Ectodermal Wnt6 is an early negative regulator of limb chondrogenesis in the chicken embryo
1 Institute of Anatomy and Cell Biology, Department of Molecular Embryology, University of Freiburg, Albertstrasse 17, D-79104 Freiburg, Germany
2 Present address: Department of Oral Health Sciences, Life Sciences Institute, University of British Columbia, Life Sciences Centre, 2350 Health Sciences Mall, Vancouver, BC, V6T 1Z3, Canada
3 Present address: Insitute of Anatomy, Department of Neuroanatomy, Nussallee, D-53115 Bonn, Germany
BMC Developmental Biology 2010, 10:32 doi:10.1186/1471-213X-10-32Published: 25 March 2010
Pattern formation of the limb skeleton is regulated by a complex interplay of signaling centers located in the ectodermal sheath and mesenchymal core of the limb anlagen, which results, in the forelimb, in the coordinate array of humerus, radius, ulna, carpals, metacarpals and digits. Much less understood is why skeletal elements form only in the central mesenchyme of the limb, whereas muscle anlagen develop in the peripheral mesenchyme ensheathing the chondrogenic center. Classical studies have suggested a role of the limb ectoderm as a negative regulator of limb chondrogenesis.
In this paper, we investigated the molecular nature of the inhibitory influence of the ectoderm on limb chondrogenesis in the avian embryo in vivo. We show that ectoderm ablation in the early limb bud leads to increased and ectopic expression of early chondrogenic marker genes like Sox9 and Collagen II, indicating that the limb ectoderm inhibits limb chondrogenesis at an early stage of the chondrogenic cascade. To investigate the molecular nature of the inhibitory influence of the ectoderm, we ectopically expressed Wnt6, which is presently the only known Wnt expressed throughout the avian limb ectoderm, and found that Wnt6 overexpression leads to reduced expression of the early chondrogenic marker genes Sox9 and Collagen II.
Our results suggest that the inhibitory influence of the ectoderm on limb chondrogenesis acts on an early stage of chondrogenesis upsteam of Sox9 and Collagen II. We identify Wnt6 as a candidate mediator of ectodermal chondrogenic inhibition in vivo. We propose a model of Wnt-mediated centripetal patterning of the limb by the surface ectoderm.