Fig. 4

Cells from the mesoderm form myosin II apico-basal cables during mesoderm invagination. a–c Time-lapse images of the ventral part of a sqhKI[eGFP] embryo during invagination (ventral and transversal views, see schemes, a). First, myosin II is accumulated apically (ventral view). Then, transient linear structures perpendicular to the apical surface are formed (white arrows, transversal view), from the onset to the end of invagination (n = 82). See Supplementary Movie 4. Scale bar: 10 µm. b Longitudinal view of the same tissue at t = 2 min showing these apico-basal structure of myosin II all along the embryo. Scale bar: 10 µm. c Enlargement of the apico-basal myosin II transient structure (white arrow) in the dashed white framed in (a) and the corresponding scheme. See Supplementary Movie 5. Scale bar: 5 µm. d Transversal views of the invaginating mesoderm in fixed embryos showing a myosin II cable formed in cells that have just constricted their apex, at early (left) and late (right) stages of invagination (white arrows). The apical and basal surfaces are outlined in white. Right of each panel: Schemes of the framed regions focusing on the apically constricted cell producing the apico-basal structure of myosin II (green line: myosin II cable, magenta dashed line: cell outline, black line: outline of the epithelium). Scale bar: 10 µm. e Time-lapse images of mesoderm invagination in a sqhKI[eGFP]; PH-mCherry embryo (ventral and transversal views, see schemes) showing the dynamics of a myosin II cable (white arrow) formed in a cell that constricts its apex (outlined by a white line in the ventral views). Dashed lines indicate the section corresponding to transversal views. Schemes recapitulate myosin II dynamics (bottom panel). See Supplementary Movie 6. Scale bar: 10 µm. An anterior, P posterior, V ventral, D dorsal, Ap apical, B basal