This method was designed for adminstering RA to the anterior side of the chick limb bud (27,28).
1. Use AG 1-X2 chromatography beads, formate form, 100-200 mesh (Bio-Rad, Hemel Hempstead, UK).
2. Select a number of uniformly sized beads (approx 200 pm) in a small Petri dish and add an aliquot of tRA in DMSO at a concentration of, for example, 1 mg/mL. Leave for 20 min. The beads are very difficult to see once in this solution, and the angle of the light source has to be altered to be able to visualize them.
3. Rinse the beads by removing the RA solution and putting on a few drops of medium, e.g., MEM. The beads will take up phenol red from the medium and become perfectly visible.
4. Repeat the rinse.
5. Incubate the beads in medium for 20 min at 37°C. They are then ready for implantation.
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