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In order to form an embryo, undifferentiated and unorganized cells must become differentiated and organized spatiotemporally. Vertebrate embryos use asymmetrical environmental cues from neighboring tissues to help generate this organized pattern of development. These neighboring tissues, called organizers, induce and pattern a field. One such organizer is Hensen’s node. In chick, a model system for studying embryonic development, Hensen’s node is located at the rostral end of the primitive streak and it is capable of inducing and patterning both head and trunk at the extended-streak stages (3d-4). Previous studies have shown that initiation of notochord formation, which correlates with the head to trunk transition, is regulated in Hensen’s node in response to signals from the midstreak and rostral endoderm. However, little is known regarding the mediation of signals regulating the head to trunk transition of the nervous system. In this study, the location of the subdivision between the head and trunk regions was evaluated. To assess regional head and trunk, we used selective transection and culture of chick embryos followed by marker gene expression in midbrain (DMBX) and caudal hindbrain (X37) by in situ hybridization. We demonstrated that the overlying rostral neuroectoderm, including the midbrain, can be induced and patterned in the absence of Hensen’s node at early gastrula stages (2-3b), suggesting that the head to trunk boundary patterns caudal to the midbrain. We further showed that the rostral ectoderm transected at gastrula stages (2-4) failed to express caudal hindbrain in the absence of the node, suggesting that the head to trunk boundary patterns rostral to the caudal hindbrain. Through the evaluation of the head to trunk boundary, we elucidated the spatiotemporal interactions regulating early signaling events involved in head/trunk axial pattern


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