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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.
Figure 1. Brachet's cleft. (a) Schematic drawing of an early gastrula showing the formation of the morphologically visible Brachet's cleft when tissue separation of mesendoderm and ectoderm takes place. (b) Histological section of a Xenopus gastrula stained for fibronectin (yellow), C-cadherin-stained cell–cell borders (red), and 4′,6-diamidino-2-phenylindole (DAPI)-stained nuclei (blue). Fibronectin marks the tissue border between mesendoderm and ectoderm. Black arrowheads mark Brachet's cleft. The white arrowhead marks the dorsal blastopore lip. Fibronectin staining and imaging were performed by C. Berger. (Reprinted with permission from Ref 7. Copyright 2004 Cold Spring Harbor)
Figure 2. Blastocoel roof assay (BCR). Schematic drawing of the BCR assay that tests for mesendodermal separation behavior and ectodermal repulsion behavior. Cell aggregates (circles) are placed on blastocoel roof. Separation behavior is scored after 30 min. Ectodermal (yellow circle) and dorsal mesodermal (purple circle) cell aggregates integrate when placed on the blastocoel roof. *Note that the dorsal mesoderm develops separation behavior only during involution and then separates, indicating that tissue separation is temporally controlled. The leading edge mesendoderm (orange circle) already achieves separation behavior and stays on top of the blastocoel roof. Photographic images: ectodermal and dorsal mesodermal aggregates (purple and yellow border) appear reintegrated into the blastocoel roof and leading edge mesendodermal aggregates (orange border) form a bulk on the surface of the blastocoel roof, indicating tissue separation. Photographic images were provided by M. Jungwirth. (Reprinted with permission from Ref 7. Copyright 2004 Cold Spring Harbor)
Figure 3. Molecular components involved in tissue separation during Brachet's cleft formation. Schematic summary of molecules involved in mesendodermal separation behavior and ectodermal repulsion behavior. Blue numbers refer to publications cited in this article.
