Early development of the thymus in Xenopus laevis

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. Developmental Dynamics, 2012. © 2012 Wiley Periodicals, Inc. Key Findings: The thymus primordia arise from the second pharyngeal pouch. Thymus formation is independent of hoxa3 expression in the pouch endoderm. foxn1 expression correlates with the initiation of thymic epithelial differentiation. Neural crest cells are not required for thymus specification or T‐cell differentiation.