Location and timing

Abstract

The third pharyngeal pouch is formed between E8.5 and E9 in mice. The thymus and the parathyroid both form from third pouch endoderm at later stages of development. Although domain specific markers have been identified, the exact pathway responsible for patterning the thymus and parathyroid from third pouch endoderm is still unknown. Furthermore, several questions remain about morphogenesis of the thymus and parathyroid once these domains are established. In this study, we take advantage of the Cre loxp system in mice to regulate genes in a tissue specific manner. By deleting or ectopically expressing genes previously identified as being required for some part of third pouch patterning or morphogenesis, we aim to learn more about the pathway in which these genes function. In our study of third pouch patterning we find that Shh signaling from neural crest mesenchyme and pouch endoderm both participate in patterning of the third pharyngeal pouch and that signaling from either source is sufficient to support parathyroid fate. We find that Shh signaling and Tbx1 expression does not guarantee Gcm2 expression although this may be caused by an additional inhibitory factor. Finally we find the first evidence that Foxn1 expression is inhibited by Tbx1 expression in vivo. In our study of thymus and parathyroid morphogenesis we find that AP-2α is required for separation of thymus from ectodermal cleft. We also find that ectodermal cleft fails to downregulate Fgf8 expression and has ectopic expression of Pax1 in the absence of AP-2a. We find that ectodermal cleft expressing endodermal identity genes is able to express Foxn1 and Gcm2; however, cleft fails to organize expression of either gene. Finally we find that thymus function is either decreased or delayed.