Birefringence Analyses Reveal Differences in Supramolecular Characteristics of Corneal Stromal Collagen Fibrils Between Falconiformes and Strigiformes.

This study aimed to assess the birefringent properties of corneal stromal collagen fibrils in birds of the orders Falconiformes (diurnal) and Strigiformes (predominantly nocturnal) to compare their supramolecular organizations. In total, 22 corneas of Falconiformes (Caracara plancus, n = 8; Rupornis magnirostris, n = 10; and Falco sparverius, n = 4) and 28 of Strigiformes (Tyto furcata, n = 16; Pseudoscops clamator, n = 6; and Athene cunicularia, n = 6) were processed histotechnically into 8-μm thick sections. Corneal optical retardation (OR) values related to the form and intrinsic fractions of the total birefringence of collagen fibrils were measured using a polarized light microscope equipped with phase compensators. In addition, the coherence coefficients that inform the local orientation of the fibrils were calculated through video image analysis. All assessments were conducted both in the anterior and posterior stroma of the cornea. Differences were significant when P < 0.05. The results showed supraorganizational differences between fibrils in the anterior stroma of Falconiformes and Strigiformes. The OR values were greater (P < 0.0001) for Falconiformes, indicating that the corneas of these birds contain more collagen fibrils or more aggregated collagen fibrils. In contrast, the coherence coefficients were higher (P = 0.016) for Strigiformes, indicating that the corneal collagen fibers in these birds are highly aligned and have few undulations. A multivariate data matrix constructed for Euclidean distance calculations showed that the dissimilarity between Falconiformes and Strigiformes corneas, in terms of the supraorganization of stromal collagen fibrils, was 4.56%. In conclusion, it is possible that the supraorganizational differences reported in this study may be sources of variation in the visual quality of Falconiformes and Strigiformes. This study provides the necessary evidence to encourage further research associating corneal optical performance to supramolecular characteristics of corneal stromal collagen.