Vitamin A-deficient Embryos Research Articles

Hensen's Node from Vitamin A-Deficient Quail Embryo Induces Chick Limb Bud Duplication and Retains Its Normal Asymmetric Expression ofSonic hedgehog(Shh)

Both Hensen's node, the organizer center in chick embryo, and exogenous retinoic acid are known to induce limb duplication when grafted or applied to the host chick limb bud. Retinoic acid is known to be present in the node and has been proposed as the putative morphogen for chick limb development. Here, we report that Hensen's node from vitamin A-deficient quail embryo induces limb duplication in the host chick embryo similar to that induced by the node from vitamin A-sufficient control embryos. We also demonstrate that the expression ofSonic hedgehog(Shh), recently shown to be the mediator of polarizing activity in the chick limb bud, is not affected by the endogenous vitamin A status of the embryo. Furthermore, whole-mountin situhybridization revealed asymmetry ofShhexpression in the Hensen's node of both vitamin A-sufficient and -deficient quail embryos. Retinoids were not detectable in the eggs from which vitamin A-deficient embryos were obtained. Extracts from normal embryos induced a level of expression of reporter gene equivalent to the presence of 3.4 pg of active retinoids per embryo, while those from vitamin A-deficient embryos induced a baseline level of reporter gene expression similar to that of the controls. Our studies suggest that endogenous retinoic acid is not involved inShhexpression nor in regulating its asymmetry during normal early avian embryogenesis and support the current view that endogenous retinoic acid may not be a direct morphogen for limb bud duplication.

The effects of retinoid status on TGF beta expression during mouse embryogenesis.

In a previous study we investigated the effects of RA excess on TGF beta protein localization in early postimplantation stages of mouse development. Here we extend this investigation by comparing the effects of retinoid deficiency with those of excess, and by comparing the effects of altered retinoid status on TGF beta protein and RNA transcript distribution. In vitamin A-deficient embryos, TGF beta 1 RNA and protein distribution were both unaltered compared with controls; conversely, TGF beta 2 protein levels were reduced while RNA levels remained normal. In RA-treated embryos, the previous study showed that intracellular TGF beta 1 levels were decreased, while those of extracellular TFG beta 1 were initially decreased but subsequently increased; here we found that TGF beta 1 RNA transcript levels were reduced following exposure to RA excess. TGF beta 2 showed a clear disparity between the effects of RA excess on protein and RNA transcript levels: RNA transcript distribution was unchanged or showed a slight increase in RA-treated embryos, whereas the previous results showed greatly reduced protein levels. The new results provide further evidence for interaction between retinoids and TGF beta s during mouse development, and indicate that retinoids are capable of differentially regulating TGF beta isoforms through mechanisms involving different stages in the process of TGF beta synthesis and secretion. The long-term nature of the effects of transient exposure to RA excess suggests that the mechanisms of RA-TGF beta interaction may be indirect.