A New Three-dimensional Model of the Organization of Proteoglycans and Collagen Fibrils in the Human Corneal Stroma

Abstract

Abstract: : Purpose: To re-evaluate the three-dimensional organization of collagen fibrils and proteoglycans (PGs) in the human corneal stroma using fresh corneas, a reproducible staining protocol and a sophisticated embedding technique. Methods: Seven corneal buttons were shortly prefixed in an aldehyde solution. One half was stained with Quinolinic phtalocyanin (QP) or Cupromeronic blue (CB). Strips of 1mm width were treated with aqueous phosphotungstic acid and further processed for electron microscopy. The other half served as control and was processed routinely with osmiumtetroxide. Ultrathin sections were cut precisely parallel (frontal sections) or perpendicular (cross sections) to the corneal surface. The sections were studied and the length of PGs and their mutual distances were measured at a calibrated final magnification of 70,000x. Results: QP- and CB-treated tissue shows well contrasted PGs near the cutting edge of the strips. In cross sections PGs form a repeating network of regular structures (~ 40 nm) around the collagen fibrils and in frontal sections they are aligned orthogonal to the collagen fibrils. The PGs are equidistantly (47 ± 5 nm) attached to the collagen fibrils along their full length and have a maximal length of approximately 55 nm . Conclusions: The observed maximal length of the PGs and the occurrence of regular structures enwrapping the collagen fibrils urged us to revisit the prevailing corneal stroma model of Maurice (1962). In the new model groups of PGs are postulated to be orthogonally attached to the surface of the hexagonally arranged collagen fibrils at regular intervals and are interconnecting the collagen fibrils with their next-nearest neighbors. In this way a regular meshwork of structures enwrapping the collagen fibrils is formed. This meshwork forms a gel-like matrix around the collagen fibrils. This will assure the regular arrangement of collagen fibrils and therefore will reduce light scattering. This arrangement might also reduce the refractive index between collagen fibrils and matrix and might increase the resistance of the cornea to compression.

Keywords: cornea: basic science • cornea: stroma and keratocytes • cornea: stroma and keratocytes