86
M. Hild et al.
6.4 Morphology of Dorsalized and Ventralized Zebrafish
Mutants
The phenotypes of zebrafish mutants, with specific defects in dorsal or
ventral specification during early dorsoventral pattern formation, could
already be predicted from the effects caused by the overexpression or
specific inhibition of the ventralizing agents Bmp2, Bmp4, or Bmp7 in
genetically wild-type embryos. Overexpression of Bmps was achieved
by microinjecting different amounts of synthetic bmp mRNAs into
zebrafish embryos of the I-cell stage, inhibition of Bmps by microinjection of mRNAs encoding one of their aforementioned inhibitors, Noggin, Chordin, or Follistatin (Bauer et al. 1998; Kishimoto et al. 1997). At
36 h postfertilization, embryos injected with the lowest amount of noggin mRNA display a partial loss of the ventral tail fin, the ventral-most
ectodermal derivative, defining the mildest degree of dorsalization, classified as Cl. Increasing amounts of injected noggin mRNA lead to a
complete loss of the ventral tail fin, accompanied by a winding up of the
body axis which is restricted to the tip of the tail in cases of mild
dorsalization (C2), but affects the entire tail in cases of a dorsalization
of intermediate strength (C3), and both tail and trunk in cases of strongest dorsalization (C4-C5). From C3 dorsalization upwards, a loss of the
blood islands, the ventral-most mesodermal derivate, is observed. The
winding up of the body axis is a consequence of a lateral expansion of
the somites which occurs only in the posterior-most in cases of mild
dorsalization, but progressively affects more anterior somites in more
strongly dorsalized embryos. In cases of strongest dorsalization (C5),
even the anterior-most somites have spread into ventral-most regions
and are fused on the ventral side of the embryo, which leads to a rupture
of the yolk sac upon somite constriction around the 15-somite stage.
A similar continuous series of phenotypes of increasing strengths is
observed upon the injection of increasing amounts of Bmp mRNAs. As
in the case of Bmp inhibition, the most sensitive structure responding to
lowest amounts of injected bmp mRNA is the ventral tail fin, which
displays a partial or complete duplication (VI). Increasing amounts of
bmp mRNA lead to a progressive enlargement of the blood islands and
the tail, and a progressive reduction in the size of head and eyes,
resulting from a progressively impaired neural induction (V2,V3). Most
strongly ventralized embryos are characterized by a complete absence
M. Hild et al.
6.4 Morphology of Dorsalized and Ventralized Zebrafish
Mutants
The phenotypes of zebrafish mutants, with specific defects in dorsal or
ventral specification during early dorsoventral pattern formation, could
already be predicted from the effects caused by the overexpression or
specific inhibition of the ventralizing agents Bmp2, Bmp4, or Bmp7 in
genetically wild-type embryos. Overexpression of Bmps was achieved
by microinjecting different amounts of synthetic bmp mRNAs into
zebrafish embryos of the I-cell stage, inhibition of Bmps by microinjection of mRNAs encoding one of their aforementioned inhibitors, Noggin, Chordin, or Follistatin (Bauer et al. 1998; Kishimoto et al. 1997). At
36 h postfertilization, embryos injected with the lowest amount of noggin mRNA display a partial loss of the ventral tail fin, the ventral-most
ectodermal derivative, defining the mildest degree of dorsalization, classified as Cl. Increasing amounts of injected noggin mRNA lead to a
complete loss of the ventral tail fin, accompanied by a winding up of the
body axis which is restricted to the tip of the tail in cases of mild
dorsalization (C2), but affects the entire tail in cases of a dorsalization
of intermediate strength (C3), and both tail and trunk in cases of strongest dorsalization (C4-C5). From C3 dorsalization upwards, a loss of the
blood islands, the ventral-most mesodermal derivate, is observed. The
winding up of the body axis is a consequence of a lateral expansion of
the somites which occurs only in the posterior-most in cases of mild
dorsalization, but progressively affects more anterior somites in more
strongly dorsalized embryos. In cases of strongest dorsalization (C5),
even the anterior-most somites have spread into ventral-most regions
and are fused on the ventral side of the embryo, which leads to a rupture
of the yolk sac upon somite constriction around the 15-somite stage.
A similar continuous series of phenotypes of increasing strengths is
observed upon the injection of increasing amounts of Bmp mRNAs. As
in the case of Bmp inhibition, the most sensitive structure responding to
lowest amounts of injected bmp mRNA is the ventral tail fin, which
displays a partial or complete duplication (VI). Increasing amounts of
bmp mRNA lead to a progressive enlargement of the blood islands and
the tail, and a progressive reduction in the size of head and eyes,
resulting from a progressively impaired neural induction (V2,V3). Most
strongly ventralized embryos are characterized by a complete absence