Morphogenetic change of the limb bud in the hand plate formation

The vertebrate hand plate is flattened and paddle shaped; that is, it is wide along the anteroposterior (AP) axis (thumb to little finger) and thin along the dorsoventral axis (back of hand to palm). To learn how the hand plate develops its three-dimensional architecture, we observed morphological changes in the distal limb bud of the chick embryo at stages 23-27 and the gecko embryo 11-13 days after oviposition. Cell population of the posterior distal limb bud expanded more than that of the anterior one in the chick embryo. Taken together with the observation that these two cell populations did not show significant differences in their expansion along the proximodistal axis, we propose that the cell population in the posterior limb bud contributes more to the morphogenetic increase along the AP axis, which widens the limb bud for the formation of the hand plate. Our observation that more mitoses were oriented anteroposteriorly than dorsoventrally in the chick embryo at around stage 25 suggests that the oriented cell division contributes to the morphogenetic increase along the AP axis.