The Questions of Developmental Biology

The formation of germ layers
After the blastoderm cells have covered about half the zebrafish yolk cell (and earlier in
fish eggs with larger yolks), a thickening occurs throughout the margin of the epibolizing
blastoderm. This thickening is called the germ ring, and it is composed of a superficial layer, the
epiblast, and an inner layer, the hypoblast. We do not understand how the hypoblast is made.
Some research groups claim that the hypoblast is formed by the involution of superficial cells
under the margin followed by their migration toward the animal pole (see Figure 11.3C). The
involution begins at the future dorsal portion of the embryo, but occurs all around the margin.
Other laboratories claim that these superficial cells ingress to form the hypoblast (see Trinkaus
1996). It is possible that both mechanisms are at work, with different modes of hypoblast
formation predominating in different species. Once formed, however, the cells of both the
epiblast and hypoblast intercalate on the future dorsal side of the embryo to form a localized
thickening, the embryonic shield (Figure 11.4).
As we will see, this shield is functionally equivalent to the dorsal blastopore lip of
amphibians, since it can organize a secondary embryonic axis when transplanted to a host embryo
(Oppenheimer 1936; Ho 1992). Thus, as the cells undergo epiboly around the yolk, they are also
involuting at the margins and converging anteriorly and dorsally toward the embryonic shield
(Trinkaus 1992). The hypoblast cells of the embryonic shield converge
and extend anteriorly, eventually narrowing along the dorsal midline
of the hypoblast. This movement forms the chordamesoderm, the
precursor of the notochord (Figure 11.4B,C).
The cells adjacent to the
chordamesoderm, the paraxial
mesoderm cells, are the precursors of
the mesodermal somites (Figure
11.4D,E). The concomitant convergence
and extension in the epiblast brings the
presumptive neural cells from all over
the epiblast into the dorsal midline,
where they form the neural keel. The
rest of the epiblast becomes the skin of
the fish.
The zebrafish fate map, then, is
not much different from that of the frog
or other vertebrates (as we will soon
see).