Figure 4

Nucleologenesis in the Rbm19^{GtXC768/GtXC768}mutant embryos. (A) Embryos from an Rbm19^GtXC768/+ intercross were sorted based on appearance and designated either wild-type-appearing (Rbm19^GtXC768/+ or Rbm19^+/+) or Rbm19^{GtXC768/GtXC768}mutants. Embryos at the indicated stages were stained with anti-B23 (green) and anti-fibrillarin (red) antibodies. (a-d) B23 and fibrillarin localization reveals the morphology of the NPB in E2.5 wild-type embryos. (e-h) At E3.5 nucleoli are smaller and more numerous than earlier stage shown in panels a-d (i-j) and (k-l). In the Rbm19^{GtXC768/GtXC768}mutant, B23 and fibrillarin, respectively, show a heterogeneous pattern, mostly resembling NPBs as seen in panel b, but with a few smaller foci suggesting partial nucleolar differentiation. (B) Electron micrograph of E3.5 wild type and Rbm19^{GtXC768/GtXC768}mutant embryos. Wild type embryos have mature nucleoli with differentiated nucleolar regions including fibrillar components (FC), dense fibrillar components (DFC), and granular components (GC). Mutant blastomeres (5 embryos examined) contained multiple fibrillar spheres (FS) that were presumed to be the nucleolar precursor bodies, but failed to form well-differentiated nucleoli.