The Questions of Developmental Biology

10. Early development and axis formation in amphibians
Amphibian embryos were the organisms of choice for experimental embryology. With
their large cells and their rapid development, salamander and frog embryos were excellently
suited for transplantation experiments. However, amphibian embryos fell out of favor during the
early days of developmental genetics, since frogs and salamanders undergo a long period of
growth before they become fertile, and their chromosomes are often found in several copies,
precluding easy mutagenesis.* However, new molecular techniques such as in situ hybridization,
antisense oligonucleotides, and dominant negative proteins have allowed researchers to return to
studying amphibian embryos and to integrate molecular analyses of development with earlier
experimental findings. The results have been spectacular, and we are enjoying our first
appreciation of how vertebrate bodies are patterned and structured.
*In the 1960s, Xenopus laevis replaced the Rana frogs and the salamanders because it could be induced to mate
throughout the year. Unfortunately, Xenopus laevis has four copies of each chromosome rather than the more usual
two, and takes 1 2 years to reach sexual maturity. These attributes make genetic studies difficult. Recently another
Xenopus species, X. tropicalis, has begun to be used in the laboratory. It has all the advantages of X. laevis, plus it is
diploid and reaches sexual maturity in a mere 6 months.
Early Amphibian Development
Cleavage in Amphibians
Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic,
just like echinoderm cleavage. The amphibian egg, however, contains much more yolk. This yolk,
which is concentrated in the vegetal hemisphere, is an impediment to cleavage. Thus, the first
division begins at the animal pole and slowly extends down into the vegetal region (Figure 10.1;
see also Figures 2.2D and 8.4). In the axolotl salamander, the cleavage furrow extends through
the animal hemisphere at a rate close to 1 mm per minute. The cleavage furrow bisects the gray
crescent and then slows down to a mere 0.02 0.03 mm per minute as it approaches the vegetal
pole (Hara 1977).