Background: After implantation, the basic body plan of the mammalian embryo is established during gastrulation when the epithelial founder tissue of the fetus, the epiblast, gives rise to new tissues by ingression through the primitive streak. Formation of the primitive streak defines the caudal aspect of the embryo and thus the anteroposterior axis. Further patterning of this axis has been attributed to signals produced by tissues arising from the primitive streak, and in particular the mesendoderm located along the midline of the embryo is thought to be responsible for the correct anteroposterior subdivision of the neurectoderm as it begins to form the central nervous system (CNS).
Results: In situ hybridization studies show that the onset of expression of the homeobox-containing gene Hesx1 coincides with the formation of the primitive streak, but occurs on the opposite side of the embryo, in a small domain of anterior endoderm. Lineage tracing using a lipophilic fluorescent label shows that the first endoderm cells to express Hesx1 are not destined to contribute to the future embryo, but instead belong to the primitive endoderm lineage and will be displaced by definitive endoderm arising from the primitive streak during gastrulation. Approximately 24 hours after Hesx1 transcripts are first detected in the endoderm, they start to appear in adjacent ectoderm that gives rise to the most anterior component of the developing CNS, the prosencephalon, which continues to express Hesx1. Eventually, Hesx1 transcripts are detectable only in Rathke's pouch as the pituitary starts to develop. Removal of endoderm cells expressing Hesx1 during the earlier stages of gastrulation either prevents or severely curtails the later expression of Hesx1 in ectoderm and neurectoderm, but does not affect gene expression in more caudal regions of the developing CNS.
Conclusions: As overt anterior pattern is present in the visceral embryonic endoderm prior to formation of any axial mesendoderm, a mechanism for bestowing anterior pattern must exist which is independent of primitive streak descendants. Furthermore, correct molecular patterning of the most rostral neurectoderm appears to depend on the presence of this anterior visceral embryonic endoderm during the early stages of gastrulation. We propose that primitive endoderm is responsible for the initial induction of rostral identity in the embryo, and in particular for the correct definition of the future prosencephalic neurectoderm. Subsequently, this identity will be reinforced and maintained by axial mesendoderm when it displaces the visceral embryonic endoderm during the course of gastrulation.