Cranial neural crest cell migration in the mammalian head. (A) Scanning electron micrograph of a rat embryo with part of the lateral ectoderm removed from the surface. Neural crest cell migration can be seen over the midbrain, and the column of neural crest cells migrating into the future first pharyngeal arch is evident. (B) Schematic drawing of cranial neural crest cell migration into the pharyngeal arches, showing Hox expression patterns. Note that the pattern is staggered between the neural tube and the pharyngeal arches, such that crest cells from the fourth rhombomere enter the second pharyngeal arch. The Hox gene expression boundaries coincide with rhombomere borders. (C) Structures formed in the human face by the neural crest-derived mesenchymal cells of the neural crest. The cartilaginous elements of the pharyngeal arches are indicated by colors, and the stippled region indicates the facial skeleton produced by anterior regions of the cranial neural crest. (A from Tan and Morriss-Kay 1985, courtesy of S.-S. Tan; B after McGinnis and Krumlauf 1992; C after Carlson 1999.)