title: Early development of the thymus in Xenopus laevis.
abstract: BACKGROUND
  Although Xenopus laevis has been a model of choice for comparative and developmental studies of the immune system, little is known about organogenesis of the thymus, a primary lymphoid organ in vertebrates. Here we examined the expression of three transcription factors that have been functionally associated with pharyngeal gland development, gcm2, hoxa3, and foxn1, and evaluated the neural crest contribution to thymus development.
  
  
  RESULTS
  In most species Hoxa3 is expressed in the third pharyngeal pouch endoderm where it directs thymus formation. In Xenopus, the thymus primordium is derived from the second pharyngeal pouch endoderm, which is hoxa3-negative, suggesting that a different mechanism regulates thymus formation in frogs. Unlike other species foxn1 is not detected in the epithelium of the pharyngeal pouch in Xenopus, rather, its expression is initiated as thymic epithelial cell starts to differentiate and express MHC class II molecules. Using transplantation experiments we show that while neural crest cells populate the thymus primordia, they are not required for the specification and initial development of this organ or for T-cell differentiation in frogs.
  
  
  CONCLUSIONS
  These studies provide novel information on early thymus development in Xenopus, and highlight a number of features that distinguish Xenopus from other organisms.
authors.affiliation: Department of Oral Anatomy, School of Dentistry and Institute of Oral Biosciences, Chonbuk National University, Jeonju, Republic of Korea.
authors.forename: Young-Hoon
authors.initials: YH
authors.lastname: Lee
authors.forename: Allison
authors.initials: A
authors.lastname: Williams
authors.forename: Chang-Soo
authors.initials: CS
authors.lastname: Hong
authors.forename: Youngjae
authors.initials: Y
authors.lastname: You
authors.forename: Makoto
authors.initials: M
authors.lastname: Senoo
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Saint-Jeannet
mesh_terms: D000818:Animals; D001483:Base Sequence; D017931:DNA Primers / Q000235:genetics; D051858:Forkhead Transcription Factors / Q000235:genetics; D018507:Gene Expression Regulation, Developmental / Q000235:genetics / Q000502:physiology*; D018398:Homeodomain Proteins / Q000378:metabolism; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D009432:Neural Crest / Q000196:embryology*; D038081:Organogenesis / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D017385:Sequence Homology; D013045:Species Specificity; D013950:Thymus Gland / Q000033:anatomy & histology / Q000196:embryology*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis / Q000196:embryology*
chemical_list: D017931:DNA Primers; D051858:Forkhead Transcription Factors; D018398:Homeodomain Proteins; D014157:Transcription Factors; C090140:Whn protein