求翻译：During the early stages of body axis extension, retinoic acid (RA) synthesized in somites by Raldh2 represses caudal FGF signaling to limit the tailbud progenitor zone. Excessive RA down-regulates Fgf8 and triggers premature termination of body axis extension, suggesting that endogenous RA may function in normal termination of body axis extension. Here, we demonstrate that Raldh2-/- mouse embryos undergo normal down-regulation of tailbud Fgf8 expression and termination of body axis extension. Interestingly, Raldh2 expression in tail somites and tailbud from E10.5 onwards does not result in RA activity monitored by RARE-lacZ. Treatment of tailbuds with physiological levels of RA or retinaldehyde induces RARE-lacZ activity, validating the sensitivity of RARE-lacZ and demonstrating deficient RA synthesis in tail somites and tailbud due to a lack of retinaldehyde synthesis. These studies demonstrate an early uncoupling of RA signaling from mouse tailbud development and show that termination of body axis extension occurs in the absence of RA signaling. 是什么意思？

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