Loss of Twist function in the cranial mesenchyme of the mouse embryo causes failure of closure of the cephalic neural tube and malformation of the branchial arches. In the Twist(-/-) embryo, the expression of molecular markers that signify dorsal forebrain tissues is either absent or reduced, but those associated with ventral tissues display expanded domains of expression. Dorsoventral organization of the mid- and hindbrain and the anterior-posterior pattern of the neural tube are not affected. In the Twist(-/-) embryo, neural crest cells stray from the subectodermal migratory path and the late-migrating subpopulation invades the cell-free zone separating streams of cells going to the first and second branchial arches. Cell transplantation studies reveal that Twist activity is required in the cranial mesenchyme for directing the migration of the neural crest cells, as well as in the neural crest cells within the first branchial arch to achieve correct localization. Twist is also required for the proper differentiation of the first arch tissues into bone, muscle, and teeth.

During mouse gastrulation, endoderm cells of the dorsal foregut are recruited ahead of the ventral foregut and move to the anterior region of the embryo via different routes. Precursors of the anterior-most part of the foregut and those of the mid-and hind-gut are allocated to the endoderm of the mid-streak-stage embryo, whereas the precursors of the rest of the foregut are recruited at later stages of gastrulation. Loss of Mixl1 function results in reduced recruitment of the definitive endoderm, and causes cells in the endoderm to remain stationary during gastrulation. The observation that the endoderm cells are inherently unable to move despite the expansion of the mesoderm in the Mixl1-null mutant suggests that the movement of the endoderm and the mesoderm is driven independently of one another.

This fate-mapping study reveals that the progenitors of all major parts of the embryonic gut are already present in endoderm of the early-head-fold to early-somite stage (1-9 somites) mouse embryo. The anterior endoderm contributes primarily to the anterior intestinal portal of the early-organogenesis stage (16-19 somites) embryo. Endoderm cells around and lateral to the node are allocated to the open "midgut" region of the embryonic gut. The posterior (post-nodal) endoderm contributes not only to the posterior intestinal portal but also the open "midgut". Descendants of the posterior endoderm span a length of the gut from the level of the 3rd-5th somites to the posterior end of the embryonic gut. The formation of the anterior and posterior intestinal portals is accompanied by similar repertoires of morphogenetic tissue movement. We also discovered that cells on contralateral sides of the anterior endoderm are distributed asymmetrically to the dorsal and ventral sides of the anterior intestinal portal, heralding the acquisition of laterality by the embryonic foregut.

Investigation of the developmental fates of cells in the endodermal layer of the early bud stage mouse embryo revealed a regionalized pattern of distribution of the progenitor cells of the yolk sac endoderm and the embryonic gut. By tracing the site of origin of cells that are allocated to specific regions of the embryonic gut, it was found that by late gastrulation, the respective endodermal progenitors are already spatially organized in anticipation of the prospective mediolateral and anterior-posterior destinations. The fate-mapping data further showed that the endoderm in the embryonic compartment of the early bud stage gastrula still contains cells that will colonize the anterior and lateral parts of the extraembryonic yolk sac. In the Lhx1(Lim1)-null mutant embryo, the progenitors of the embryonic gut are confined to the posterior part of the endoderm. In particular, the prospective anterior endoderm was sequestered to a much smaller distal domain, suggesting that there may be fewer progenitor cells for the anterior gut that is poorly formed in the mutant embryo. The deficiency of gut endoderm is not caused by any restriction in endodermal potency of the mutant epiblast cells but more likely the inadequate allocation of the definitive endoderm. The inefficient movement of the anterior endoderm, and the abnormal differentiation highlighted by the lack of Sox17 and Foxa2 expression, may underpin the malformation of the head of Lhx1 mutant embryos.