cell mass population, and fewer of them formed viable embryos after implant ation. The size of post-implantation 3/4 embryos was initially smaller than controls of the same gestational age, but compensatory growth, a chieved by increasing cell numbers at above the normal rate and beyond the normal duration, took place between 6.5 and 11.5 days, resulting in a complete restoration of body size. During compensatory growth the 3/4 embryos rescheduled events of gastrulation and morphogenesis in k eeping with cell number or body size appropriate for each developmenta l stage. The formation of the correct number of somites was accomplish ed by changing the rate of somite segmentation and by an adjustment of the size of individual somites and somitomeres proportional to the av ailable amount of precursor tissues. Morphogenesis and pattern formati on in embryos recovering from earlier cell losses are therefore regula ted in accordance to tissue volume (or cell number) instead of chronol ogical age or some intrinsic cellular clock.