The Questions of Developmental Biology

11. The early development of vertebrates: Fish, birds, and mammals
This final chapter on the processes of early development will extend our survey of
vertebrate development to include fish, birds, and mammals. The amphibian embryos described
in the previous chapter divide by means of radial holoblastic cleavage. Cleavage in bird, reptile,
and fish eggs is meroblastic, with only a small portion of the cytoplasm being used to make cells.
Mammals modify their holoblastic cleavage to make a placenta, which enables the embryo to
develop inside another organism. Although methods of gastrulation also differ among the
vertebrate classes, there are some underlying principles in common throughout the vertebrates.
Early Development in Fish
In recent years, the teleost fish Danio rerio, known as the zebrafish, has become a
favorite organism of those who wish to study vertebrate development. Zebrafish have large
broods, breed all year, are easily maintained, have transparent embryos that develop outside the
mother (an important feature for microscopy), and can be raised so that mutants can be readily
screened and propagated. In addition, they develop rapidly, so that at 24 hours after fertilization,
the embryo has formed most of its tissues and organ primordia and displays the characteristic
tadpole-like form (see Granato and Nüsslein-Volhard 1996; Langeland and Kimmel 1997).
Therefore, much of the description of fish development below is based on studies of this species
Cleavage in Fish Eggs
In fish eggs, cleavage occurs only in the blastodisc, a thin region of yolk-free cytoplasm
at the animal cap of the egg. Most of the egg cell is full of yolk. The cell divisions do not
completely divide the egg, so this type of cleavage is called meroblastic (Greek, meros, "part").
Since only the cytoplasm of the blastodisc becomes the embryo, this type of meroblastic cleavage
is called discoidal. Scanning electron micrographs show beautifully the incomplete nature of
discoidal meroblastic cleavage in fish eggs (Figure 11.1). The calcium waves initiated at
fertilization stimulate the contraction of the actin cytoskeleton to squeeze non-yolky cytoplasm
into the animal pole of the egg. This converts the spherical egg into a more pear-shaped structure,
with an apical blastodisc (Leung et al. 1998). Early cleavage divisions follow a highly
reproducible pattern of meridional and equatorial cleavages.