Gcm2 and Foxn1 mark early parathyroid- and thymus-specific domains in the developing third pharyngeal pouch

The thymus and parathyroids originate from a common primordium that develop s from the third pharyngeal pouch in mice and humans. The molecular mechani sm that specifies this primordium into distinct organ domains is not known. The Gcm2 and Foxn1 transcription factors are required for development of t he parathyroid and thymus respectively, and are attractive candidates for t his role. However, their embryonic expression patterns during pharyngeal po uch development and early thymus and parathyroid organogenesis have not bee n described. Hen we report that Gcm2 is expressed specifically in the devel oping second and third pharyngeal pouches at E9.5, and is further confined to a small domain of the third pouch endoderm by E10.5. In contrast, Foxn1 is not expressed until after the common primordium is formed, beginning at E11.25. Our results show that Gcm2 and Foxn1 expression mark two complement ary domains that prefigure para thyroid and thymus regions within the commo n primordium before morphological distinctions are present. (C) 2001 Elsevi er Science Ireland Ltd. All rights reserved.