Fig. 7. Possible models of events leading to the somite border formation and the establishment of RC polarity. (A) Mesp2 or Mespb might regulate these two events using different genetic pathways. Mesp2 is known to suppress Dll1 via the Notch signaling pathway. (B) However, the pathway might be important for the normal expression of EphA4, Lfng and PAPC, which is required for the border formation. (C) Finally the suppression of Dll1 required for the establishment of RC polarity might play a role in the border formation. In all cases, Notch signaling is required for the autoregulation of Mesp2, and RC polarity is required for the maintenance of the segmental border. Only the anterior most bands of Lfng and PAPC are Mesp-dependent.