The Questions of Developmental Biology

If the chordino gene is mutated, the neural tube fails to form. It is hypothesized (Nguyen
et al. 1998) that different concentrations of BMP2B pattern the ventral and lateral regions of the
zebrafish ectoderm and mesoderm, and that the ratio between Chordino and BMP2B may specify
the position along the dorsal-ventral axis. In fishes, however, the notochord may not be the only
structure capable of producing the proteins that block BMP2B. If the notochord fails to form (as
in the floating head or no tail mutations), the neural tube will still be produced. It is possible that
the notochordal precursor cells (which are produced in these mutations) are still able to induce the
neural tube, or that the dorsal portion of the somite precursors can compensate for the lack of a
notochord (Halpern et al. 1993; 1995; Hammerschmidt et al. 1996).
The embryonic shield appears to acquire its organizing ability in much the same way as
its amphibian counterparts. In amphibians, the endoderm cells beneath the dorsal blastopore lip
(i.e., the Nieuwkoop center) accumulate β-catenin. This protein is critical in amphibians for the
ability of the endoderm to induce the cells above them to become the dorsal lip (organizer) cells.
In zebrafish, the nuclei in that part of the yolk syncytial layer that lies beneath the cells that will
become the embryonic shield similarly accumulate β-catenin. This protein distinguishes the
dorsal YSL from the lateral and ventral YSL regions (Figure 11.7; Schneider et al. 1996).
Inducing β-catenin accumulation on the ventral side of the egg
causes dorsalization and a second embryonic axis (Kelly et al.
1995). In addition, just prior to gastrulation, the cells of the
dorsal blastopore margin synthesize and secrete Nodal-related
proteins. These induce the precursors of the notochord and
prechordal plate to activate goosecoid and other genes (Sampath
et al. 1998; Gritsman et al. 2000.) Thus, the embryonic shield is
considered equivalent to the amphibian organizer, and the dorsal
part of the yolk cell can be thought of as the Nieuwkoop center
of the fish embryo.
Anterior-posterior axis formation: two signaling centers
As is evident from Figure 11.5, when a second dorsal-ventral axis is experimentally
induced in zebrafish eggs, both the regular and the induced axes have the same anterior-posterior
polarity. Both heads are at the former animal cap, and both tails are located vegetally. Indeed, the
anterior-posterior axis is specified during oogenesis, and the animal cap marks the anterior of the
embryo. This axis becomes stabilized during gastrulation through two distinct signaling centers.
First, a small group of anterior neural cells at the border between the neural and surface ectoderm
(a region that become the pituitary gland, nasal placode, and anterior forebrain) secrete
compounds that cause anterior development. If these anterior neural cells are experimentally
placed more posteriorly in the embryo, they will cause the neural cells near them to assume the
characteristics of forebrain neurons. The second signaling center, in the posterior of the embryo,
consists of lateral mesendoderm precursors at the margin of the gastrulating blastoderm. These
cells produce caudalizing compounds , most likely Nodal-related proteins and activin (Figure
11.6C; Woo and Fraser 1997; Houart et al. 1998; Thisse et al. 2000). If transplanted adjacent to
anterior neural ectoderm, this tissue will transform the presumptive forebrain tissue into
hindbrain-like structures.