Expression of AV-1, a position-specific molecule, in response to retinoic acid beads and ZPA grafts in chick limb development.

Local application of retinoic acid (RA) to the anterior-distal margin of chick limb buds causes digit pattern duplications identical to those that result from zone of polarizing activity (ZPA) grafts. The AV-1 antigen is a cell membrane glycoprotein which is expressed during chick limb bud development in a stage- and position-specific manner that correlates with cartilage pattern formation. In order to further understand the limb patterning mechanisms affected by RA and by ZPA grafts, we first examined the effects of RA treatment on AV-1 expression. We then compared the effect of RA with those of ZPA grafts. When beads soaked in varying concentrations of RA were applied to the anterior-distal margin of wing buds, changes in the pattern of AV-1 antigen expression were observed. AV-1 expression appears to correlate with regions that will give rise to a digit 3. These results suggest that the AV-1 protein is one of the molecules which may be involved in the process of cartilage pattern formation during limb development. ZPA grafts produced similar alterations in AV-1 expression, but the initial changes were observed earlier with ZPA grafts than with RA beads. These results support the idea that RA changes anterior limb bud cells into ZPA cells.