Dorsoventral Patterning of the Zebrafish Embryo
83
the early zebrafish embryos are established and interpreted by the respective cells, which genes are involved in these processes, and how the
various gene products interact.
6.2 What We Know from Studies in the Frog
Early dorsoventral patterning processes have been intensively studied in
amphibian embryos (Fig. 1). Under maternal control, which means governed by factors that have been generated by the mother and deposited
in the egg, a coarse initial dorsoventral pattern is set up in the developing mesoderm. The mesoderm is induced in the equatorial/marginal
region of the amphibian blastula by signals emanating from vegetal
blastomeres which themselves will form endoderm. Ventral mesoderm,
which gives rise to blood, is induced in a broad equatorial region
spanning most of the dorsoventral axis, while dorsal mesoderm, which
gives rise to the notochord, is induced in a rather small dorsal equatorial
domain. This initial dorsoventral pattern is subsequently refined under
the control of zygotic signals generated by the embryo itself. Dorsalizing signals from the dorsal mesoderm, also called the Spemann organizer, have two functions: firstly, they convert mesoderm in the dorsolateral equatorial region, initially specified as ventral, into a whole
spectrum of intermediate fates, so that instead of blood, somitic muscle
or pronephros is formed; secondly, they induce neural specification in
dorsal animal cells which would otherwise give rise to epidermal fates.
These dorsalizing signals are counteracted by zygotic ventralizing signals generated in the rest of the embryo. Here, we will deal with the
second phase of dorsoventral patterning, the zygotic refinement of the
initial, coarse dorsoventral pattern.
Several candidates for the dorsalizing and ventralizing signals have
been identified by different means: the bone morphogenetic proteins
Bmp2, Bmp4, and/or Bmp7 - members of the TGF~ superfamily of
growth factors, which have ventralizing activities and, when applied to
amphibian or fish embryos, lead to an expansion of ventral fates at the
expense of dorsal fates - and their antagonists Chordin, Noggin, and
Follistatin (for reviews see Harland and Gerhart 1997; Heasman 1997;
Thomsen 1997) - which have dorsalizing activities and, when applied
to embryos, lead to an expansion of dorsal fates at the expense of ventral
83
the early zebrafish embryos are established and interpreted by the respective cells, which genes are involved in these processes, and how the
various gene products interact.
6.2 What We Know from Studies in the Frog
Early dorsoventral patterning processes have been intensively studied in
amphibian embryos (Fig. 1). Under maternal control, which means governed by factors that have been generated by the mother and deposited
in the egg, a coarse initial dorsoventral pattern is set up in the developing mesoderm. The mesoderm is induced in the equatorial/marginal
region of the amphibian blastula by signals emanating from vegetal
blastomeres which themselves will form endoderm. Ventral mesoderm,
which gives rise to blood, is induced in a broad equatorial region
spanning most of the dorsoventral axis, while dorsal mesoderm, which
gives rise to the notochord, is induced in a rather small dorsal equatorial
domain. This initial dorsoventral pattern is subsequently refined under
the control of zygotic signals generated by the embryo itself. Dorsalizing signals from the dorsal mesoderm, also called the Spemann organizer, have two functions: firstly, they convert mesoderm in the dorsolateral equatorial region, initially specified as ventral, into a whole
spectrum of intermediate fates, so that instead of blood, somitic muscle
or pronephros is formed; secondly, they induce neural specification in
dorsal animal cells which would otherwise give rise to epidermal fates.
These dorsalizing signals are counteracted by zygotic ventralizing signals generated in the rest of the embryo. Here, we will deal with the
second phase of dorsoventral patterning, the zygotic refinement of the
initial, coarse dorsoventral pattern.
Several candidates for the dorsalizing and ventralizing signals have
been identified by different means: the bone morphogenetic proteins
Bmp2, Bmp4, and/or Bmp7 - members of the TGF~ superfamily of
growth factors, which have ventralizing activities and, when applied to
amphibian or fish embryos, lead to an expansion of ventral fates at the
expense of dorsal fates - and their antagonists Chordin, Noggin, and
Follistatin (for reviews see Harland and Gerhart 1997; Heasman 1997;
Thomsen 1997) - which have dorsalizing activities and, when applied
to embryos, lead to an expansion of dorsal fates at the expense of ventral