Preparation as a Solid

An alternative to administering retinoids as a solution is to administer them as neat powder in a depot. In this case, a rubber compound called Silastic 382 Medical Elastomer (Dow Corning Corporation, Midland, MI) acts as the depot. tRA is released from this compound at a constant rate over the first 24 h and then the release plateaus by which time about 65-70% of the total content has been released (26).

1. Measure 1 mL of Silastic, either as a volume from a syringe or by weight, into a small Petri dish.

2. Add the required amount of tRA, e.g., 10 mg to the Silastic.

3. Mix thoroughly for 10 min until an even consistency is obtained—easy to judge with a yellow compound like tRA.

4. Add one drop of catalyst, stannous octoate, mix, and leave to set for an hour at room temperature.

5. The silastic can now be cut up into any shape and size for administering to the embryo. This is performed down a dissecting microscope with a graticule on the stage plate. The required size is normally determined empirically by the dose or by the size of the embryo or embryonic field to be treated. The amount of retinoid administered can be calculated from the size of the block, e.g., a 200-|im cube has a volume of 8 x 106 |im3. If the concentration of the mixture is 10 mg/mL = 10 mg/1012 |im3 then this block will have (8 x 106)/1012 x 10 mg = 80 ng of RA in it.

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