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Fig. 2. Regulation of Hox gene expression throughout mouse embryogenesis.
Expression of a 3' Hox gene, Hoxb1 (red), and of a 5' Hox
gene, Hoxb8 (blue), at different developmental stages in mouse
embryos. Posterior (P) is towards the left of the E10.5 embryo, owing to axial
rotation, which mouse embryos undergo between E8.5 and E9.0. (A) E7.2, late
primitive streak stage embryo, with the primitive streak (grey) reaching the
node at the distal tip of the embryo. The red arrow shows the direction of the
anterior expansion of the Hoxb1 expression domain. (B) E7.7, late
head fold stage embryo, showing a maximally extended Hoxb1 expression
domain (red) and an early expression field for Hoxb8 (in blue
overlaying Hoxb1 expression). The blue arrow indicates the
anteriorwards spread of the Hoxb8 expression domain; the orange line
indicates retinoic acid (RA) in the mesoderm. (C) E8.0, five-somite stage
embryo; the remnant of the primitive streak is shown in black
medioposteriorly, with the node region at its anterior end. White circle
indicates posterior stem cell zone. (D) E10.5 embryo; the remnant of the
primitive streak is in the tailbud. Hoxb1 expression is downregulated
and remains strong in rhombomere 4 anteriorly, and in the tailbud posteriorly.
Hoxb8 expression is about to be induced by RA to extend rostrally
into the posterior hindbrain (blue arrow). The role of Wnt and Fgf at the
early stages (A,B) is assumed, but not definitively documented. See
Fig. 8 for more detail on the
role of the early locus enhancer, and Fig.
7 for that of the chromatin events. PSM, presomitic mesoderm.
Green triangle indicates posterior-to-anterior Fgf and Wnt gradients. trxG and
PcG, trithorax group and polycomb group protein complexes, which activate and
repress Hox gene expression, respectively. Scale bars: 100 µm for A-C; 75
µm for D.
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