The role of the polarizing zone in the pattern of experimental chondrogenesis in the chick embryo interdigital space.

We have previously shown that removal of the apical ectodermal ridge of the third interdigital space of the chick leg bud at stages 28 and 29 is followed by the appearance of ectopic cartilage, which in the course of development gives rise to extra digits. These in vivo studies suggest that the pattern of skeletal morphogenesis in the limb depends on the inhibitory effect of the ectoderm. In the present study we tested whether zone polarizing activity (ZPA) exerted an effect on the pattern of experimental chondrogenesis in the interdigital space of the leg bud in stage 29 HH chick embryos. A small fragment of tissue from the ZPA in chick embryos in which ZPA activity was most intense was grafted onto the interdigital space in which chondrogenesis had previously been experimentally induced. No significant changes were observed in the course of differentiation of the recipient interdigital spaces with ZPA grafts, leading us to conclude that the graft failed to modify the morphogenetic fate of interdigital tissue.