Cellular and cis-regulation of En-2 expression in the mandibular arch.

Investigations into early muscle development have focused primarily on somite derived cells. Cranial mesoderm does not undergo somitogenesis, and muscle formation in this region is less well understood. In the present study, we have focused upon the expression of engrailed in mandibular arch myoblasts. We demonstrate that En-2 is expressed in mandibular arch myoblasts of the mouse. The activity of the En-2 enhancer is maintained in several functionally related muscles that arise from the first arch. Through the use of reporter transgenics, we demonstrate that local cell-cell interactions are important in maintaining En-2 expression in the mandibular arch cells. En-2 enhancer activity in the first arch requires a combination of cis-acting sequences that includes a motif which is identical to one found in the Otx2 enhancer and which is sufficient to direct expression in the first arch. These data support the notion that cranial muscle development is regulated by local cell-cell interactions which distinguish distinct anatomical and functional muscle groups.