Figure 3

Schematic diagram illustrating expression of the genes that are involved in RA biosynthesis and transportation at gastrula (stage 11) and neurula (stage 14) stages of Xenopus embryos. rdh10 is expressed in the circumblastoporal region of Xenopus gastrula, and the signals at the dorsal side form a zone extending anteriorly. At stage14, the signals are found in trunk paraxial mesoderm region [46]. rdh10 is not expressed in the notochord. raldh2 is expressed in a form of a ring around the vegetal pole with signals more intense in the dorsal blastpore lip. During neurula stages, raldh2 signals are mainly distributed in the trunk paraxial mesoderm, expanding ventrally [38]. rdh10 and raldh2 display overlapping expression pattern in the trunk paraxial mesoderm, with rdh10 expressed more anteriorly than raldh2. cyp26a is expressed in two primary domains at stage11, the posterior domain surrounding the blastopore and the anterior domain covering the anterior part of prospective neural plate. At stage 14, the anterior cyp26a transcripts is developed into three elements corresponding to the cement gland anlage, the mid-/hindbrain boundary, and the auditory placodes, while the posterior expression domain remains in the circumblastoporal area, and the developing neural plate is also covered by a gradient of cyp26a signals with the highest present in posterior region [92]. The expression domains of radlh2 and cyp26a do not overlap at gastrula and neurula stages of Xenopus embryos. rarα2 is expressed in the involuting surface layer surrounding the blastopore, and becomes stronger as the gastrulation proceeds. At stage 14, rarα2 expression is expressed predominantly in the posterior neural plate of the embryos [167]. The expression of the carbp-II is defined into an anterior and a posterior domain at gastrula stages. In the anterior domain, Xenopus crabp-II is limited to the dorsal area which generates prospective head structures. At the neurula stages, crabp-II is expressed in the prospective telencephalon and rhombencephalon, and the most posterior region of the embryos [168]. The dhrs3 signals form a circumblastoporal ring which is similar to rdh10 at stage 11. The signals in the neural plate form two signaling zones and gradually converge towards the midline, forming two signal strips extending posteriorly. In addition, dhrs3 is expressed in the notochord at neurula stages [169]. All these drawings are shown in dorsal view and the blue color represents expression signals.