The Questions of Developmental Biology

While these cells are dividing, numerous cell adhesion molecules keep the blastomeres
together. One of the most important of these molecules is EP-cadherin. The mRNA for this
protein is supplied in the oocyte cytoplasm. If this message is destroyed (by injecting antisense
oligonucleotides complementary to this mRNA into the oocyte), the EP-cadherin is not made, and
the adhesion between the blastomeres is
dramatically reduced (Heasman et al. 1994a,b),
resulting in the obliteration of the blastocoel
(Figure 10.4).
Amphibian Gastrulation
The study of amphibian gastrulation is
both one of the oldest and one of the newest areas of experimental embryology.
Even though amphibian gastrulation has been extensively studied for the past century, most of
our theories concerning the mechanisms of these developmental movements have been revised
over the past decade. The study of amphibian gastrulation has been complicated by the fact that
there is no single way amphibians gastrulate. Different species employ different means toward the
same goal (Smith and Malacinski 1983; Minsuk and Keller 1996). In recent years, the most
intensive investigations have focused on the frog Xenopus laevis, so we will concentrate on its
mode of gastrulation.
The fate map of Xenopus
Amphibian blastulae are faced with the same tasks as the invertebrate blastulae we
followed in Chapters 8 and 9 namely, to bring inside the embryo those areas destined to form
the endodermal organs, to surround the embryo with cells capable of forming the ectoderm, and
to place the mesodermal cells in the proper positions between them. The movements whereby this
is accomplished can be visualized by the technique of vital dye staining (see Chapter 1).
Fate mapping by Løvtrup (1975; Landstrom and Løvtrup 1979) and by Keller (1975,1976) has
shown that cells of the Xenopus blastula have different fates depending on whether they are
located in the deep or the superficial layers of the embryo (Figure 10.5).
In Xenopus, the mesodermal precursors exist mostly in the deep layer of cells, while the
ectoderm and endoderm arise from the superficial layer on the surface of the embryo. Most of the
precursors for the notochord and other mesodermal tissues are located beneath the surface in the
equatorial (marginal) region of the embryo. In urodeles (salamanders such as Triturus and
Ambystoma) and in some frogs other than Xenopus, many more of the notochord and mesoderm
precursors are among the surface cells (Purcell and Keller 1993).