The Questions of Developmental Biology

Sonic hedgehog is the most widely used of the three vertebrate homologues. Made by
the notochord, it is processed so that only the amino-terminal two-thirds of the molecule is
secreted. This peptide is responsible for patterning the neural tube such that motor neurons are
formed from the ventral neurons and sensory neurons are formed from the dorsal neurons (see
Chapter 12; Yamada et al. 1993). Sonic hedgehog is also responsible for patterning the somites so
that the portion of the somite closest to the notochord becomes the cartilage of the spine (Fan and
Tessier-Lavigne 1994; Johnson et al. 1994). As we will see in later chapters, Sonic hedgehog has
been shown to mediate the formation of the left-right axis in chicks, to initiate the anteriorposterior
axis in limbs, to induce the regionally specific differentiation of the digestive tube, and
to induce feather formation (see Figures 6.11
and 6.6). Sonic hedgehog often works with
other paracrine factors, such as Wnt and FGF
proteins. In the developing tooth, Sonic
hedgehog, FGF4, and other paracrine factors
are concentrated in the region where cell
interactions are creating the cusps of the teeth
(see Figure 13.9; Vaahtokari et al. 1996a).
The Wnt family
The Wnts constitute a family of cysteinerich
glycoproteins. There are at least 15 members
of this family in vertebrates. Their name comes
from fusing the name of the Drosophila segment
polarity gene wingless with the name of one of its
vertebrate homologues, integrated.
While Sonic hedgehog is important in patterning
the ventral portion of the somites (causing the
cells to become cartilage), Wnt1 appears to be
active in inducing the dorsal cells of the somites to
become muscle (McMahon and Bradley 1990;
Stern et al. 1995). Wnt proteins also are critical in
establishing the polarity of insect and vertebrate
limbs, and they are used in several steps of
urogenital system development (Figure 6.12).
The TGF-β superfamily
There are over 30 structurally related members of the TGF-b superfamily, and they
regulate some of the most important interactions in development (Figure 6.13). The proteins
encoded by TGF-β superfamily genes are processed such that the carboxy-terminal region
contains the mature peptide. These peptides are dimerized into homodimers (with themselves) or
heterodimers (with other TGF-β peptides) and are secreted from the cell. The TGF-β superfamily