title: Development of somites and their derivatives in amphioxus, and implications for the evolution of vertebrate somites.
abstract: BACKGROUND
  Vertebrate somites are subdivided into lineage compartments, each with distinct cell fates and evolutionary histories. Insights into somite evolution can come from studying amphioxus, the best extant approximation of the chordate ancestor. Amphioxus somites have myotome and non-myotome compartments, but development and fates of the latter are incompletely described. Further, while epithelial to mesenchymal transition (EMT) is important for most vertebrate somitic lineages, amphioxus somites generally have been thought to remain entirely epithelial. Here, we examined amphioxus somites and derivatives, as well as extracellular matrix of the axial support system, in a series of developmental stages by transmission electron microscopy (TEM) and in situ hybridization for collagen expression.
  
  
  RESULTS
  The amphioxus somite differentiates medially into myotome, laterally into the external cell layer (a sub-dermal mesothelium), ventrally into a bud that forms mesothelia of the perivisceral coelom, and ventro-medially into the sclerotome. The sclerotome forms initially as a monolayered cell sheet that migrates between the myotome and the notochord and neural tube; subsequently, this cell sheet becomes double layered and encloses the sclerocoel. Other late developments include formation of the fin box mesothelia from lateral somites and the advent of isolated fibroblasts, likely somite derived, along the myosepta. Throughout development, all cells originating from the non-myotome regions of somites strongly express a fibrillar collagen gene, ColA, and thus likely contribute to extracellular matrix of the dermal and axial connective tissue system.
  
  
  CONCLUSIONS
  We provide a revised model for the development of amphioxus sclerotome and fin boxes and confirm previous reports of development of the myotome and lateral somite. In addition, while somite derivatives remain almost entirely epithelial, limited de-epithelialization likely converts some somitic cells into fibroblasts of the myosepta and dermis. Ultrastructure and collagen expression suggest that all non-myotome somite derivatives contribute to extracellular matrix of the dermal and axial support systems. Although amphioxus sclerotome lacks vertebrate-like EMT, it resembles that of vertebrates in position, movement to surround midline structures and into myosepta, and contribution to extracellular matrix of the axial support system. Thus, many aspects of the sclerotome developmental program evolved prior to the origin of the vertebrate mineralized skeleton.
authors.affiliation: Department of Biology, Barnard College, Columbia University, 3009 Broadway, New York, NY 10027 USA.
authors.forename: Jennifer H
authors.initials: JH
authors.lastname: Mansfield
authors.affiliation: Department of Integrative Biology, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620 USA.
authors.forename: Edward
authors.initials: E
authors.lastname: Haller
authors.affiliation: Marine Biology Research Division, Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093 USA.
authors.forename: Nicholas D
authors.initials: ND
authors.lastname: Holland
authors.affiliation: Department of Biology, Barnard College, Columbia University, 3009 Broadway, New York, NY 10027 USA.
authors.forename: Ava E
authors.initials: AE
authors.lastname: Brent
keywords: Amphioxus; Chordate; Connective tissue; Sclerotome; Skeleton; Somite evolution; Tendon