The Questions of Developmental Biology

This remaining patch of endoderm is called the yolk plug; it, too, is eventually
internalized (Figure 10.9). At that point, all the endodermal precursors have been brought into the
interior of the embryo, the ectoderm has encircled the surface, and the mesoderm has been
brought between them.
The midblastula transition: preparing for gastrulation
Now that we have seen an overview of amphibian gastrulation, we can look more deeply
into its mechanisms. The first precondition for gastrulation is the activation of the genome. In
Xenopus, the nuclear genes are not transcribed until late in the twelfth cell cycle (Newport and
Kirschner 1982a,b). At that time, different genes begin to be transcribed in different cells, and the
blastomeres acquire the capacity to become motile. This dramatic change is called the midblastula
transition (see Chapters 8 and 9). It is thought that different transcription factors (such as the
VegT protein, mentioned above) become active in different cells at this time, giving the cells new
properties. For instance, the vegetal cells (probably under the direction of the maternal VegT
protein) become the endoderm and begin secreting the factors that cause the cells above them to
become the mesoderm (Wylie et al. 1996).
Positioning the blastopore
The vegetal cells are critical in determining the location of the blastopore, as is the point
of sperm entry. The microtubules of the sperm direct cytoplasmic movements that empower the
vegetal cells opposite the point of sperm entry to induce the blastopore in the mesoderm above
them. This region of cells opposite the point of sperm entry will form the blastopore and become
the dorsal portion of the body.
In Chapter 7, we saw that the internal cytoplasm of the fertilized egg remains oriented
with respect to gravity because of its dense yolk accumulation, while the cortical cytoplasm
actively rotates 30 degrees animally ("upward"), toward the point of sperm entry (see Figure
7.33). In this way, a new state of symmetry is acquired. Whereas the unfertilized egg was radially
symmetrical about the animal-vegetal axis, the fertilized egg now has a dorsal-ventral axis. It has
become bilaterally symmetrical (having right and left sides). The inner cytoplasm moves as well.
Fluorescence microscopy of early embryos has shown that the cytoplasm of the presumptive
dorsal cells differs from that of the presumptive ventral cells (see Figure 7.35; Danilchik and
Denegre 1991). These cytoplasmic movements activate the cytoplasm opposite the point of sperm
entry, enabling it to initiate gastrulation. The side where the sperm enters marks the future ventral
surface of the embryo; the opposite side, where gastrulation is initiated, marks the future dorsum
of the embryo (Gerhart et al. 1981, 1986; Vincent et al. 1986). If cortical rotation is blocked, there
is no dorsal development, and the embryo dies as a mass of ventral (primarily gut) cells (Vincent
and Gerhart 1987).
Although the sperm is not needed to induce these movements in the egg cytoplasm, it is
important in determining the direction of the rotation. If an egg is artificially activated, the
cortical rotation still takes place at the correct time. However, the direction of this movement is
unpredictable. The directional bias provided by the point of sperm entry can be overridden by
mechanically redirecting the spatial relationship between the cortical and internal cytoplasms.
When a Xenopus egg is turned 90 degrees, so that the point of sperm entry faces upward, the
cytoplasm rotates such that the embryo initiates gastrulation on the same side as sperm entry
(Gerhart et al. 1981; Cooke 1986). One can even produce two gastrulation initiation sites by
combining the natural sperm-oriented rotation with an artificially induced rotation of the egg.