Department of Zoology; University of Oxford
Evidence from Drosophila suggests that the insulin like growth factor system is critical for cell number and size determination. In mouse, disruption of the paternal allele (Igf2-m/-p) of the imprinted embryonic gene coding for Insulin-like growth factor II (IGF-II), results in a 40% reduction in weight compared to wild-type (WI) controls (1). Differences in weight are detectable from embryonic day (E) 11, and are maintained throughout life (2). Using methods to generate single cell suspensions of whole embryos suitable for flow cytometry, we were able to time litters based on an equation determined from WT cell number accumulation, as opposed to timing by plugging. Here we show that Igf2+m/-p and WT whole embryos have a similar total cell number up to E9.25 (3x105 cells with >2C DNA content). At this time a striking increase in cell death occurs in Igf2+m/-p embryos (subG1 counts p=0.003, FAM-VAD-FMK p=.044) reverting to WT levels by E9.75. This is followed by significant changes in the proportion of cells in S-phase and G2 at E9.75 (S-phase counts p=0.001, BrdU pulse p=0.006). Cell numbers begin to diverge by E9.5 and significant differences occur by E11 (75% of WI). Cell size was unaffected throughout this period as assessed by Coulter Multisizer analysis. Furthermore, no differences in morphological development (E8.5-11.5) were detected, based on somite counts and the appearance of limb buds, otic pits and branchial arches. We conclude that one important mechanism of growth control by IGF-II concerns modification of cell survival and proliferation during a discrete phase of mouse development (E9.25 to E9.75).
De Chiara, T.M. et at. A growth-deficiency phenotype in heterozygous mice carrying an insulin-like growth factor II gene disrupted by targeting. Nature 345: 78-80, 1990.
Baker, J. et al. Role of insulin-like growth factors in embryonic and postnatal growth. Cell 75: 73-82, 1993.