Basement membranes (BMs) are considered to be uniform, approximately 100 nm-thin extracellular matrix sheets that serve as a substrate for epithelial cells, endothelial cells and myotubes. To find out whether BMs maintain their ultrastructure, protein composition and biophysical properties throughout life the natural aging history of the human inner limiting membranes (ILM) was investigated. The ILM is a BM at the vitreal surface of the retina that connects the retina with the vitreous. Transmission electron microscopy (TEM) showed that the ILM steadily increases in thickness from 70 nm at fetal stages to several microns at age 90. By the age of 20, the ILM loses its laminated structure to become an amorphous and very irregular extracellular matrix layer. Atomic force microscopy (AFM) showed that the native, hydrated ILMs are on average 4-fold thicker than the dehydrated ILMs as seen by TEM and that their thickness is prominently determined by its water-binding proteoglycans. The morphological changes are accompanied by age-related changes in the biochemical composition, whereby the relative concentrations of collagen IV and agrin increase, and the concentration of laminin decreases with age. Force-indentation measurements by AFM also showed that ILMs become increasingly stiffer with advancing age. The data suggest that BMs from other human tissues may undergo similar age-related changes.
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