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Fig. 8. Models for IZ formation based on dose dependence for Him + Her5 inhibitory
activity. In each panel, a half IZ is represented and color-coded (red: MIZ or
LIZ, light grey: intermediate field, dark grey: ngn1-positive areas),
and the level of Him + Her5 inhibitory activity is indicated by black
horizontal bars. The threshold required for ngn1 inhibition within
the LIZ is lower than the MIZ threshold (white bars). Thus, if Him + Her5
activity is below the LIZ threshold level, ectopic neurogenesis will occur
both laterally and medially, and the MIZ and LIZ are lost (i). This situation
corresponds to the absence of activity of both Her5 and Him. When Him + Her5
activity reaches the LIZ threshold level (one dose of Her5 or Him is
sufficient), ngn1 expression is prevented laterally but not medially,
and the LIZ forms but not the MIZ (ii). Finally, if Him + Her5 activity is
above the MIZ threshold (
1 dose of Him and 1 dose of Her5 or three
doses of Him), the whole IZ is formed properly (normal development of
wild-type embryos, (iii). The persisting absence of ngn1 expression
within the intermediate field located between the MIZ and LIZ (light grey
triangle) could result from additional inhibitor(s) expressed along the AP
axis, in register with longitudinal non-differentiation stripes of the
rhombomere and spinal cord. Flat-mounted embryos at three-somite stage, with
ngn1 expression in dark and pax 2.1 in red, corresponding to
each situation are shown adjacent to the scheme. The combinations of
inhibitory factors producing the appropriate level of inhibitory activity for
each situation are listed (green letters), as well as the genetic ways to
obtain this particular inhibitory activity (black letters for genotype and
orange for interfering agents).
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