96
M. Hild et al.
6.9 Long-Range Effects of Chordino Signaling
The rearrangement of the bmp2b and bmp7 mRNA distribution from the
uniform to the graded pattern takes place in the entire embryo, including
ventral-most regions very distant from the Spemann organizer, the site
of Chordino production. Thus, if Chordino function was indeed responsible for this transformation, one has to propose that Chordino has
long-range signaling effects. To test this hypothesis, chimeric embryos
were generated by transplanting labeled wild-type cells into chordino
mutant host embryos (Hammerschmidt et al. 1996b). When wild-type
cells were transplanted into the future ventral side of a late blastula
chordino mutant embryo, the mutant phenotype was maintained at
normal strength, whereas transplantation into the future dorsal side led
to a striking rescue of the phenotype to almost wild-type condition. This
effect was achieved even when only a few cells (approximately 20) were
transplanted. Although after 48 h of development, the descendants of
these cells were all located in very anterior positions, the chimeric
embryo displayed a rescue of the chordino mutant phenotype over the
entire length of its body axis, indicating that Chordino must act in a
non-cell autonomous fashion with long-range effects, influencing even
the fate of very distant cells. With molecular markers, this long-range
effect of the Chordino-generating wild-type cells can already be revealed at midgastrula stages, a few hours after cells had been transplanted (Fig. 30-1). Wild-type cells were transplanted directly into the
Spemann organizer of an early gastrula mutant embryo. After 2 h, the
wild-type cells were located in the presumptive notochord anlage. Such
chimeric embryos displayed a significant retraction of the expression of
the ventrolateral marker gene eve} into lateral regions. Even mutant
cells many cell diameters away from the wild-type cells, the source of
functional Chordino protein, had lost their eve} mRNA expression
(Fig. 3H). No such effect was observed upon transplantation of chordino mutant cells. This indicates that Chordino signaling can spread
over about a quarter of the entire dorsoventral axis of an early gastrula
embryo within 2 h.
It is currently unclear whether this long-range effect is caused by
Chordino proteins directly, or whether secondary signals are involved.
In any case, the long-range signaling, which occurs on the level of the
protein, is further potentiated by a progressive lateral expansion of the
M. Hild et al.
6.9 Long-Range Effects of Chordino Signaling
The rearrangement of the bmp2b and bmp7 mRNA distribution from the
uniform to the graded pattern takes place in the entire embryo, including
ventral-most regions very distant from the Spemann organizer, the site
of Chordino production. Thus, if Chordino function was indeed responsible for this transformation, one has to propose that Chordino has
long-range signaling effects. To test this hypothesis, chimeric embryos
were generated by transplanting labeled wild-type cells into chordino
mutant host embryos (Hammerschmidt et al. 1996b). When wild-type
cells were transplanted into the future ventral side of a late blastula
chordino mutant embryo, the mutant phenotype was maintained at
normal strength, whereas transplantation into the future dorsal side led
to a striking rescue of the phenotype to almost wild-type condition. This
effect was achieved even when only a few cells (approximately 20) were
transplanted. Although after 48 h of development, the descendants of
these cells were all located in very anterior positions, the chimeric
embryo displayed a rescue of the chordino mutant phenotype over the
entire length of its body axis, indicating that Chordino must act in a
non-cell autonomous fashion with long-range effects, influencing even
the fate of very distant cells. With molecular markers, this long-range
effect of the Chordino-generating wild-type cells can already be revealed at midgastrula stages, a few hours after cells had been transplanted (Fig. 30-1). Wild-type cells were transplanted directly into the
Spemann organizer of an early gastrula mutant embryo. After 2 h, the
wild-type cells were located in the presumptive notochord anlage. Such
chimeric embryos displayed a significant retraction of the expression of
the ventrolateral marker gene eve} into lateral regions. Even mutant
cells many cell diameters away from the wild-type cells, the source of
functional Chordino protein, had lost their eve} mRNA expression
(Fig. 3H). No such effect was observed upon transplantation of chordino mutant cells. This indicates that Chordino signaling can spread
over about a quarter of the entire dorsoventral axis of an early gastrula
embryo within 2 h.
It is currently unclear whether this long-range effect is caused by
Chordino proteins directly, or whether secondary signals are involved.
In any case, the long-range signaling, which occurs on the level of the
protein, is further potentiated by a progressive lateral expansion of the