The distribution of endogenous retinoic acid in the chick embryo: implications for developmental mechanisms

The aim of these experiments was to determine the endogenous distribution o f retinoic acid (RA) across a wide range of embryonic stages in the chick e mbryo. By high pressure liquid chromatography, it was revealed that didehyd roRA is the most prevalent retinoic acid in the chick embryo and that the t issues of the stage 24 embryo differed widely in their total RA content (di dehydroRA + all-trans-RA). Some tissues such as the heart had very little R A and some such as the neural tube had very high levels, the total variatio n between these two being 29-fold. We shelved that these tissues also synth esised RA and released it into the medium, thus validating the use of the F 9 reporter cell system for further analyses of younger staged embryos. With these F9 cells, me showed that, at stage 4, the posterior end of the embry o had barely detectably higher levels of RA than the anterior end, but that a significant level of RA generation was detected as soon as somitogenesis began. Then a sharp on/off boundary of RA was present at the level of the first somite, We could find no evidence for a posterior-to-anterior gradien t of RA. Throughout further development, various consistent observations we re made: the developing brain did not generate RA, but the spinal part of t he neural tube generated it at very high levels so there must be a sharp on /off boundary in the region of the hindbrain/spinal cord junction; the mese nchyme surrounding the hindbrain generated RA whereas the hindbrain itself did not; there was a variation in RA levels from the midline outwards with the highest levels of RA in the spinal neural tube followed by lower levels in the somites followed by lower levels in the lateral plate; the posterio r half of the limb bud generated higher levels than the anterior half. With these observations, we were able to draw maps of endogenous RA throughout these early stages of chick embryogenesis and the developmental implication s of these results are discussed.