The objectives of my research are to characterize the movements of all craniofacial mesenchymal cells in the avian embryo, from their origins through the establishment of fully differentiated tissues, and to analyze the interactions that regulate patterned assembly of mesenchymal populations into craniofacial tissues and organs. Data generated during the current funding period revealed the following heretofore unreported features of avian craniofacial development: (1) a series of deep, segmental pathways by which neural crest cells move into paraxial mesoderm, (2) multiple origins and invasive movements of angioblasts, and their dependency upon cues in head mesenchyme for assembly into blood vessels, (3) a mesenchymal population that develops at the endoderm:mesoderm interface shortly before the onset of blood vessel formation but is only visible in SEM preparations, (4) patterns of myotube alignment that appear early in muscle development, are unique in each branchial arch muscle, and presage the subsequent patterns of segregation of individual muscles, and (5) precise details of cell movements and patterns of nerve fiber growth during development of the trigeminal sensory-motor complex. The proposed new experiments will extend analyses of each of these tissues, and focus greater attention on defining the tissue interactions necessary for their coordinated formation. Specific proposed experiments will examine the relative contributions of different angioblast precursors in development of craniofacial blood vessels, particularly those in the hindbrain, face, and heart wall. These will be determined by studying the formation of transplanted quail endothelial precursors, which can be recognized immunocytochemically, developing in chick host embryos. To define myogenic interactions, TEM and fiber type analyses will be performed on muscles developing from transplanted myogenic precursors that normally would express different features. Finally, interactions underlying specification of trigeminal sensory nerves, as evidence by their ability to form specific central and peripheral projections, will be assessed using several fiber-tracing methods on embryos in which the ganglionic precursors (neural crest or placodal tissues) have been transplanted to different locations or at different times.