Forward spreading in the establishment of a vertebrate Hox expression boundary: The expression domain separates into anterior and posterior zones, and the spread occurs across implanted glass barriers

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Abstract
By use of wholemount in situ hybridization, we show how expression of the chicken homeobox gene Hoxd-4 commences in the posterior part of the primitive streak and then spreads forward, covering most of the primitive streak by the 2 somite stage, covering the entire primitive streak by the 5 somite stage, reaching the somite 1/somite 2 level of the neural tube by the 9 somite stage, and reaching the rhombomere 6/rhombomere 7 junction of the hindbrain by the 15 somite stage. Forward spreading does not depend upon cell migration, as was evidenced by vital dye (DiI) cell marking experiments. Furthermore, forward spreading does not apparently require tissue continuity since it could not be blocked by impermeable (glass) barriers surgically implanted to divide embryonic tissues. As forward spreading of chick Hoxd-4 proceeds, the domain of expression separates, at late primitive streak stages, into “anterior” and “posterior zones,” with an intervening “intermediate zone” of weak or non-expression. Clear anterior and posterior zones were also found for Hoxa-3 and a-4 expression in late primitive steak stage mouse embryos. We present evidence that the anterior zone corresponds with the “definitive” domain of Hox gene expression, as has earlier been extensively characterized in midgestation embryos. The posterior zone is transitory, probably persisting only for the duration of the primitive streak, and it is a region of intense Hox expression in primitive streak tissue, Hensen's node, and adjacent regions of neurectoderm and mesoderm. We suggest that the posterior zone marks the source of a morphogen which is the primary activator of Hox gene expression, and we discuss possible models for the mechanism of forward spreading in expression.