Initial retinoid requirement for early avian development coincides with retinoid receptor coexpression in the precardiac fields and induction of normal cardiovascular development

Abstract

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.

Vitamin A–Not for Your Eyes Only: Requirement for Heart Formation Begins Early in Embryogenesis

“…The absolute essentiality of vitamin A for early embryogenesis is unambiguously demonstrated in the vitamin A-deficient (VAD) avian embryo, an ideal model to study the function of vitamin A in a vertebrate embryo during early development. These completely VAD embryos develop gross abnormalities in the cardiovascular and central nervous systems and trunk and die early in embryonic life [31,32,66,67,68,69]. Significantly, the VAD avian embryo can be rescued and normal development restored by the administration of the physiological ligand for RARs, all- trans -retinoic acid, or its precursor, retinol, during a critical, RA-requiring time window in which important developmental events are specified.…”

“…Presently there is no understanding of the molecular basis of cardiovascular birth defects. The critical vitamin A-requiring developmental events that take place in the early avian embryo during heart formation [67] coincide with the first 2-3 weeks of human pregnancy and may be severely compromised if maternal vitamin A intake is marginal or if there is interference with vitamin A function during pregnancy. The high incidence of vitamin A deficiency in developing countries may account for the increased incidence of heart malformations in these populations [74].…”

“…The hallmark of vitamin A deficiency in the avian embryo is a grossly abnormal cardiovascular system, characterized by an absence of vascular networks and by a ballooned, non-compartmentalized, randomly-positioned heart, without an inflow tract at the posterior site of the heart [79]; these embryos die by embryonic day four; b) . The discovery of a critical time window when the presence of RA is absolutely essential for normal embryogenesis to proceed; this time coincides with the initiation of heart morphogenesis and the organization of endothelial cells into vascular networks [67] and corresponds to the first 2–3 weeks of human pregnancy. If RA is administered to the VAD quail embryo during this critical time or prior to it, the embryo is rescued and develops normally.…”

“…In the VAD avian embryo, the predominant feature of the VAD phenotype is a failure of cardiovascular development, with embryo lethality linked directly to the absence of the cardiac inflow tracts, i.e., these embryos lack the sino atrial tissue and their hearts are closed posteriorly, thus there is no blood supply to the embryo [32,79,80]. The administration of RA or retinol during or prior to the critical developmental time window when RA requirement begins in the early avian embryo ( i.e., the HH8 4/5 somite stage) rescues the VAD embryo, and it now develops with a completely normal phenotype [66,67,68]. …”

“…However, cardiomyocyte differentiation is not altered and the primitive heart tube forms in these VAD embryos, thus providing evidence that RA signaling is not required for these early heart morphogenesis events [68]; c) Additional support for the role of vitamin A in IFT building was obtained from the localization of Raldh2, the main RA synthesizing enzyme, in the posterior heart region during quail embryogenesis [85] and from the demonstration of expression of all retinoid receptors in the heart forming regions of quail embryos [67,84,85,86]; and d) A systematic examination of the early development in the VAD quail embryos revealed that in these embryos one of the earliest observable morphological defects is an abnormal formation of the heart IFTs which gradually become more narrow and finally close up, thus resulting in the absence of a connection between the developing heart and the extraembryonal blood supply to the heart [82]. …”