A role for epithelial-mesenchymal interactions in tail growth/morphogenesis and chondrogenesis in embryonic mice.
Neurulation involves development from primary germ layers before any differentiation of embryonic mesenchyme. Subsequently, secondary organogenesis is via epithelial-mesenchymal interaction. It is unclear whether formation of the caudal body axis and tail bud in vertebrate embryos is by temporal and causal extension of primary neurulation, by secondary neurulation, or by secondary induction (epithelial-mesenchymal interactions) as seen in organogenesis of the limb buds, kidneys, heart and other embryonic regions. Reports of a ventral ectodermal ridge (VER) associated with tail bud development in rodent embryos imply that tail bud development may share features with limb bud development, in which the apical ectodermal ridge (AER) directs limb bud outgrowth and skeletal patterning. Organ culture or grafting to the chorioallantoic membranes of host chick embryos, of tail bud mesenchyme with or without tail epithelium, demonstrates that both survival and growth of tail mesenchyme depend on the presence of tail epithelium. Initiation of chondrogenesis of tail mesenchyme was similarly dependent on tail epithelium until 10.5 days of gestation, which is when the VER is at its maximal extent. Initiation of myogenesis was independent of the presence of tail epithelium. These results are discussed in relation to the similarity of tail bud to limb bud developed, and to the different mechanisms employed in differentiation of the cranial and caudal ends of vertebrate embryos. Secondary induction of the caudal body region is argued to be fundamental in vertebrate embryogenesis.