Figure 8.

The challenge of embryonic tissue expansion within a confining basement membrane. A, The basement membrane barrier between epithelial and mesenchymal tissues must be able to expand along with bud expansion or extension during branching morphogenesis. B, Although degradation and thinning of the basement membrane are known to occur, absence of a controlled process would lead to tissue fragmentation and mixing of the highly motile epithelial cells into the surrounding mesenchyme—but this mixing does not occur in vivo. C, Basement membranes can become flexible by the formation of numerous microscopic holes or perforations that produce a lace‐like meshwork of basement membrane that can expand while still confining the epithelial cells [Colour figure can be viewed at wileyonlinelibrary.com]