Development of Important Model Species II: Vertebrates
Amphibians represent the archetype of vertebrate development. Figure 5.1 quotes the phylogenetic positions of vertebrates that are often investigated in the laboratory. From the Ancient World (Aristotle, Box1.1) up to the eighteenth century incubated chick eggs were the gold standard for animal development; since 1900 amphibians took over this position. The modified development of reptiles, birds and mammals can be deduced readily from the amphibian model exemplified by frogs and newts. The amphibian egg is rich in yolk and large (often 1–2 mm in diameter), yet the egg is able to undergo holoblastic cleavage, and cleavage converts a typical blastula to a gastrula displaying textbook features. In addition, amphibian embryos develop outside the mother and are therefore accessible to experimentation at all stages. The transparent jelly coat can easily be removed, surgically or chemically (after fertilization the envelope can be removed with 2.5 % cysteine-HCl, pH7.4). Pieces cut out from an embryo are able to continue development in sterile salt solutions without added nutrients, thanks to a supply of yolk in each cell. The eggs are suitable for microsurgical operations, which can be performed by hand with glass needles prepared in the laboratory; no complicated and expensive apparatus is needed.