Expression pattern of Hoxa-1 in whole-mount in situ hybridisations. (A,B) 50% epiboly. A is the view from the animal pole dorsal side towards the bottom, B is the same embryo as in A but tilted to show more of the dorsal side. (C,D) About 70% and 80% epiboly respectively, viewed from the dorso-vegetal side, note the broadening of the domain of expression towards the vegetal pole and also the closing of the gap at the dorsal midline. (E,F) 100% epiboly. E is a dorsal view and F is the same embryo viewed form the side, dorsal to the right and rostral end of the embryo is at the top. (G,H) Progressively older embryos after the tailbud stage viewed from the dorsal side. Note the sharp anterior boundary of the expression domain and the increase in signal at the dorsal midline where the cells are converging to form the spinal cord. (I,J) 20 hour old embryo showing the strong signals from the spinal cord, the endoderm (arrowheads), the tailbud, the regions perpendicular to the A/P body axis at the level of the otic vesicle. (K-O) Embryos in a sequence from 10 to 16 hours showing the expression of Hoxa-1 and krx-20. The boundary of Hoxa-1 at the time krx-20 first expresses is just caudal to r3. By 16 hours, this rostral Hoxa-1 boundary has receded to the spinal cord/hindbrain border. Expression can be seen in the lateral regions of r4 prior to the signal receding – arrowed in M. Scale bar, 100 μm.
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