The chicken mutant talpid3 (tu3) has polydactylous limbs with up to 7-8 morphologically similar digits. This lack of antero-posterior polarity in digit pattern is correlated with symmetrical expression of genes of the HoxD complex. We determined the distribution of polarizing activity in limb buds of the chick mutant tu3 by assessing the ability of mesenchyme from various positions along the antero-posterior axis to induce digit duplications when grafted anteriorly into a normal limb. Cells with highest polarizing activity were found at the posterior margin of the wing as in the polarizing region of normal limb buds. However, in contrast to normal limb buds, tu3 anterior mesenchyme also had low polarizing activity. Application of retinoic acid or a polarizing region graft to the anterior of tu3 limb buds changed digit morphology but did not induce digit duplications or digits with any characteristic a-p pattern. To determine which genes are associated with polarizing activity and which are associated with patterning of the digits, we examined expression of the genes Sonic hedgehog (shh), Bmp-2, and Bmp-7, whose expression is normally confined to the posterior margin of the early wing bud and is associated with the polarizing region. In addition, we determined the distribution of Fgf-4 transcripts which in normal limb buds are restricted to the posterior part of the apical ectoderma1 ridge. In tu3 limb buds, shh expression is restricted to the posterior limb mesenchyme, which has high polarizing activity, but is not expressed in regions which have low polarizing activity. In contrast, Bmp-2 and Bmp-7 are expressed uniformly along the a-p axis. Fgf4 transcripts are present throughout the apical ectodermal ridge in tu3 limb buds. In the tu3 mutant, there is both an abnormal distribution of signalling activity and response to polarizing signals. In addition, the dissociation between the expression of shh and Bmps suggests distinct roles for the encoded molecules in signalling and response in a-p patterning of limb buds. o 1995 Wiley-Liss, Inc.

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Using a silver-impregnation method, the occurrence and significance of Golgi apparatus orientation has been studied in cells contributing to the cartilage condensations in the developing skeleton of the chick limb bud, both in normal embryos and in the talpid mutant, in which the pattern of condensation in situ, and cell behaviour in vitro, is abnormal. Analysis of photographed sections made up as photomontages with a final magnification of X 1000, indicates a sequence of changing Golgi orientation in the course of establishing cartilage condensations in the mesenchyme in normal limb buds, including at an early stage an orientation of one population of cells towards the condensation centre and of another population in the contrary direction, and a modification of the sequence in the mutant. The changing patterns of cell orientation has been further analysed in scanning electron microscope studies on the formation of cartilage condensations in vitro.