The process of nucleologenesis was examined in preimplantation bovine embryos by using ethanolic phosphotungstic acid (E-PTA) at the electron microscopic level. E-PTA binds basic lysine-rich proteins and thus makes possible detection of their localization and distribution. In blastomere nuclei of 2-, 4- and early 8-cell embryos, only nucleolus precursor body/bodies (NPBs) appear, being formed from a mass which is homogeneously stained with E-PTA. In cow embryos, the whole nucleologenesis is situated in the 8-cell stage. During the following step of nucleologenesis, the NPB with a big central area (named NPB vacuole) is formed. Fibrillar mass around the central vacuole is intensively stained, especially in the regions in close vicinity to the central vacuole. Many clumps of E-PTA-positive, regularly dispersed material are found inside the vacuole. The next step of nucleologenesis is characterized by the presence of NPB with secondary vacuoles that are also filled with clumps of E-PTA-positive material. Small, intensively stained areas are visible at sites that are probably identical with those where a dense fibrillar component is formed. Until the end of the 8-cell stage, as well as in the morula and early blastocyst, typical fibrillogranular nucleoli are present. These nucleoli have 3 basic components--fibrillar centres (FC), dense fibrillar components (DFC) and granular components (GC) which stain with different increasing intensity. FC and DFC show a strong (particular) reaction while the GC are stained to a lesser degree. In all examined stages of embryonal development, the E-PTA positivity was found within the perinucleolar chromatin and on the clumps of heterochromatin. An analogical localization of basic and acidic regulatory proteins in the examined developmental stages is discussed.

Research Institute of Animal Production Praha