Ultrastructural localization of basic lysine-rich proteins during the nucleologenesis in preimplantation mouse...

The paper describes nucleologenesis in preimplantation mouse embryos studied by electron microscopy after staining with ethanolic phosphotungstic acid (E-PTA), a substance which binds to basic lysine-rich proteins and makes it thus possible to follow their localization and distribution. A nucleolar precursor body (NPB) homogeneously stained with E-PTA first appears in pronuclei of a fertilized oocyte. In 2-cell stage embryos, the staining intensity of the central part of NPB is reduced, whereas the outer zone retains the PTA positivity. In the course of further development, reticulation takes place in the peripheral zone and all structures of a functional nucleolus are gradually formed. Tiny spherical bodies, called "remnant NPBs", can still be observed in reticular nucleoli of cells at the morula stage. The similarities in reaction and morphological appearance of the NPB and the corresponding structures in oocytes of the mouse and rat antral follicle are discussed.