Understanding craniofacial development requires that many embryonic processes be defined. Among these are the embryonic origins of all facial tissues, including muscles, cartilages and bones. In addition, developmental patterning of craniofacial tissues must be analyzed. Patterning includes the mechanisms by which each structure forms in the correct location and with the appropriate initial shape. Recently I discovered that cephalic voluntary muscles in avian embryos are derived from paraxial mesoderm, not from branchial mesoderm as is stated in all embryology texts. These data negate the concept of branchiomeric muscles, which is a key aspect of many models of head development. Muscles in different parts of the head differ only with respect to the origin of their connective tissue components. Neural crest cells migrate into branchial arch primordia and are subsequently transformed from cytologically homogeneous populations into a series of skeletal elements, each having a shape unique to its position within the arch and also appropriate for the particular arch. Following substitution of presumptive first arch (lower jaw) neural crest cells for presumptive second arch (hyoid) primordia, chick embryos developed with two complete sets of lower jaw tissues, the normal jaw and an ectopic jaw in the location of the second arch. These data indicate that the basis for patterning of some craniofacial skeletal structures is inherent in the nural crest population before the onset of migration. In these embryos many of the voluntary muscles were also rearranged, suggesting that muscles are dependent upon patterning information residing within the connective tissue primordia. The objectives of this research are to complete the mapping of normal muscle origins in the avian embryo, and to experimentally analyze the mechanisms by which craniofacial connective tissues, muscles, and also motor nerves and blood vessels are normally patterned. These processes are of fundamental importance in understanding the basis for both normal development and craniofacial dysmorphologies.
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