Expression patterns of Fgf-8 during development and limb regeneration of the axolotl

Fgf-8 is one of the key signaling molecules implicated in the initiation, o utgrowth, and patterning of vertebrate limbs. However, it is not clear whet her FGF-8 plays similar role in development and regeneration of urodele lim bs. We isolated a Fgf-8 cDNA from the Mexican axolot1 (Ambystoma mexicanum) through the screening of an embryo cDNA library. The cloned 1.26-kb cDNA c ontained an open reading frame encoding 212 amino acid residues with 84%, 8 6%, and 80% amino acid identities to those of Xenopus, chick, and mouse, re spectively. By using the above clone as a probe, we examined the temporal a nd spatial expression patterns of Fgf-8 in developing embryos and in regene rating larval limbs. In developing embryos, Fgf-8 was expressed in the neur al fold, midbrain-hindbrain junction, tail and limb buds, pharyngeal clefts , and primordia of maxilla and mandible. In the developing axolotl limb, Fg f-8 began to be expressed in the prospective forelimb region at pre-limb-bu d and limb bud stages. Interestingly, strong expression was detected in the mesenchymal tissue of the limb bud before digit forming stages. In the reg enerating limb, Fgf-8 expression was noted in the basal layer of the apical epithelial cap (AEC) and the underlying thin layer of mesenchymal tissue d uring blastema formation stages. These data suggest that Fgf-8 is involved in the organogenesis of various craniofacial structures, the initiation and outgrowth of limb development, and the blastema formation and outgrowth of regenerating limbs. In the developing limb of axolotl, unlike in Xenopus o r in amniotes such as chick and mouse, the Fgf-8 expression domain was loca lized mainly in the mesenchyme rather than epidermis. The unique expression pattern of Fgf-8 in axolotl suggests that the regulatory mechanism of Fgf- 8 expression is different between urodeles and other higher species. The ex pression of Fgf-8 in the deep layer of the AEC and the thin layer of underl ying mesenchymal tissue in the regenerating limbs support the previous noti on that the amphibian AEC is a functional equivalent of the AER in amniotes . (C) 2001 Wiley-Liss, Inc.