ENDOGENOUS DISTRIBUTION OF RETINOIDS DURING NORMAL DEVELOPMENT AND TERATOGENESIS IN THE MOUSE EMBRYO

We have analysed the endogenous retinoids present in whole mouse embry os from day 9 to day 14 of development and in individual components of the embryo at two stages, day 10.5 and day 13, by HPLC. We can only d etect two retinoids, all-trans-RA (tRA) and all-trans-retinol (t-retin ol), and t-retinol is 5-10-fold in excess over tRA. We cannot detect 9 -cis-RA or any didehydroretinoids; thus mammalian embryos seem to diff er in their retinoid content from other embryos such as chick, Xenopus , and fish. The levels of tRA do not change significantly over the 6 d ays of development analysed, whereas t-retinol rises sharply as the li ver develops. Within the embryo, tRA is present at high levels in the developing spinal cord and at very low levels in the forebrain; indeed there is a gradient of endogenous tRA from the forebrain to the spina l cord. Other parts of the embryo had intermediate levels of tRA. When a teratogenic dose of RA was administered to day 10.5 embryos, the le vels of tRA present in individual tissues of the embryo rose dramatica lly-from 175-fold to 1,400-fold-and the levels rose in all tissues not in any exclusive areas. We then determined which areas of the embryo were malformed by such a teratogenic dose. The lower jaw, palate, vert ebrae, tail, and limbs were consistently abnormal, and since these are as received a dose of tRA no higher than any other it was concluded th at cell-specific factors must determine the teratogenic response of th ese tissues. We then considered whether cellular retinoic acid-binding protein I or II (CRABP I or II) played any role in this response by d etermining their relative levels in each of the tissues analysed. Ther e was no correlation between the presence of CRABP I and II and the di stribution of administered RA. Neither was there a clear correlation i n detail between the presence of CRABP I and II and the sites of terat ogenesis. We therefore conclude that other factors, for example, nucle ar factors, must be responsible for the teratogenic response to RA. (C ) 1995 Wiley-Liss, Inc.