Proteoglycan arrays in the cochlear basement membrane

Indirect immunofluorescence and transmission electron microscopy were used to investigate the composition and assembly of proteoglycans in the basemen t membranes of the spiral limbus, basilar membrane, spiral ligament, Reissn er's membrane, myelinated nerve fibers, and blood capillaries of the spiral ligament and stria vascularis in the chinchilla cochlea. Four types of bas ement membrane components: laminin, entactin/nidogen, type IV collagen and heparan sulfate proteoglycans were immunolocalized in all basement membrane s in association with heparan sulfate proteoglycans. beta 1 and alpha 1 int egrin subunits were also detected along these basement membranes. The conce ntration of the basement membrane-associated proteins and integrin subunits differed according to the adjacent cell type. Electron microscopy showed t hat all basement membranes, with exception of those of stria vascularis, co nsist of two layers: lamina lucida and lamina densa. In the stria vasculari s only a homogeneous lamina densa was observed. Cuprolinic blue treatment r evealed heterogeneity in the ultrastructure and arrangement of proteoglycan s in the cochlear basement membranes. Proteoglycans of the subepithelial ba sement membrane in the spiral limbus and spiral ligament formed quasi-regul ar, linear arrays within the lamina lucida, or were located at both sides o f the lamina densa in the basilar membrane and Reissner's membrane. In the basement membranes of nerve fibers, and capillaries in the spiral ligament and stria vascularis, proteoglycans were scattered throughout these basemen t membranes, but showed different concentration and ultrastructural appeara nce, which may be related to different filtration and mechanical properties . In the basilar membrane, PGs were located above and below the lamina dens a. An additional layer of PGs below the lamina densa may function as increa sed mechanical support of organ of Corti by its interaction with underlying fibrillar collagen layer. In the stria vascularis capillaries, PGs were st ained considerably less with Cuprolinic blue and were scattered through the lamina densa of the basement membrane compared to capillaries of spiral li gament. This observation is compatible with a higher permeability of the st rial capillaries. (C) 2001 Elsevier Science B.V. All rights reserved.