Fate and function of the ventral ectodermal ridge during mouse tail development

In the mouse embryo, the body axis continues to develop after gastrulation as a tail forms at the posterior end of the embryo. Little is known about what controls outgrowth and patterning of the tail, but it has been speculated that the ventral ectodermal ridge (VER), a morphologically distinct ectoderm on the ventral surface near the tip of the tail, is a source of signals that regulate tail development (Grüneberg, H. (1956). Nature 177, 787-788). We tested this hypothesis by ablating all or part of the VER and assessing the effects of such ablations on the development of tail explants cultured in vitro. The data showed that the VER produces signals necessary for somitogenesis in the tail and that the cells that produce these signals are localized in the middle and posterior region of the VER. Dye labeling experiments revealed that cells from these regions move anteriorly within the VER and eventually exit it, thereby colonizing the ventral surface ectoderm anterior to the VER. In situ hybridization analysis showed that the genes encoding the signaling molecules FGF17 and BMP2 are specifically expressed in the VER. Assays for gene expression in VER-ablated and control tails were performed to identify targets of VER signaling. The data showed that the VER is required for expression of the gene encoding the BMP antagonist noggin in the tail ventral mesoderm, leading us to speculate that one of the major functions of the VER in tail development is to regulate BMP activity.