Bmp4 and Noggin expression during early thymus and parathyroid organogenesis.

The thymus and parathyroids originate from the third pharyngeal pouches, which form as endodermal outpocketings in the pharyngeal region beginning on embryonic day 9 (E9.0) of mouse development. Using organ-specific markers, we have previously shown that thymus and parathyroid-specific organ domains are established within the primordium prior to formation of the organs proper: Gcm2 expression defines the prospective parathyroid cells in the dorsal pouch from E9.5, while Foxn1 is expressed in the thymus domain from E11.25. Bmp (bone morphogenetic protein) signaling has been implicated in thymic epithelial cell differentiation and thymus organogenesis. In the present study, we report expression patterns of Bmp4 and Noggin, a Bmp4 antagonist, in the third pharyngeal pouch using two lacZ transgenic mouse strains. Results from this gene expression study revealed localization of Bmp4 expression to the ventral region of the third pharyngeal pouch endoderm at E10.5 and E11.5, in those cells that will express Foxn1 and form the thymus. Conversely, the expression of Noggin was confined to the dorsal region of the pouch and primordium at these stages, and thus appeared to be co-expressed with Gcm2 in the parathyroid domain. This represents the first detailed study of Bmp4 and Noggin expression during the early stages of thymus and parathyroid organogenesis.