Surprisingly, the earliest steps of animal development are among the most variable, even within a phylum. A frog, a chicken, and a mammal, for example, even though they develop in similar ways later, make eggs that differ radically in size and structure, and they begin their development with different sequences of cell divisions and cell specializations. Gastrulation occurs in all animal embryos, but the details of its timing, of the associated pattern of cell movements, and of the shape and size of the embryo as gastrulation proceeds are highly variable. Likewise, there is great variation in the time and manner in which the primary axes of the body become marked out. However, this polarization of the embryo usually becomes discernible very early, before gastrulation begins: it is the first step of spatial patterning.
Three axes generally have to be established. The animal-vegetal (A-V) axis, in most species, defines which parts are to become internal (through the movements of gastrulation) and which are to remain external. (The bizarre name dates from a century ago and has nothing to do with vegetables.) The anteroposterior (A-P) axis specifies the locations of future head and tail. The dorsoventral (D-V) axis specifies the future back and belly.
Figure 21–14 The frog egg and its asymmetries. (a) Side view of a Xenopus egg photographed just before fertilization.
(B) the asymmetric distribution of molecules inside the egg, and how this changes following fertilization so as to define a dorsoventral as well as an animal-vegetal asymmetry. Fertilization, through a reorganization of the microtubule cytoskeleton, triggers a rotation of the egg cortex (a layer a few μm deep) through about 30° relative to the core of the egg; the direction of rotation determined by the site of sperm entry. Some components are carried still further to the future dorsal side by active transport along microtubules. the resulting dorsal concentration of Wnt11 mrNa leads to dorsal production of the Wnt11 signal protein and defines the dorsoventral polarity of the future embryo. Vegetally localized Vegt defines the vegetal source of signals that will induce endoderm and mesoderm. (a, courtesy of tony Mills.)
At one extreme, the egg is spherically symmetrical, and the axes only become defined during embryogenesis. The mouse comes close to being an example, with little obvious sign of polarity in the egg. Correspondingly, the blastomeres produced by the first few cell divisions seem to be all alike and are remarkably adaptable. If the early mouse embryo is split in two, a pair of identical twins can be produced—two complete, normal individuals from a single cell. Similarly, if one of the cells in a two-cell mouse embryo is destroyed by pricking it with a needle and the resulting “half-embryo” is placed in the uterus of a foster mother to develop, in many cases a perfectly normal mouse will emerge.
At the opposite extreme, the structure of the egg defines the future axes of the body. This is the case for most species, including insects such as Drosophila, as we shall see shortly. Many other organisms lie between the two extremes. The egg of the frog Xenopus, for example, has a clearly defined A-V axis even before fertilization: the nucleus near the top defines the animal pole, while the mass of yolk (the embryo’s food supply, destined to be incorporated in the gut) toward the bottom defines the vegetal pole. Several types of mRNA molecules are already localized in the vegetal cytoplasm of the egg, where they produce their protein products. After fertilization, these mRNAs and proteins act in and on the cells in the lower and middle part of the embryo, giving the cells there specialized characters, both by direct effects and by stimulating the production of secreted signal proteins. For example, mRNA encoding the transcription regulator VegT is deposited at the vegetal pole during oogenesis. After fertilization, this mRNA is translated, and the resulting VegT protein activates a set of genes that code for signal proteins that induce mesoderm and endoderm, as discussed later.