In vertebrate development, Vg genes may be required for an evolutionarily conserved early step in positioning or induction of the axis.

Vitamin A requirement for early embryonic development is clearly evident in the gross cardiovascular and central nervous system abnormalities and an early death of the vitamin A-deficient quail embryo. This retinoid knockout model system was used to examine the biological activity of various natural retinoids in early cardiovascular development. We demonstrate that all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol and all-trans-retinol induce and maintain normal cardiovascular development as well as induce expression of the retinoic acid receptor beta2 in the vitamin A-deficient quail embryo. The expression of RARbeta2 is at the same level and at the same sites where it is expressed in the normal embryo. Retinoids provided to the vitamin A-deficient embryo up to the 5-somite stage of development, but not later, completely rescue embryonic development, suggesting the 5-somite stage as a critical retinoid-sensitive time point during early avian embryogenesis. Retinoid receptors RARalpha, RARgamma, and RXRalpha are expressed in both the precardiac endoderm and mesoderm in the normal and the vitamin A-deficient quail embryo, while the expression of RXRgamma is restricted to precardiac endoderm. Vitamin A deficiency downregulates the expression of RARalpha and RARbeta. Our studies provide strong evidence for a narrow retinoid-requiring developmental window during early embryogenesis, in which the presence of bioactive retinoids and their receptors is essential for a subsequent normal embryonic development.

Follistatin, a secreted glycoprotein, has been shown to act as a potent neural inducer during early amphibian development. The function of this protein during embryogenesis in higher vertebrates is unclear, and to further our understanding of its role we have cloned, sequenced, and performed an in-depth expressional analysis of the chick homologue of follistatin. In addition we also describe the expression pattern of activin beta A and activin beta B, proteins that have previously been shown to be able to interact with follistatin. In this study we show that the expression of follistatin and the activins do not always overlap. Follistatin was first detected in Hensen's node and subsequently in the region described by Spratt [1952] as the neuralising area. In older embryos it was also expressed in a highly dynamic manner in the hindbrain as well as in the somites. We also present evidence that follistatin may have a later role in the resegmentation of the somites. We were unable to detect the expression of activin beta A during early embryogenesis, whereas activin beta B was first expressed in the extending primitive streak and subsequently in the neural folds. The results from this study are consistent with a role for follistatin in neural induction but suggest it has additional functions unrelated to its inhibitory actions on activins.

We have examined the distribution of the retinoid X receptor gamma (RXRgamma) in the developing chicken retina by using in situ hybridization and RNase protection assays. We detected RXRgamma transcripts as early as 4 days of embryonic development (d4) in central regions of the retina, spreading to more peripheral regions by d8. The first few RXRgamma-positive cells were scattered within the depth of the retinal neuroepithelium, but as they increased in number they became localized predominantly to the apical (outer, ventricular) layer. The identity of the RXRgamma-positive cells at these stages is unknown, due to the lack of cell type-specific markers. By d10, when photoreceptors and ganglion cells have been generated and begun to establish their definitive layers, RXRgamma-positive cells were virtually restricted to the photoreceptor layer, and maintained this distribution to posthatch stages. RNase protection assays were performed on whole retinae to verify the temporal pattern of in situ hybridization results and showed that between d5 and d16 there was a significant increase in the mRNA levels of the RXRgamma2 isoform. Between d16 and early posthatch stages the level of RXRgamma2 mRNA did not change significantly. Consistent with previous studies, mRNA levels of the RXRgamma1 isoform were substantially lower than mRNA levels of the RXRgamma2 isoform at all time points examined. These results demonstrate that RXRgamma mRNA is expressed in photoreceptors in the developing chicken retina and implicate RXRgamma as the earliest marker of photoreceptor differentiation documented to date.

Vitamin A requirement for early embryonic development is clearly evident in the gross cardiovascular and central nervous system abnormalities and an early death of the vitamin A‐deficient quail embryo. This retinoid knockout model system was used to examine the biological activity of various natural retinoids in early cardiovascular development. We demonstrate that all‐trans‐, 9‐cis‐, 4‐oxo‐, and didehydroretinoic acids, and didehydroretinol and all‐trans‐retinol induce and maintain normal cardiovascular development as well as induce expression of the retinoic acid receptor β2 in the vitamin A‐deficient quail embryo. The expression of RARβ2 is at the same level and at the same sites where it is expressed in the normal embryo. Retinoids provided to the vitamin A‐deficient embryo up to the 5‐somite stage of development, but not later, completely rescue embryonic development, suggesting the 5‐somite stage as a critical retinoid‐sensitive time point during early avian embryogenesis. Retinoid receptors RARα, RARγ, and RXRα are expressed in both the precardiac endoderm and mesoderm in the normal and the vitamin A‐deficient quail embryo, while the expression of RXRγ is restricted to precardiac endoderm. Vitamin A deficiency downregulates the expression of RARα and RARβ. Our studies provide strong evidence for a narrow retinoid‐requiring developmental window during early embryogenesis, in which the presence of bioactive retinoids and their receptors is essential for a subsequent normal embryonic development. Dev. Dyn. 1998;213:188–198. © 1998 Wiley‐Liss, Inc.