Brachet's cleft: a model for the analysis of tissue separation in Xenopus

Tissue border formation is an important process that prevents mixing of cells during embryonic development. The establishment of tissue borders is not a trivial problem, particularly in early embryos when cells and tissues are not fully differentiated. An example of an early tissue separation process is the formation of Brachet's cleft in Xenopus. During early gastrulation, this morphologically visible cleft separates mesendoderm and ectoderm. Over the last decade, it was recognized that morphogenetic processes, including tissue separation, can be experimentally uncoupled from embryonic patterning events. In this study, we summarize the data explaining the regulation of Brachet's cleft and introduce the experimental arsenal that was used for this analysis. The formation of Brachet's cleft involves the activity of transcription factors, cell adhesion molecules, and signaling modules, which act in a complex regulatory network. According to the current state of knowledge, Rho signaling seems to be the central player during this process. The mechanisms that regulate Rho during tissue separation and the experimental approaches to monitor Rho activity are discussed.