Ultrastructural features of corneas with pellucid marginal degeneration

Pellucid marginal degeneration (PMD) of the cornea is a rare ectatic disorder which typically affects the inferior or superior peripheral cornea in a crescentic fashion. We report histological and ultrastructural features of three PMD corneas.

The following three patients were diagnosed with PMD corneas: (A) one 41-year-old male, (B) one 56-year-old female, and (C) one 31-year-old male. The patients underwent keratoplasty and the excised corneas were processed for light and electron microscopy to study the ultrastructural features.

Degenerated corneas were observed in the region adjacent to the limbus. In the degenerated region of the cornea, the Bowman's layer had been replaced by collagenous pannus and the anterior stroma contained degenerated collagen fibrils (CFs) with very large proteoglycans (462±420 nm(2)). The lamellae were fused and keratocytes appeared like fibroblast. The prelimbal region of the PMD cornea had a degenerated Bowman's layer and thin undulating lamellae in the stroma. The CFs of the Bowman's layer and the stroma were replaced by very fine microfilaments. The mean of the minimum CF diameter was 19±3 nm in London Rsin White-embedded tissue.

Our observations of the disorganization and degeneration of CFs suggest that PMD could be related to a disorder in the synthesis of CF. This disorder was more severe in the cornea adjacent to the limbus compared to the cornea further away from the limbus.