Cellular and subcellular localization of stathmin during oocyte and preimplantation embryo development

Stathmin is a 19 kDa cytosolic phosphoprotein, proposed to act as a relay i ntegrating diverse intracellular signaling pathways involved in regulation of cell proliferation, differentiation, and function. To gain further infor mation about its significance during early development, we analyzed stathmi n expression and subcellular localization in mouse oocytes and preimplantat ion embryos. RT-PCR analysis revealed a low expression of stathmin mRNA in unfertilized oocytes and a higher expression at the blastocyst stage. A fin e cytoplasmic punctuate fluorescent immunoreactive stathmin pattern was det ected in the oocyte, while it evolved toward an increasingly speckled patte rn in the two-cell and later four- to eight-cell embryo, with even larger s peckles at the morula stage. In blastocysts, stathmin immunoreactivity was fine and intense in inner cell mass cells, whereas it was low and variable in trophectodermal cells. Electron microscopic analysis allowed visualizati on with more detail of two types of stathmin immunolocalization: small clus ters in the cytoplasm of oocytes and blastocyst cells, together with loosel y arranged clusters around the outer membrane of cytoplasmic vesicles, corr esponding to the immunofluorescent speckles in embryos until the morula sta ge. In conclusion, it appears from our results that maternal stathmin is ac cumulated in the oocyte and is relocalized within the oocyte and early prei mplantation embryonic cell cytoplasm to interact with specific cytoplasmic membrane formations. Probably newly synthesized, embryonic stathmin is expr essed in the blastocyst, where it is localized more uniformly in the cytopl asm mostly of inner cell mass (ICM) cells. These expression and localizatio n patterns are probably related to the particular roles of stathmin at the successive steps of oocyte maturation and early embryonic development. They further support the proposed physiologic importance of stathmin in essenti al biologic regulation. Mel. Reprod. Dev. 53:306-317, 1999. (C) 1999 Wiley- Liss, Inc.