The Questions of Developmental Biology

Eventually, like a wave leaving shells upon a beach, its expression recedes caudally,
while the most anterior expression region remains. The caudalmost region of this anterior
expression band correlates with the posterior terminus of the next somite to be formed. This
process can be seen in Figure 14.4.
Thus, the expression pattern of the hairy gene correlates with the positioning of the place
where a somite will separate from the unsegmented mesoderm. Labeling of cells with diI shows
that this wave of hairy expression is not caused by the migration of particular cells that express
the hairy gene. Rather, the wavelike pattern of hairy expression is an autonomous property of the
presomitic mesoderm. Even if the presomitic mesoderm is separated from the caudalmost
mesoderm, the ectoderm, neural tube, notochord, Hensen's node, and lateral plate regions, the
dynamic expression of the hairy gene remains (Palmeirim et al. 1997; Jouve et al. 2000).
Separation
The hairy gene encodes a transcription factor
(indeed, one that is also used for forming the
segmental units of Drosophila see Chapter 9), but it
is not known what its targets are. One set of possible
targets (directly or indirectly) are the genes for ephrin
and its receptor. The Eph receptor proteins and their
ephrin ligands were discussed in Chapter 13 as being
able to cause cell-cell repulsion between the posterior
somite and migrating neural crest cells. Ephrin and
Eph also may be critical for separating the somites.
In the zebrafish, the boundary between the most
recently separated somite and the presomitic
mesoderm forms between ephrinB2 in the posterior of
the somite and EphA4 in the most anterior portion of
the presomitic mesoderm (Figure 14.5A; Durbin et al.
1998). As somites form, this pattern of gene
expression is reiterated caudally. Interfering with this
signaling (by injecting embryos with RNA encoding
dominant negative Ephs) leads to abnormal somite
boundary formation.