Two phases in the establishment of HOX expression domains.

Vertebrate HOX genes are thought to play a role in the transmission of anteroposterior (A-P) positional information to the cellular precursors of embryonic axial and paraxial structures. We present an in situ analysis of the expression of two adjacent HOX genes, Hox-2.3 and Hox-2.4. We discuss the data on early gene expression in the light of what is known about cell behavior, cellular fate maps, and A-P regionalization during gastrulation. After the genes are switched on in the most posterior part of the embryo at the late primitive streak stage, the expression domains spread, in a way that does not correlate with cell lineage, toward more cranial positions within and along the streak in mesoderm and ectoderm. A second phase of HOX gene expression takes place at the early somite stage after the expression domains have reached the region around and lateral to the anterior part of the streak (the equivalent of the chicken Hensen's node). HOX gene expression during this second phase is probably clonally transmitted and would be directly associated with the acquisition of A-P positional identity by the progenitors of paraxial mesoderm and neurectoderm which are located anterolaterally to the node.