Han M J, An J Y, Kim W S
Department of Life Science, Sogang University, Seoul, Korea.
Dev Dyn. 2001 Jan;220(1):40-8. doi: 10.1002/1097-0177(2000)9999:9999<::AID-DVDY1085>3.0.CO;2-8.
Fgf-8 is one of the key signaling molecules implicated in the initiation, outgrowth, and patterning of vertebrate limbs. However, it is not clear whether FGF-8 plays similar role in development and regeneration of urodele limbs. We isolated a Fgf-8 cDNA from the Mexican axolotl (Ambystoma mexicanum) through the screening of an embryo cDNA library. The cloned 1.26-kb cDNA contained an open reading frame encoding 212 amino acid residues with 84%, 86%, and 80% amino acid identities to those of Xenopus, chick, and mouse, respectively. By using the above clone as a probe, we examined the temporal and spatial expression patterns of Fgf-8 in developing embryos and in regenerating larval limbs. In developing embryos, Fgf-8 was expressed in the neural fold, midbrain-hindbrain junction, tail and limb buds, pharyngeal clefts, and primordia of maxilla and mandible. In the developing axolotl limb, Fgf-8 began to be expressed in the prospective forelimb region at pre-limb-bud and limb bud stages. Interestingly, strong expression was detected in the mesenchymal tissue of the limb bud before digit forming stages. In the regenerating limb, Fgf-8 expression was noted in the basal layer of the apical epithelial cap (AEC) and the underlying thin layer of mesenchymal tissue during 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 or in amniotes such as chick and mouse, the Fgf-8 expression domain was localized 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 expression of Fgf-8 in the deep layer of the AEC and the thin layer of underlying mesenchymal tissue in the regenerating limbs support the previous notion that the amphibian AEC is a functional equivalent of the AER in amniotes.
Fgf - 8是参与脊椎动物肢体起始、生长和模式形成的关键信号分子之一。然而，尚不清楚FGF - 8在有尾目动物肢体的发育和再生中是否发挥类似作用。我们通过筛选墨西哥钝口螈（美西螈）胚胎cDNA文库分离出一个Fgf - 8 cDNA。克隆得到的1.26 kb cDNA包含一个开放阅读框，编码212个氨基酸残基，与非洲爪蟾、鸡和小鼠的氨基酸同一性分别为84%、86%和80%。以上述克隆为探针，我们检测了Fgf - 8在发育胚胎和再生幼体肢体中的时空表达模式。在发育胚胎中，Fgf - 8在神经褶、中脑 - 后脑交界处、尾巴和肢芽、咽裂以及上颌和下颌原基中表达。在发育中的美西螈肢体中，Fgf - 8在肢芽前期和肢芽期开始在前肢预期区域表达。有趣的是，在指形成阶段之前，在肢芽的间充质组织中检测到强表达。在再生肢体中，在芽基形成阶段，在顶端上皮帽（AEC）的基底层和下方薄层间充质组织中观察到Fgf - 8表达。这些数据表明Fgf - 8参与各种颅面结构的器官发生、肢体发育的起始和生长以及再生肢体的芽基形成和生长。在美西螈发育中的肢体中，与非洲爪蟾或鸡和小鼠等羊膜动物不同，Fgf - 8表达域主要定位于间充质而非表皮。Fgf - 8在美西螈中的独特表达模式表明，有尾目动物和其他高等物种之间Fgf - 8表达的调控机制不同。Fgf - 8在再生肢体的AEC深层和下方薄层间充质组织中的表达支持了先前的观点，即两栖动物的AEC在功能上等同于羊膜动物的顶外胚层嵴（AER）。
