## 2 Understanding the Basement Membrane  
The BM is a specialized form of an extracellular matrix (ECM) found underneath all polarized epithelial cells of eumetazoans [11; 12; 13]. It is a dynamic structure that is constantly deposited and modified by the epithelial cells, endothelial cells and fibroblasts [12; 14]. The BM not only provides mechanical support to the overlying tissues but also regulates tissue architecture and its function by interacting with the cells and the surrounding environment [12]. Hence, there is a variation in the BM thickness, composition and function with its location (cell type), physiological state and pathophysiological state [12; 15]. BM is broadly divided into three layers, namely lamina lucida, lamina densa and lamina fibroreticularis. Of the three, lamina densa is the most important layer, which is separated from the cells and the extracellular matrix by lamina lucida and lamina fibroreticularis, respectively [11]. Lamina lucida is a less dense zone, through which the components of lamina densa interact with the cell surface receptors (integrin and non-integrin proteins) [11; 16] The thickness of lamina fibroreticularis, on the other hand, varies from place to place with a well-demarcated structure under the respiratory epithelium to entirely absent in capillary walls of the kidneys [11]. The core components of lamina densa are type IV collagen, laminin, nidogen, and heparin sulfate proteoglycan/perlecan of which type IV collagen and laminin form two independent, self-polymerizing networks that are connected to one another by nidogen and perlecan [17]. Laminin is a heterotrimer made of \(\alpha\), \(\beta\) and \(\gamma\) chains, which form a cruciform structure, whereas type IV collagen is a heterotrimer made of three different types of alpha chains [16]. The complex assembly of the BM is initiated by the binding of laminin to cell surface receptors such as sulfated glycolipid, integrin, and dystroglycan [18]. Once bound, a cascade of reactions takes place starting with self-polymerization of laminin, accumulation of other core components, bridging of laminin to type IV collagen by nidogen and polymerization of type IV collagen to form the covalently stable network [18]. Although this network provides the BM with its characteristic mechanical strength, their receptor binding and cell signaling properties differ with the kind of core component isoforms [18]. For instance, there are three isoforms of type IV collagen and 16 different isoforms of laminin distributed in different BM in a tissue-specific manner, imparting it with its unique structure and function [19]. Other components of BM are collagen III, collagen V, collagen VI, collagen VII, collagen XVII and collagen XVIII [19]. The BM is also a reservoir filled with growth factors such as transforming growth factor \(\beta\), fibroblast growth factor, vascular endothelial growth factor, and heparin epidermal growth factor, as well as various proteases and cytokines, making it the most complex structure to be synthetically replicated in-vitro [11; 19; 20]. Moreover, their dynamic nature and modification with age and disease adds to the challenge. Nanotopography of the BM plays a very important role in cell growth and function [21]. The nanoscale variations on the surface present the cell with contact guidance, which modulates the cellular response in many ways [21]. Similarly, the size, geometry, roughness, stiffness, and porosity of the synthetic scaffold significantly control the cell behavior [22; 23]. Hence, to efficiently fabricate a synthetic membrane in vitro, it is important to understand the topography of the native BM. Generally, BM is considered to be of \(<\)100 nm thickness with 400 nm long collagen IV forming a flat horizontal stretch along the interconnected network [24; 25]. However, this configuration of the BM does not provide the amount of flexibility required during tissue growth and for organs (e.g., skin, lung, and vasculature), which are constantly changing their shape [26].