title: Ancestral mesodermal reorganization and evolution of the vertebrate head.
abstract: BACKGROUND
  The vertebrate head is characterized by unsegmented head mesoderm the evolutionary origin of which remains enigmatic. The head mesoderm is derived from the rostral part of the dorsal mesoderm, which is regionalized anteroposteriorly during gastrulation. The basal chordate amphioxus resembles vertebrates due to the presence of somites, but it lacks unsegmented head mesoderm. Gastrulation in amphioxus occurs by simple invagination with little mesodermal involution, whereas in vertebrates gastrulation is organized by massive cell movements, such as involution, convergence and extension, and cell migration.
  
  
  RESULTS
  To identify key developmental events in the evolution of the vertebrate head mesoderm, we compared anterior/posterior (A/P) patterning mechanisms of the dorsal mesoderm in amphioxus and vertebrates. The dorsal mesodermal genes gsc, bra, and delta are expressed in similar patterns in early embryos of both animals, but later in development, these expression domains become anteroposteriorly segregated only in vertebrates. Suppression of mesodermal involution in vertebrate embryos by inhibition of convergence and extension recapitulates amphioxus-like dorsal mesoderm formation.
  
  
  CONCLUSIONS
  Reorganization of ancient mesoderm was likely involved in the evolution of the vertebrate head.
authors.affiliation: Kuratani Evolutionary Morphology Laboratory, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, Kobe, 650-0047 Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Onai
authors.affiliation: Pattern Formation Group, Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871 Japan.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Aramaki
authors.affiliation: Laboratory for Axial Pattern Dynamics, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, Kobe, 650-0047 Japan.
authors.forename: Hidehiko
authors.initials: H
authors.lastname: Inomata
authors.affiliation: Kuratani Evolutionary Morphology Laboratory, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, Kobe, 650-0047 Japan.
authors.forename: Tamami
authors.initials: T
authors.lastname: Hirai
authors.affiliation: Kuratani Evolutionary Morphology Laboratory, RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuo-ku, Kobe, 650-0047 Japan.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kuratani
keywords: Amphioxus; Head mesoderm; Somites; Vertebrate body plan