Fig. 6. Inner medial cells become dispersed and intercalate towards the ectoderm. (A) We define inner medial (IM) cells as cells at the centre of the furrow, which will be situated at the innermost point (top) of the invaginated epithelial tube (dark grey) after gastrulation. (B) Photoactivated IM cell at gastrulation. The midline is indicated by the radial line. (C) The same cell 5 minutes later is now situated at the top of the invaginated epithelial tube. Photoactivated fluorescence (red), Neurotactin (green). (D) Before gastrulation, embryos were monitored using the low levels of pre-activation fluorescence until the onset of furrowing was detected. The furrow was first detected as a slight flattening at the anterior and posterior ends of the nascent furrow. (E) Two cells at the centre of the developing furrow were first scanned with 488 nm laser at zoom x14.5. A polygon was then drawn around them and they were scanned with the UV laser. (F) This resulted in the two cells being brightly labelled, with some neighbouring cells also being weakly labelled. The dotted line shows the midline. (G,G') One minute 45 seconds later the furrow has formed and the photolabelled cells are internalised. This morphological stage is used as the zero time-point for all timings in movies. A reconstructed cross-section through the dotted line is shown in G'. (H-J) A time-lapse sequence at a constant focal plane showing the appearance of inner cell progeny at approximately 1 hour post-gastrulation. (H'-J') Reconstructed cross-sections of z-series in upper panels. (H) At 0:46:00 the inner medial cell progeny are too internal to be detected. White blurred areas on the right are yolk. (I) At 0:56:00 minutes the inner medial cell progeny are first detected. (J) By 1:06:00 the cells are clearer, and are dispersed across the mesoderm. As seen in the reconstructed cross-section panel below, the cells are at a depth typical of cells adjacent to the ectoderm. (K) An isolated inner cell in a fixed control embryo shortly before the second division. Dotted line shows midline. Scale bars: 20 µm in B-D,F-K; 2 µm in E.