Basement membranes (BM) are specialized extracellular matrices (ECM) which serve as complex interfaces between epithelia, peripheral nerves, or muscle cells and their surrounding tissue microenvironments. Their composition is known to include type IV collagen, laminin, entactin, heparan sulfate proteoglycan (HSPG, perlecan), and chondroitin sulfate proteoglycan (CSPG). By immunohistochemistry, collagen IV, laminin, and entactin are detectable from day 14 of gestation on, and become progressively more prominent with time. Perleaan has not been examined in prentatal lungs, but is widely distributed and abundant in all lung MBs from birth throughout development. CSPG has a somewhat discontinuous and lightly reactive appearance in alveolar BMs at birth but the staining becomes continuous and darker in the adult. This contrasts with glycosaminoglycan, chondroitin sulfate, which is prominently expressed in prenatal and early postnatal stages, but progressively diminishes with advancing development. As an interface between cell populations and surrounding ECMs, BMs act as a physical barrier to some cells and molecules, while serving as attachment points and binding sites for others. Basic fibroblast growth factor is an example of the latter, because it localizes with all BM components by immunostaining throughout development and reflects the multifactorial array of potential effectors in the complex processes of proliferation and differentiation.