Fig. 8.

Model for formation of neural and mesodermal tissues from the NMps. A model based on work presented here and from other studies. The NMps co-express Sox2 and Brachyury/Tbxta. Cells that migrate into a Wnt-free environment retain Sox2 expression and differentiate as neural cells. Cells that enter a high Wnt environment retain Brachyury expression and activate Tbx16 (Tbx6 in amniotes), which together with Msgn1 and Tbx16l (Morrow et al., 2017) activates mesodermal gene expression. Tbx16 also represses both Sox2 and later Wnt, to promote mesodermal differentiation (Bouldin et al., 2015). Brachyury has an essential role in creating the niche for mesoderm formation by activating canonical Wnt expression and inhibiting RA from the posterior end by inducing the expression of Cyp26a1. We propose that the Hox13 proteins act within the NMps to enhance the niche-promoting effects of Brachyury.