Development of the hypothalamic infundibulum in the early chick embryo
In the posterior neural tube of vertebrate embryos, the secreted signalling molecules Sonic hedgehog (Shh) and bone morphogenetic proteins (BMPs) are expressed at opposite poles of the neural tube, and act antagonistically to pattern the dorso-ventral axis of the neural tube. In contrast, in the ventral diencephalon, in regions that will give rise to the hypothalamus, BMPs and Shh are initially co-expressed. Subsequently, Shh shows a dynamic pattern of expression, first undergoing a medio-Iateral expansion and then disappearing from ventro-medial cells that will form the infundibulum. In this study, I address how Shh is regulated within the hypothalamic infundibulum. Using a combination of fate mapping and gene expression analysis, I demonstrate that cells that initially co-express Shh and BMP7 give rise to the Shh-negative infundibulum. Furthermore, I demonstrate, both in vitro and in vivo, that BMP7 is necessary and sufficient to cause the downregulation of Shh in the infundibulum. Recent studies on the mouse Shh promoter (Jeong and Epstein, 2003) have revealed that aT-box binding site is necessary for the down-regulation of Shh in the infundibulum. Through analysis of Tbx genes, I have found that Tbx2 is expressed in the prospective infundibulum. Furthermore, my studies reveal that its expression is dependent on BMP activity Finally, my studies show that, in addition to their role in regulating Shh expression, BMPs and Tbx2 may also playa role in regulating the size of the infundibulum. Analysis with the M-phase marker, PH3, reveals that, following exposure to BMPs, prospective infundibular cells are transiently cell cycle arrested, entering a synchronised cell cycle once BMP activity is lost, and Tbx2 is expressed. Together, my results suggest that BMPs act through Tbx2 in order to control the domain of Shh expression within the forming hypothalamus, whilst simultaneously controlling the size of this progenitor domain.