The dynamic spatial and temporal relationships between the phalanx-forming region and the interdigits determine digit identity in the chick limb autopod

Interdigits (IDs) determine digit identity in chick limbs. They are located between the digital rays and act as secondary signaling centers downstream of sonic hedgehog to provide positional information for determining digit identity in the phalanx-forming region (PFR). We examined the dynamic developmental mechanism by which PFR cells obtain positional information from IDs to determine the identity of individual digits in the chick hindlimb.

The digital rays are naïve during the initial stages of their development, at which time digit identity is not determined. To determine digit identity, each PFR cell shows a unidirectional response to obtain positional information specifically from the IDs located posterior to the PFR, regardless of the signal strength from the anteriorly located IDs.

We identified the specific region of the IDs responsible for determining digit identity and showed that PFR cells actively receive positional information only from the posteriorly, and not the anteriorly, located IDs. We also demonstrated that digits 1, 2, and 3 are interchangeable with each other, but not with digit 4. Finally, we found that both ID4 and digital ray 4 are necessary for determining digit 4 identity. Conclusions: The digital rays are naïve during the initial stages of their development, at which time digit identity is not determined. To determine digit identity, each PFR cell shows a unidirectional response to obtain positional information specifically from the IDs located posterior to the PFR, regardless of the signal strength from the anteriorly located IDs.