Induction and prepatterning of the zebrafish pectoral fin bud requires axial retinoic acid signaling

Abstract

Vertebrate forelimbs arise as bilateral appendages from the lateral plate mesoderm (LPM). Mutants in aldh1a2 (raldh2), an embryonically expressed gene encoding a retinoic acid (RA)-synthesizing enzyme, have been used to show that limb development and patterning of the limb bud are crucially dependent on RA signaling. However, the timing and cellular origin of RA signaling in these processes have remained poorly resolved. We have used genetics and chemical modulators of RA signaling to resolve these issues in the zebrafish. By rescuing pectoral fin induction in the aldh1a2/neckless mutant with exogenous RA and by blocking RA signaling in wild-type embryos, we find that RA acts as a permissive signal that is required during the six- to eight-somite stages for pectoral fin induction. Cell-transplantation experiments show that RA production is not only crucially required from flanking somites, but is sufficient to permit fin bud initiation when the trunk mesoderm is genetically ablated. Under the latter condition, intermediate mesoderm alone cannot induce the pectoral fin field in the LPM. We further show that induction of the fin field is directly followed by a continued requirement for somite-derived RA signaling to establish a prepattern of anteroposterior fates in the condensing fin mesenchyme. This process is mediated by the maintained expression of the transcription factor hand2, through which the fin field is continuously posteriorized, and lasts up to several hours prior to limb-budding. Thus, RA signaling from flanking somites plays a dual early role in the condensing limb bud mesenchyme.

Transcriptomic, proteomic, and metabolomic landscape of positional memory in the caudal fin of zebrafish

“…This difference may be due to genetic or morphological differences among species. However, RA signaling is important for fin development and regeneration (67)(68)(69)(70). We identified several members of the RA signaling pathway with opposing patterns of differential expression along the proximodistal axis of uninjured fins.…”

Fate of retinoic acid–activated embryonic cell lineages

“…In any event, our data show that the distal limb mesenchyme or ''progress zone,'' although actively devoid of RA due to the activity of the CYP26B1 enzyme (Yashiro et al, 2004), is not a population that has never been exposed to the RA signal. Thus, in light of recent models describing the role of RA during limb budding (Gibert et al, 2006;Zhao et al, 2009), it seems that the distal region of the forming bud progressively clears all the RA required at earlier stages for limb bud initiation.…”

Regulation of limb bud initiation and limb‐type morphology

“…In chick as well as in zebrafish, application of chemical inhibitors of retinaladehyde dehydrogenases, enzymes essential for RA synthesis, prevents limb bud formation (Stratford et al, 1996;Grandel et al, 2002;Gibert et al, 2006). Mice lacking the retinaladehyde dehydrogenase, Raldh2, do not form forelimb buds and die by embryonic day (E) 10 (Niederreither et al, 1999).…”

“…Mice lacking the retinaladehyde dehydrogenase, Raldh2, do not form forelimb buds and die by embryonic day (E) 10 (Niederreither et al, 1999). Similarly, the zebrafish raldh2 mutants, no fin and necklace, fail to form pectoral fins (the equivalent of the forelimb in fish) (Grandel et al, 2002;Gibert et al, 2006). Zebrafish raldh2 mutants die during early larval stages (Grandel et al, 2002) prior to the stage of pelvic fin budding that occurs after three weeks of development (Grandel and Schulte-Merker, 1998) and, consequently, the role of RA during pelvic fin (the equivalent of the hindlimb in fish) development has not been addressed.…”