How astrocytes close a critical period During the visual critical period, brain circuits are rewired to adjust to sensory input. Closure of the critical period stabilizes the circuits. Looking at development in the mouse visual cortex, Ribot et al. found that astrocytes increase their expression of the gap junction channel subunit connexin 30, which in turn inhibits expression of a matrix-degrading enzyme (see the Perspective by Kofuji and Araque). As the matrix stabilizes, inhibitory interneurons mature, and the unusual flexibility of the critical period comes to an end. Science , abf5273, this issue p. 77 ; see also abj6745, p. 29