title: The midline, oral ectoderm, and the arch-0 problem.
abstract: In most versions of theories of the segmentation of the vertebrate head, a premandibular segment is present rostral to the jaw-forming mandibular segment. These theories posit that in ancient fishes this segment included a gill and a gill-supporting skeleton, which then was modified to support the anterior brain. However, we find no recent evidence for existence of such a premandibular segment. Rather, new findings from studies of fate mapping and gene expression show that the "premandibular" territory is in fact the maxillary region of the mandibular arch. A signaling cascade, beginning with dorsal midline mesoderm in the gastrula and relayed through neural ectoderm and then oral ectoderm, greatly expands the skeletal derivatives of maxillary neural crest in a manner fully consistent with the Gans-Northcutt theory of the vertebrate new head.
authors.affiliation: Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA.
authors.forename: Charles B
authors.initials: CB
authors.lastname: Kimmel
authors.forename: Johann K
authors.initials: JK
authors.lastname: Eberhart