The Drosophila central nervous system (CNS) is covered by a thick basement membrane that mediates interactions with glial cells and governs the shape of the tissue. Basement membranes are formed from a meshwork of secreted extracellular matrix (ECM) proteins and must be continually remodelled to accommodate growth during development. Metalloproteinases, such as those in the AdamTS family, can break down ECM proteins in the basement membrane and thus allow for tissue expansion. On p. 3102, James Skeath and colleagues identify one protein in the family, AdamTS-A, that is critical for both maintaining the structural integrity of the Drosophila CNS and keeping cell lineages anchored in the tissue. When the function of this protein is reduced, neurons escape the CNS and invade the peripheral tissues of the developing larva. AdamTS-A acts through protease-dependent and possibly protease-independent mechanisms to retain neurons within the developing CNS, and regulates tissue stiffness by restricting Collagen IV/Viking accumulation in the basement membrane. These findings demonstrate the crucial role for extracellular proteases in tissue development and highlight the importance of the basement membrane in shaping the nervous system.