Retinoic acid signalling centres in the avian embryo identified by sites of expression of synthesising and catabolising enzymes

Abstract

Retinoic acid is an important signalling molecule in the developing embryo, but its precise distribution throughout development is very difficult to determine by available techniques. Examining the distribution of the enzymes by which it is synthesised by using in situ hybridisation is an alternative strategy. Here, we describe the distribution of three retinoic acid synthesising enzymes and one retinoic acid catabolic enzyme during the early stages of chick embryogenesis with the intention of identifying localized retinoic acid signalling regions. The enzymes involved are Raldh1, Raldh2, Raldh3, and Cyp26A1. Although some of these distributions have been described before, here we assemble them all in one species and several novel sites of enzyme expression are identified, including Hensen's node, the cardiac endoderm, the presumptive pancreatic endoderm, and the dorsal lens. This study emphasizes the dynamic pattern of expression of the enzymes that control the availability of retinoic acid as well as the role that retinoic acid plays in the development of many regions of the embryo throughout embryogenesis. This strategy provides a basis for understanding the phenotypes of retinoic acid teratology and retinoic acid-deficiency syndromes.

Retinoic acid down‐regulates Tbx1 expression in vivo and in vitro

“…The explanation for this discrepancy between the mouse and avian data may be that, in the VAD embryos, there is no dietary vitamin A. Thus, all the embryonic RA production pathways, including those mediated by retinol dehydrogenases, e.g., adh1 and adh4 (Rossant et al, 1991;Vonesch et al, 1994;Haselbeck and Duester, 1998), and retinaldehyde dehydrogenases such as Raldh1, 2 and 3 (Blentic et al, 2003;Fan et al, 2003) are blocked. In the hypomorphic raldh2 mouse mutants, only one RA-producing enzymatic pathway is partially blocked, perhaps resulting in a less severe effect upon Tbx1 expression.…”

Generating gradients of retinoic acid in the chick embryo: Cyp26C1 expression and a comparative analysis of the Cyp26 enzymes

“…The neuroepithelium, ectoderm, and endoderm are devoid of expression. This expression domain of Cyp26C1 in the mesoderm is remarkably similar to that of Cyp26A1, which is expressed in the neural plate (Swindell et al, 1999;Blentic et al, 2003), and to determine whether these two Cyps do in fact overlap, we performed a double in situ with these two genes. Figure 1D shows that Cyp26C1 expression (in turquoise) extends more laterally than that of Cyp26A1 (in purple).…”

“…great interest to determine its spatial relationship to Raldh2, the enzyme that generates RA in the mesoderm. The double in situ in Figure 1H reveals that there is a gap between the anterior border of Raldh2 (in purple) at the level of the presumptive first somite (Blentic et al, 2003) and Cyp26C1 (in turquoise). The red arrowheads in Figure 1H mark this region, which is where the posterior hindbrain will develop in the neural plate above.…”

“…The first member of the CYP26 family to be identified, Cyp26A1, has been described in the Xenopus, chick, and mouse embryo (Fujii et al, 1997;Hollemann et al, 1998;de Roos et al, 1999;Swindell et al, 1999;Blentic et al, 2003). It has a dynamic expression pattern in the anterior epiblast, the tail bud, and the dorsal neural tube.…”

“…This finding has led to the idea that a gradient of RA could be generated between these two domains across which the posterior hindbrain will develop and be patterned (Swindell et al, 1999;Maden, 1999). However, more detailed analyses have revealed that these two enzymes are expressed in completely different germ layers, Raldh2 is expressed in the mesoderm (subsequently the paraxial mesoderm), whereas Cyp26A1 is expressed in the epiblast (subsequently the anterior neuroepithelium; Blentic et al, 2003). This difference in tissue type is an unlikely scenario for generating a gradient.…”