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Fig. 7. Steps in stem zone specification and maintenance. At HH4, an overlap
between pan-neural (Sox2) and early mesodermal gene (bra)
expression is observed in epiblast cells close to the anterior primitive
streak. Underlying paraxial mesoderm begins to express Raldh2, while
Fgf8 expression is restricted to the primitive streak. From HH7, stem
zone-specific gene expression (Sax1) commences in epiblast cells
close to the primitive streak. Sax1 onset requires signals from the
paraxial mesoderm and an intact FGF pathway, which works at least in part by
inhibiting the retinoid pathway (see text). At this time, Raldh2 is
rostrally restricted by FGF8 in newly emerged paraxial mesoderm
(Diez del Corral et al.,
2003). Retinoic acid can also attenuate Fgf8 levels
(Diez del Corral et al., 2003)
and so a fine balance between FGF and retinoid signalling is required for
formation of the stem zone. Sax1 induction can take place without
prior creation of an ambivalent mesodermal/neural cell state. At HH10,
Sax1 and Fgf8 expression are also present in the transition
zone and Sax1 is maintained by FGF/MAPK signalling. There is a
persisting overlap of neural and mesodermal genes in the stem zone, which may
be the location of bipotent stem cells.
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