/. Embryol. exp. Morph. 90,197-209 (1985) 197 Printed in Great Britain © The Company of Biologists Limited 1985 A monoclonal antibody against mouse oocyte cytoskeleton recognizing cytokeratin-type filaments E. LEHTONEN Department of Pathology, University of Helsinki, Haartmaninkatu 3, SF-00290 Helsinki, Finland SUMMARY Monoclonal antibodies were raised to detergent-extracted cytoskeleton preparations of mouse oocytes. In immunofluorescence microscopy, one oi : the antibodies, OCS-1, localizes exclusively to epithelial cells in frozen tissue sections, including various simple and stratified epithelia. The antibody decorates a keratin-type of fibrillax, vinblastine-resistant network in various cultured, epithelial-type cells, but not in myoid or fibroblastoid cells. In mouse oocytes and cleavage-stage embryos, the OCS-1 antibody gives a diffuse, spotty staining pattern. In blastocyst-stage embryos, the antibody reveals a keratin-type filamentous organization in the trophectoderm cells. In immunoelectron microscopy, the OCS-1 antibody decorates 10 nm-thick filaments, often identifiable as desmosome-attached tonofilaments, in detergent-treated trophectoderm cells. The antigen(s) recognized by the OCS-1 antibody is apparently present in, or closely associated with, cytokeratin filaments. In addition to mouse oocytes and early embryos, a wide variety of epithelial cells in various species seem to share this antigen(s). The present results suggest that at the early stages, the cytokeratin-related antigen(s) defined by the OCS-1 antibody are stored in a non-fibrillar form which is then converted into afibrillarnetwork at the blastocyst stage. A pre-existing supply of cytokeratin-related protein may be essential for the development of the blastocyst. INTRODUCTION Formation of different types of intermediate filaments (IFs) is associated with cell differentiation, and IF subclasses serve as markers for different cell types (Lazarides, 1982; Osborn & Weber, 1983). Special interest should therefore be addressed to the IF cytoskeleton of germ cells and early embryonic cells. The first type of IF proteins expressed during early mouse development is cytokeratin (Jackson et al. 1980; Paulin, Babinet, Weber & Osborn, 1980; Lehtonen et al. 1983c), and another type of IF, vimentin, appears in the newly forming mesoderm cells late on embryonic day 8 (Jackson et al 1981; Franke et al. 1982). In the preimplantation mouse embryo, the presence of cytokeratin filaments in the trophectoderm cells of the blastocyst has been confirmed by several authors (Brulet, Babinet, Kemler & Jacob, 1980; Paulin et al 1980; Jackson et al 1980; Lehtonen et al 1983c; Oshima et al 1983). In a previous report, we have demonstrated, with electron microscopy, the presence of detergent-resistant Key words: cytoskeleton, cytokeratin, monoclonal antibodies, oocyte, immunoelectron microscopy, mouse.