PATTERNS OF EXPRESSION OF HOXA-11 IN MICROMASS CULTURES OF CHICK LIMBMESENCHYME FROM VARIOUS STAGES SUGGEST A ROLE FOR HOXA-11 IN THE SPECIFICATION OF THE ZEUGOPOD

During chick limb development the expression of Hoxa-11 is specificall y associated with the prospective zeugopod (radius + ulna) region. A s eries of micromass cultures were performed using mesenchyme from whole stage 18/19 or 20/21 limbs or mesenchyme from the tips of stage 22/23 , 23/24 or 25 limbs. In such cultures, limb mesenchyme undergoes chond rogenesis paralleling its fate in vivo. Hoxa-11 and collagen type II m RNA were examined by dot blot and in situ analysis at different time p oints. The highest level of Hoxa-11 mRNA was found in micromass cultur es of the tips of stage 22/23 and 23/24 limbs at 48 hours. Between sta ges 22 and 24, the determining events occur which subsequently lead to the formation of the zeugopod by the mesenchyme present in the tips a t these stages. This correlation further supports a possible role of H oxa-11 in the specification of the zeugopod.