Fine Structure of the Interface between the Anterior Limiting Lamina and the Anterior Stromal Fibrils of the Human Cornea

To use transmission electron microscopy (TEM) to investigate further the ultrastructural details of the collagen fibrils linking the anterior limiting lamina (ALL; Bowman's membrane) of the human cornea to the anterior stromal lamellae. Six disease-free corneas from donors aged 42 to 82 years were fixed (2% glutaraldehyde in 80 mM sodium cacodylate) and processed for TEM within 72 hours postmortem. A series of overlapping images, at 10,204x magnification, of the central corneal ALL-stroma interface were assembled. The features of the terminal ends of fibril bundles at the interface with the anterior stroma were quantitatively assessed. TEM revealed apparently terminating anterior stromal fibril bundles adjacent to the ALL. These terminating lamellae (7.8 per 100 mum) were embedded in an electron-dense material within the surrounding stromal matrix and were termed electron-dense formations (EDFs). The mean width of these stromal features was 1.6 mum. At intervals, anterior stromal lamellae approached the ALL and, in a shallow manner, inserted into the ALL. Such projections (5.4 per 100 mum) into the ALL were, on average, less than 1 mum. Numerous fibrils (29.8 per 100 mum) extended from the ALL into the stroma with a mean length of 0.8 mum. The interface the ALL forms with the anterior stroma is complex, and TEM revealed at least three different types of fibrillar arrangements, which may serve optical requirements rather than provide a structural function.