Fig. 6.

Declining cell size during development does not induce YP activation. (A) Large cell size was apparent as decreased nuclear density (DAPI, green) in the prosencephalon of a late neurula treated with hydroxyurea/aphidicolin (HU/APH; bottom), as compared with control siblings (top). Scale bar: 100 μm. (B) Overexpression of CycA2 and ^6mycCdk2 increased the frequency of mitotic cells in multiple lineages of late neurulae (top, uninjected control in bottom panels). Transcripts encoding the two proteins were injected into the animal pole at the two-cell stage. Cells expressing ^6mycCdk2 were identified with anti-myc 9E10 antibody (α-myc), mitotic nuclei by anti-phospho-Histone H3 antibody (α-PH3). Right, single channel images; left, merged images. In contrast to a previous report (Richard-Parpaillon et al., 2004), CycA2/^myc6Cdk2 increased the mitotic index of the somitogenic mesoderm and of the normally postmitotic notochord (notochord, 7.4% versus 0%; somitogenic mesoderm, 5.2% versus 1.0% mitotic index; injected versus uninjected, n>160 cells). Scale bar: 100 μm. (C-E) The effects of HU/APH treatment and CycA2/^6mycCdk2 overexpression on mitotic index, cell size and YP activation were quantified. HU/APH and DMSO control data are derived from the prosencephalon of late neurulae (stage 17-20), whereas CycA2/^6mycCdk2 overexpression and uninjected control data are derived from the neural tube of late neurulae (stage 18-19). The average ±1 s.e. is presented. For HU/APH and DMSO control, n=9 embryos for cell size and YP activation and n=6 embryos for mitotic index. For CycA2/^6mycCdk2 overexpression and uninjected control, n=8 embryos for all parameters. ^*P<0.01, Student's t-test. pr, prosencephalon; ac, archentron ceiling; ng, neural groove; nt, notochord; s, somitogenic mesoderm.