Catala M
Service d'Histologie, Embryologie et Cytogénétique, Groupe Hospitalier Pitié-Salpêtrière, Paris, France.
Morphologie. 2000 Jun;84(265):17-23.
Limbs are formed after a series of complex interactions between tissues derived from the embryonic layers (surface ectoderm and somatopleural mesoderm). Data at both cellular and molecular levels are numerous. The developing limb becomes thus one of the best known system in the field of developmental biology. The limb bud derives from numerous embryonic tissues (surface ectoderm, somatopleural mesoderm, neural crest cells, somites, hematopoietic cells deriving from the splanchnopleura that gives rise to the aortic buds). Multiple interactions take place between these tissues. So, the definitive shape of a structure could depend on only one of its component. For example, the muscular shape is not dependent on the origin of muscle fibers but on the origin of somatopleural cells. Superior (or anterior) and inferior (or posterior) limbs differ by their shape. Some genes expressed by only one of the limb are known but genes specifying the identity of the limbs are still totally unknown. Three axes develop during the formation of the limb buds: proximo-distal, antero-posterior and ventro-dorsal. The formation of the proximo-distal axis is due to the induction of the apical ectodermal ridge, a specialized region of the surface ectoderm in which cells are prismatic and associated by gap junctions. The family of secreted FGF and their receptors play a major role in the regulation of the development of this axis. Furthermore, some genes from Hox complexes a and d participate into the regulation of the positional identity along this axis. The antero-posterior axis is determined by the zone of polarizing activity that secretes Sonic hedgehog. This molecules fashions a gradient of concentration that differentiates the future antero-posterior derivatives. At last, the ventro-dorsal axis depends on the surface ectoderm that have received first positional informations from the mesoderm. The dorsal region is specified by Wnt7a and Lmx1 whereas Engrailed 1 gene plays a role in ventral specification.
肢体是在源自胚胎层（表面外胚层和体壁中胚层）的组织之间进行一系列复杂相互作用后形成的。细胞和分子水平的数据众多。因此，发育中的肢体成为发育生物学领域中最知名的系统之一。肢芽源自众多胚胎组织（表面外胚层、体壁中胚层、神经嵴细胞、体节、源自产生主动脉芽的脏壁的造血细胞）。这些组织之间发生多种相互作用。所以，一个结构的最终形状可能仅取决于其一个组成部分。例如，肌肉形状不取决于肌纤维的起源，而是取决于体壁细胞的起源。上肢（或前肢）和下肢（或后肢）在形状上有所不同。一些仅在一个肢体中表达的基因是已知的，但决定肢体特征的基因仍然完全未知。在肢芽形成过程中会形成三个轴：近-远轴、前-后轴和腹-背轴。近-远轴的形成是由于顶端外胚层嵴的诱导，顶端外胚层嵴是表面外胚层的一个特殊区域，其中细胞呈棱柱形并通过间隙连接相连。分泌型FGF家族及其受体在该轴的发育调节中起主要作用。此外，来自Hox复合体a和d的一些基因参与了沿该轴的位置特征调节。前-后轴由分泌音猬因子的极化活性区决定。该分子形成一个浓度梯度，使未来的前后衍生物得以区分。最后，腹-背轴取决于首先从中胚层接收位置信息的表面外胚层。背侧区域由Wnt7a和Lmx1指定，而 engrailed 1基因在腹侧指定中起作用。
