The primary mouth - the initial opening connecting the embryonic foregut to the outside world - forms from a mesoderm-free domain at the front of deuterostome embryos (in which the mouth forms after the anus). Early in the mouth's formation, the basement membrane between the ectoderm and endoderm in this domain disappears. Dickinson and Sive now report that Wnt antagonists locally regulate this process in Xenopus embryos (see ). The researchers show that frzb-1 and crescent - which encode secreted frizzled-related proteins (sFRPs), a class of Wnt antagonists - are transiently and locally expressed in the primary mouth anlage. Other experiments indicate that sFRP function is crucial for primary mouth formation and that Frzb-1 overexpression decreases the expression of the basement membrane genes fibronectin and laminin, whereas Wnt-8 overexpression increases their expression. These data, which are the first to connect Wnt signalling and basement membrane integrity during primary mouth development, suggest that the modulation of Wnt signalling might regulate basement membrane remodelling during other developmental processes.
