Formation of cytoskeletal elements during mouse embryogenesis. III. Primary mesenchymal cells and the first appearance of vimentin filaments.

The cytoskeletal composition of the 'primitive streak' stage of mouse embryos, i.e. at late day 8 ('day 8.5') of gestation, has been examined by electron microscopy, using thin sections of fixed and embedded embryos, and by immunofluorescence microscopy, using cryostat sections of frozen embryos. At this stage primary mesenchymal cells are observed in the posterior part of the embryo which seem to migrate toward the anterior region. For most of the embryo, these mesenchymal cells are separated from the embryonic ectoderm by a continuous basal lamina. Frequently mesenchymal cells form cytoplasmic projections many of which make contact with this basal lamina, with surfaces of proximal endoderm cells, or with other mesenchymal cells. Primary mesenchymal cells contain sparse individual intermediate-sized filaments (IF), but closely packed IF bundles as they occur as tonofibrils in both embryonic epithelia, ectoderm and proximal endoderm, have not been found. Mesenchymal cells also can form junctions of the fascia adhaerens-type but appear to be devoid of desmosomes. Antibodies to cytokeratins reveal strong fibrillar fluorescence in cells of the proximal endoderm and weak, predominantly subapical staining in embryonic ectoderm. Correspondingly, antibodies to desmoplakins, the major proteins of the desmosomal plaque, show punctate fluorescence in both embryonic epithelia. These epithelial cells are not significantly stained with antibodies to other IF proteins such as vimentin and desmin. However, antibodies to vimentin show positive fluorescence, often in fibrillar tangles, in primary mesenchymal cells which in turn are negative with cytokeratin and desmin antibodies. This first detection of expression of vimentin in embryogenesis has been confirmed by two-dimensional gel electrophoresis of cytoskeletal proteins from 35S-methionine-labelled embryos. The observations indicate that during embryogenesis synthesis of vimentin occurs, for the first time, in the primitive streak stage and is restricted to the primary mesenchymal cells. Concomitantly, these cells cease to produce cytokeratins and desmoplakin. Possible mechanisms effective in this rapid change from epithelial to mesenchymal character, i.e. from cytokeratin IF to vimentin IF, are discussed.