![]() Our result implies that, in transplantation assay, embryos have potential to form ectopic tail without graft. We showed that this ectopic tail formation requires physical contact between BCR and blastocoel floor (BCF), and wound per se seems not needed. Analyses of marker gene expression revealed that this process of ectopic tail formation reflects that of original tail formation. In the present study, we found that an ectopic tail is formed when a slit is made on the BCR at the early gastrula stage in Xenopus. Hence, those reports led us to hypothesize that a secondary tail is formed when blastocoel roof (BCR) is simply wounded during normal development. ( 2009) reported that a mechanically relaxed embryo made by orthotopic transplantation of a suprablastoporal area at gastrula stage often possesses a tail-like protrusion and wounding activates ERK (LaBonne & Whitman 1997 Christen & Slack 1999), which is downstream of FGF signaling. In many studies, tail organizer activity has been assessed by the Einsteck procedure (Gont et al. 2001), the mechanism of early tail formation is still only partially understood. Whereas the mechanism of tail bud outgrowth has begun to be revealed (Beck & Slack 1999 Beck et al. ![]() In addition, Tucker and Slack presented a model for tail formation in Xenopus that a tail bud will be determined when the junction of mesodermal and neural regions of the posterior neural plate directly overlies the underlying dorsal mesoderm (Tucker & Slack 1995a). Conversely, the injection of flrt3, which promotes FGF signaling, can generate ectopic tails (Bottcher et al. Blocking of fibroblast growth factor (FGF) signaling in Xenopus embryo by the expression of a dominant negative mutant form of the FGF type I receptor (dnFGFR1) can give rise to embryos with normal heads and anterior trunk tissues, but not tails (Amaya et al. The molecular mechanism of tail formation is gradually being revealed by recent loss-of-function experiments. After the discovery of the Spemann–Mangold organizer, many studies were performed to identify the head-inducing factor (Nakamura & Kawakami 1977), whereas few focused on tail induction. The tail is a continuation of the structures of the main body axis posterior to the anus and contains the neural tube, the notochord, and somites. However, it is reported that the results obtained by those two methods differ (Nakamura & Kawakami 1977) and hence, we should consider the results carefully. Because of ease of use, the Einsteck procedure is applied to various grafts, such as injected animal caps (ACs) (Ruiz i Altaba & Melton 1989). One involves graft transplantation into the ventral side of a host gastrula, and the other is the Einsteck procedure, which involves graft insertion into the host gastrula blastocoel. ![]() There are two main transplantation methods to assess organizer activity. These important findings were derived from transplantation assays. Thereafter, Spemann showed that the organizer activity changes by developmental stage: the dorsal lip of early gastrulae has head-to-tail inducing activity, whereas that of late gastrulae can induce only the tail structure (Spemann 1931). In 1924, Spemann and Mangold revealed that the dorsal blastopore lip of amphibians can induce a new body axis (Spemann & Mangold 1924), and today, this region is called the Spemann–Mangold organizer. Our results imply that the secondary tail formation reflects the developmental processes of the original tail, indicating that simple wounding of BCR is useful for the analysis of tail formation in normal development. This secondary tail was similar to the original tail in the expression pattern of tail genes, and in the fact that the inhibition of fibroblast growth factor signaling prevented the secondary tail induction. Therefore, the secondary tail might be induced by the contact between BCR and BCF due to the leak of blastocoel fluid from the wound. We revealed that the ectopic expression of Xbra was blocked by inhibiting the contact between BCR and blastocoel floor (BCF), and wounding per se seemed to be not directly related to the secondary tail formation. However, we found that simple wounding of blastocoel roof (BCR) made it possible to form secondary tails without any transplantation in Xenopus laevis. The tail organizer has been assessed by such transplantation methods as the Einsteck procedure.
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