Splanchnic (visceral) mesoderm has limb-forming ability according to the position along the rostrocaudal axis in chick embryos

Resolving the mechanisms underlying epithelial-to-mesenchymal transition of the lateral plate mesoderm

Formation of the vertebrate limb buds begins with a localized epithelial-to-mesenchymal transition (EMT) of the somatic lateral plate mesoderm (LPM). While the processes that drive proliferation and outgrowth of the limb mesenchyme are well established, the fundamental mechanisms that precede this process and initiate EMT are less understood. In this review, we outline putative drivers of EMT of the LPM, drawing from analyses across a range of vertebrates and developmental models. We detail the expression patterns of key EMT transcriptional regulators in the somatic LPM of the presumptive limb fields, and their potential role in producing a mesenchymal cell fate. These include a putative cooperative role between the EMT inducers PRRX1 and TWIST1, supported by evidence in zebrafish and chicken models but unconfirmed data from mice. As such, additional functional data are required to definitively determine the mechanisms that initiate and drive EMT of the somatic LPM, a critical transition preceding formation of the limb bud mesenchyme.

How the embryo makes a limb: determination, polarity and identity

The vertebrate limb with its complex anatomy develops from a small bud of undifferentiated mesoderm cells encased in ectoderm. The bud has its own intrinsic polarity and can develop autonomously into a limb without reference to the rest of the embryo. In this review, recent advances are integrated with classical embryology, carried out mainly in chick embryos, to present an overview of how the embryo makes a limb bud. We will focus on how mesoderm cells in precise locations in the embryo become determined to form a limb and express the key transcription factors Tbx4 (leg/hindlimb) or Tbx5 (wing/forelimb). These Tbx transcription factors have equivalent functions in the control of bud formation by initiating a signalling cascade involving Wnts and fibroblast growth factors (FGFs) and by regulating recruitment of mesenchymal cells from the coelomic epithelium into the bud. The mesoderm that will form limb buds and the polarity of the buds is determined with respect to both antero-posterior and dorso-ventral axes of the body. The position in which a bud develops along the antero-posterior axis of the body will also determine its identity - wing/forelimb or leg/hindlimb. Hox gene activity, under the influence of retinoic acid signalling, is directly linked with the initiation of Tbx5 gene expression in the region along the antero-posterior axis of the body that will form wings/forelimbs and determines antero-posterior polarity of the buds. In contrast, Tbx4 expression in the regions that will form legs/hindlimbs is regulated by the homeoprotein Pitx1 and there is no evidence that Hox genes determine antero-posterior polarity of the buds. Bone morphogenetic protein (BMP) signalling determines the region along the dorso-ventral axis of the body in which both wings/forelimbs and legs/hindlimbs develop and dorso-ventral polarity of the buds. The polarity of the buds leads to the establishment of signalling regions - the dorsal and ventral ectoderm, producing Wnts and BMPs, respectively, the apical ectodermal ridge producing fibroblast growth factors and the polarizing region, Sonic hedgehog (Shh). These signals are the same in both wings/forelimbs and legs/hindlimbs and control growth and pattern formation by providing the mesoderm cells of the limb bud as it develops with positional information. The precise anatomy of the limb depends on the mesoderm cells in the developing bud interpreting positional information according to their identity - determined by Pitx1 in hindlimbs - and genotype. The competence to form a limb extends along the entire antero-posterior axis of the trunk - with Hox gene activity inhibiting the formation of forelimbs in the interlimb region - and also along the dorso-ventral axis.