Micromere signaling to adjacent cells involves primarily:
Primary inductive molecular signal from micromeres to overlying Veg2 cells is highly conserved Delta-Notch juxtacrine pathway operating through direct cell contact. Micromeres express high levels of Delta ligand after Pmar1/HesC double-negative gate derepression liberates Delta expression. Adjacent Veg2 cells express Notch receptor; interaction triggers proteolytic cleavage releasing Notch intracellular domain, activating Suppressor of Hairless target genes including Gcm that specifies pigment and other secondary mesenchyme fates. Blocking Delta morpholino or inhibiting γ-secretase eliminates secondary mesenchyme without affecting primary mesenchyme specification. FGF, BMP and Hedgehog operate later for skeletal patterning but initial micromere induction depends on Delta-Notch signaling.
Ref: Sherwood & McClay, Development 1999; Oliveri et al., 2008 GRN - Delta-Notch requirement for secondary mesenchyme.