The Questions of Developmental Biology

Fig. 22.15
Fig. 22.16
The formation of the proximal-distal axis also proceeds similarly in vertebrates and
invertebrates. In insects, the wing margin forms at the border between the dorsal cells and the
ventral cells. The dorsal cells express Apterous protein, and this activates the expression of
Fringe. The interaction of the Fringe protein with the ventral cells leads to the growth of the wing
blade outward from the body wall. Unexpectedly, a very similar cascade induces the outgrowth of
the vertebrate limb. The apical ectodermal ridge (AER) forms at the junction of the dorsal cells
with the ventral cells. The dorsal cells express Radical fringe, a vertebrate homologue of the
Fringe protein. This protein is critical in forming the AER. It is interesting that the ways in which
these proteins become localized differ enormously between these phyla. Radical fringe, for
instance, is induced by fibroblast growth factors, and it induces the AER to produce more
fibroblast growth factors for the outgrowth of the limb. Fibroblast growth factors play no role in
insect limb development. Yet, the instructions for limb outgrowth and polarity appear to be
essentially the same in insects and vertebrates. Nature may have evolved the mechanism to form a
limb only once, in the PDA, and both arthropods and vertebrates use modifications that process to
this day.