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Fig. 1. Translocation of the maternal dorsalizing activity. (A-D) Translocation of
the dorsalizing activity leads to ß-catenin stabilization and the
formation of the dorsal organizer. Between the time of fertilization and the
first embryonic cell division, a maternally deposited dorsalizing activity
(red) moves from (A) the vegetal pole to (B) the prospective dorsal region.
(C) By the two-cell stage, maternal ß-catenin (yellow) has become
asymmetrically stabilized in the region that has received the dorsalizing
activity. (D) Stabilized ß-catenin activates genes of the dorsal
organizer (green circle; also called the Nieuwkoop and Spemann organizers) in
the dorsal equatorial region, as shown in an early gastrula embryo. (E,F) The
dorsalizing activity translocates in the same direction as cortical rotation.
(E) The dorsalizing activity (red) resides in the shear zone, an area of
looser cytoplasm that forms between the outer cortex of the egg and the dense
core cytoplasm following fertilization. The black bars at the vegetal pole
mark the starting positions of the core and the cortex early in the first cell
cycle. (F) During the first cell cycle, the cortex rotates relative to the
core, moving about 30° towards the dorsal side in the same direction as
the dorsalizing activity. This process is called cortical rotation, as
represented by the displacement of the outer black bar. D, dorsal; V,
ventral.
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