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Zheng et al. 2005). Laminins are large heterotrimeric in BM composition result from modifications in the glycoproteins consisting of aa, b, andg chain, encoded byDOI: 10.1530/REP-07-0561specific genes named Lama, Lamb, and Lamc respectively. q 2008 Society for Reproduction and Fertilitybalance between ECM component synthesis and degradation, and two major groups of enzymes areIntroduction Collagen type IV, laminins, nidogens, and heparan sulfate proteoglycans are essential components of basement membranes (BMs). BMs are specialized structures of the extracellular matrix (ECM) with multiple functions, including proliferation, migration, survival, and differen-tiation (Timpl 1996, Timpl & Brown 1996). Collagen IV is essential for BM stability. It is composed of three a chains assembled in a triple helix. They are six different chains of collagen type IV, a1–a6, each encoded by a separate gene, but only three specific protomers, a1a1a2, a3a4a5, and a5a5a6 (Timpl 1996). While a1(IV) and a2(IV) chains are ubiquitously present in BMs, the a3–a6 (chains) generally have a more restricted distribution (Hudson et al. 2003, To date, five a chains, three b chains, and three g chains have been identified, from which 16 laminin heterotrimers have been characterized in vivo (Aumailley et al. 2005). Laminincan bind tocollagen IV, entactin/nidogen, heparin sulfate proteoglycan and itself, and is thought to have an organizing role in matrix assembly and the resulting supramolecular architecture (Colognato & Yurchenko 2000). Thus, the composition of BMs can be highly variable depending on the types of laminin and collagen type IV, and it is now recognized that the unique composition of each BM contributes to its specific functional properties. In addition, its composition may change during development and growth, or in response to injury or an environmental or hormonal stimulus (Ender