The Questions of Developmental Biology

If one conceptually opens a Xenopus blastula at the vegetal pole and stretches the opening
into a marginal ring, the resulting fate map closely resembles that of the zebrafish embryo at the
stage when half of the yolk has been covered by the blastoderm (see Figure 1.9; Langeland and
Kimmel 1997).
Axis Formation in Fish Embryos
Dorsal-ventral axis formation: the embryonic shield
The embryonic shield is critical
in establishing the dorsal-ventral axis in
fishes. It can convert lateral and ventral
mesoderm (blood and connective tissue precursors) into dorsal
mesoderm (notochord and somites), and it can cause the ectoderm to
become neural rather than epidermal. This was shown by
transplantation experiments in which the embryonic shield of one
early-gastrula embryo was transplanted to the ventral side of another
(Figure 11.5; Oppenheimer 1936; Koshida et al. 1998). Two axes
formed, sharing a common yolk cell. Although the prechordal plate
and the notochord were derived from the donor embryonic shield, the
other organs of the secondary axis came from host tissues that would
have formed ventral structures. The new axis has been induced by the
donor cells. In the embryo that had had its embryonic shield removed,
no dorsal structures formed, and the embryo lacked a nervous system.
These experiments are similar to those performed on amphibian
gastrulae by Spemann and Mangold (1924; see Chapter 10), and they
demonstrate that the embryonic shield is the homologue of the dorsal blastopore lip, the
amphibian organizer.
Like the amphibian dorsal blastopore lip, the embryonic shield forms the prechordal plate
and the notochord of the developing embryo. The precursors of these two regions are responsible
for inducing the ectoderm to become neural ectoderm. Moreover, the presumptive notochord and
prechordal plate appear to do this in a manner very much like that of their homologous structures
in amphibians.* In both fishes and amphibians, BMP proteins made in the ventral and lateral
regions of the embryo would normally cause the ectoderm to become epidermis. The notochord
of both fishes and amphibians secretes factors that block this induction and thereby allow the
ectoderm to become neural. In fishes, the BMP that ventralizes the embryo is BMP2B. The
protein secreted by the chordamesoderm that binds with and inactivates BMP2B is a chordin-like
paracrine factor called Chordino (Figure 11.6B; Kishimoto et al. 1997; Schulte-Merker et al. 1997).