FORWARD SPREADING IN THE ESTABLISHMENT OF A VERTEBRATE HOX EXPRESSIONBOUNDARY - THE EXPRESSION DOMAIN SEPARATES INTO ANTERIOR AND POSTERIOR ZONES, AND THE SPREAD OCCURS ACROSS IMPLANTED GLASS BARRIERS

By use of wholemount in situ hybridization, we show how expression of the chicken homeobox gene Hoxd-4 commences in the posterior part of th e primitive streak and then spreads forward, covering most of the prim itive streak by the 2 somite stage, covering the entire primitive stre ak 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/rhomb omere 7 junction of the hindbrain by the 15 somite stage. Forward spre ading does not depend upon cell migration, as was evidenced by vital d ye (Dil) cell marking experiments. Furthermore, forward spreading does not apparently require tissue continuity since it could not be blocke d by impermeable (glass) barriers surgically implanted to divide embry onic tissues. As forward spreading of chick Hoxd-4 proceeds, the domai n of expression separates, at late primitive streak stages, into ''ant erior'' and ''posterior zones,'' with an intervening ''intermediate zo ne'' of weak or non-expression. Clear anterior and posterior zones wer e also found for Hoxa-3 and a-4 expression in late primitive streak st age mouse embryos. We present evidence that the anterior zone correspo nds with the ''definitive'' domain of Hox gene expression, as has earl ier been extensively characterized in midgestation embryos. The poster ior zone is transitory, probably persisting only for the duration of t he primitive streak, and it is a region of intense Hox expression in p rimitive streak tissue, Hensen's node, and adjacent regions of neurect oderm and mesoderm. We suggest that the posterior zone marks the sourc e of a morphogen which is the primary activator of Hox gene expression , and we discuss possible models for the mechanism of forward spreadin g in expression. (C) 1994 Wiley-Liss, Inc.