The Questions of Developmental Biology

As we have seen, the unfertilized egg has a polarity along the animal-vegetal axis. Thus,
the germ layers can be mapped onto the oocyte even before fertilization. The surface of the
animal hemisphere will become the cells of the ectoderm (skin and nerves), the vegetal
hemisphere surface will form the cells of the gut and associated organs (endoderm), and the
mesodermal cells will form from the internal cytoplasm around the equator. This general fate map
is thought to be imposed upon the egg by the transcription factor VegT and the paracrine factor
Vg1. The mRNAs for these proteins are located in the cortex of the vegetal hemisphere of
Xenopus oocytes, and they are apportioned to the vegetal cells during cleavage (see Figure 5.33).
By using antisense oligonucleotides, Zhang and colleagues (1998) were able to deplete maternal
VegT protein in early embryos. The resulting embryos lacked the normal fate map. The animal
third of the embryo produced only ventral epidermis, while the marginal cells (which normally
produced mesoderm) generated epidermal and neural tissue. The vegetal third (which usually
produces endoderm) produced a mixture of ectoderm and mesoderm (Figure 10.6). Joseph and
Melton (1998) demonstrated that embryos that lacked functional Vg1 lacked endoderm and dorsal
mesoderm.
These findings tell us nothing, however, about which part of the egg will form the belly
and which the back. The anterior-posterior, dorsal-ventral, and left-right axes are specified by the
events of fertilization and are realized during gastrulation.
Cell movements during amphibian gastrulation
Before we look at the process of gastrulation in detail, let us first trace the movement
patterns of the germ layers. Gastrulation in frog embryos is initiated on the future dorsal side of
the embryo, just below the equator in the region of the gray crescent (Figure 10.7). Here, the cells
invaginate to form a slitlike blastopore. These cells change their shape dramatically. The main
body of each cell is displaced toward the inside of the embryo while the cell maintains contact
with the outside surface by way of a slender neck (Figure 10.8). These bottle cells line the
archenteron as it forms. Thus, as in the gastrulating sea urchin, an invagination of cells initiates
archenteron formation. However, unlike gastrulation in sea urchins, gastrulation in the frog
begins not at the most vegetal region, but in the marginal zone: the zone surrounding the equator
of the blastula, where the animal and vegetal hemispheres meet. Here the endodermal cells are
not as large or as yolky as the most vegetal blastomeres.