title: Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish.
abstract: The hagfish, a group of extant jawless fish, are known to lack true vertebrae and, for this reason, have often been excluded from the group Vertebrata. However, it has yet to be conclusively shown whether hagfish lack all vertebra-like structures, and whether their somites follow developmental processes and patterning distinct from those in lampreys and gnathostomes. Here we report the presence of vertebra-like cartilages in the in-shore hagfish, Eptatretus burgeri. These elements arise as small nodules occupying anatomical positions comparable to those of gnathostome vertebrae. Examination of hagfish embryos suggests that the ventromedial portion of a somite transforms into mesenchymal cells that express cognates of Pax1/9 and Twist, strikingly similar to the pattern of sclerotome development in gnathostomes. We conclude that the vertebra-like elements in the hagfish are homologous to gnathostome vertebrae, implying that this animal underwent secondary reduction of vertebrae in most of the trunk.
authors.affiliation: Laboratory for Evolutionary Morphology, RIKEN Center for Developmental Biology, Kobe, Japan. otakinya@gate.sinica.edu.tw
authors.forename: Kinya G
authors.initials: KG
authors.lastname: Ota
authors.forename: Satoko
authors.initials: S
authors.lastname: Fujimoto
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Oisi
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kuratani
mesh_terms: D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D002356:Cartilage / Q000033:anatomy & histology / Q000196:embryology*; D003001:Cloning, Molecular; D005260:Female; D006196:Hagfishes / Q000033:anatomy & histology / Q000196:embryology*; D006652:Histological Techniques; D017403:In Situ Hybridization; D007564:Japan; D008297:Male; D008648:Mesoderm / Q000378:metabolism; D008969:Molecular Sequence Data; D051761:Paired Box Transcription Factors / Q000378:metabolism; D017422:Sequence Analysis, DNA; D019170:Somites / Q000196:embryology*; D013131:Spine / Q000033:anatomy & histology / Q000196:embryology*; D051879:Twist-Related Protein 1 / Q000378:metabolism
chemical_list: D051761:Paired Box Transcription Factors; D051879:Twist-Related Protein 1