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).