Morphological changes of the dorsal forerunner cell cluster at the end of epiboly. (A-D′) Three-dimensional renderings of the dorsal forerunner cell (DFC) cluster in a Tg(sox17:GFP) embryo as dorsal views (A-D; animal pole to the top) and sagittal views (A′-D′; enveloping surface layer to the left). (A,A′) DFC cluster at 75% epiboly stage. Cluster size is shown along the following axes: x (medio-lateral, yellow dotted lines in A), y (animal-vegetal, red dotted lines in A′), and z (surface-depth, green dotted line in A′). At this stage, the cluster is composed of approximately 36 cells. Note that the DFC cluster is still in contact with the enveloping layer (EVL) (white arrow in A′). (B,B′) After 60 minutes, the cluster has narrowed along the x and y axes (B and B′), and extended along the z axis (B′). At this stage, the number of DFCs had increased to 41, but remained approximately constant during the remainder of the recording. Note that the dorsal blastoderm (not labelled) has started to move between the DFC cluster and EVL (animal concavity in B′). (C,C′) After 120 minutes, the cluster has begun to extend along the x axis while further narrowing along the y (C) and extending along the z (C′) axes. Note that the DFC cluster is still partially attached to the EVL (white arrow in C′). (D,D′) After 180 minutes, the DFC cluster exhibits a spheroid shape with wider x, shorter y and thicker z dimensions (D and D′). Note that the DFC cluster is now completely detached from the EVL (white arrow in D′). (E) Quantification of the changes in DFC cluster size along the Y (yellow), Y (red) and Z (green) axes. Scale bar: 20 μm.
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