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Reconciling different models of forebrain induction and patterning: a dual role for the hypoblast

Ann C. Foley, Isaac Skromme and Claudio D. Stern

Department of Genetics and Development, Columbia University, 701 West 168th Street #1602, New York, NY 10032, USA

Fig. 6A

Control embryo

Time-lapse sequence of the embryo in Fig. 6A. The posterior end of the embryo lies at the top left hand corner of the frame. The embryo was explanted at stage XII and the centre of the prospective forebrain labelled with DiI. One frame corresponds to 10 minutes of real time. In addition to the movement of label, other cell movements of gastrulation can be seen. In particular, note the 'Polonaise' movements that begin before the appearance of the primitive streak, and consist of convergence of epiblast cells towards the posterior pole, and simultaneously a forwards movement along the midline. After the primitive streak appears, the mesoderm and endoderm can be seen migrating away from the axis of the streak. At the end of the film (about stage 8) the label is seen in the forebrain-midbrain region. Note that in the last few frames the exposure was increased to reveal the entire labelled region.

Fig. 6C

Hypoblast rotation 1

Time-lapse sequence of the embryo in Fig. 6C. The posterier end of the embryo lies at the top left of the frame, and the hypoblast has been rotated by 90°, so that it now lies at the top of the frame. One frame corresponds to 10 minutes. Note that the movements of gastrulation (cf. Control embryo) begin as in the control embryo, but there is also a displacement downwards within the frame (to the right of the embryo), which leads to a bending of the axis. Labelled cells end up in the forebrain region. Again in the last few frames the exposure was increased to reveal the entire labelled region.

Fig. 6D

Hypoblast rotation 2

Time-lapse sequence of the embryo in Fig. 6D. Same orientation and frame interval as the other sequences. In this embryo, in addition to the normal movements of gastrulation (cf. other sequences), a regionof the epiblast close to the label seems 'confused' and rotates around before settling. At the end of the film, the label ends up in the forebrain and adjacent head mesenchyme and head ectoderm. Other features are the same as in the other two sequences.

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