Age-related changes in the basement membrane of the human corneal epithelium.

An ultrastructural evaluation and morphometric analysis of the basement membrane of the normal human corneal epithelium in 45 specimens from subjects ranging in age from 17 weeks of gestation to 93 years was conducted. It was found that thickening of the corneal epithelial basement membrane takes place by two main processes: continued membrane deposition (forming unilaminar membranes) and membrane reduplication (forming multilaminar membranes). During the fetal period and for the first two decades of postnatal life, membrane thickening occurs exclusively by continued membrane deposition so that these unilaminar membranes increase in thickness by 3 nm each year. Membrane reduplication in focal areas after the age of 20 and with increasing frequency and greater involvement of the membrane thereafter was observed. Since anchoring fibrils and hemidesmosomes are important in the adhesion of the epithelium to Bowman's layer, we paid particular attention to these structures. We could find no relation between hemidesmosome content and the formation of reduplicated or multilaminar membranes. However, the anchoring fibrils become disrupted with increasing age, and the membrane thickness exceeds fibril length in some specimens. It is proposed that this phenomenon effectively blocks linkage between the anchoring fibrils and Bowman's layer. The evidence for cell injury and death as a promoter of membrane reduplication was compared to the evidence for a primary deterioration of the basement membrane.