Fig. 7. A shift in the plane of cell divisions occurs in the DM prior to complete dissociation. (A-C) Frequency histograms of the orientation of mitotic spindles in the DM at stages HH16-20 of development. An angle of 0° represents a fully parallel orientation of mitosis vis-à-vis the mediolateral plane of the DM epithelium, whereas an angle of 90° reflects perpendicular cell divisions. (D) Quantification of the orientations of cell divisions as a function of embryonic stage. All measurements concerned the flank level of the axis. (E-G) Transverse sections of embryos aged E2.5, E3 and E3.5, respectively, double-labeled with -tubulin (red dots) and Hoechst nuclear stain (blue). Note in E and F the epithelial structure of the DM. In G the central portion of the epithelium is dissociating. Dividing cells are enclosed in inserts and high magnifications of the inserts highlight the planes of cell division. Note that in addition to centrosome-specific staining, -tubulin immunolabeling is also apparent throughout the cytoplasm of tissues such as myotome and ectoderm. DM, dermomyotome; M, myotome, NT, neural tube. Scale bar: 12 µm in E,F; 10 µm in G; 6 µm in E,G (inserts); 7 µm in F.