This is found in the chick limb bud at 5-6 times higher amounts than tRA (4,19). It is generated by a parallel pathway to tRA from 3,4-didehydroretinol via 3,4-didehydroretinal. It is equipotent with tRA at inducing duplications in the chick wing bud (19).
This is a more polar metabolite of tRA, originally thought to be a breakdown product, but present in Xenopus embryos (20). It is more potent than tRA in inducing neural defects in Xenopus and a more potent mammalian teratogen (21). It binds and activates RARP, but only weakly activates RXRa, resembling tRA in this respect (20).
This is a typical esterified form of retinol used as storage in the liver. It is convenient for administration to amphibians, since it is available in a soluble form attached to corn starch and can be simply added to the tank water. It is eightfold less potent than tRA at inducing limb duplications during regeneration as assayed by this method of administration (22).
These are examples of synthetic derivatives of tRA, which usually have far higher potencies in biological assays than tRA, including embryological assays, such as duplication of the chick limb bud (23-25), the regenerating amphibian limb (8,25), and feather transformation in chick skin (10). This may be because they are much more stable in cells than tRA and are not broken down by the tRA metabolism pathway.
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