title: A Comparative Study of Vertebrate Corneal Structure: The Evolution of a Refractive Lens.
abstract: OBJECTIVE
  Although corneal curvature plays an important role in determining the refractive power of the vertebrate eye, the mechanisms controlling corneal shape remain largely unknown. To address this question, we performed a comparative study of vertebrate corneal structure to identify potential evolutionarily based changes that correlate with the development of a corneal refractive lens.
  
  
  METHODS
  Nonlinear optical (NLO) imaging of second-harmonic-generated (SHG) signals was used to image collagen and three-dimensionally reconstruct the lamellar organization in corneas from different vertebrate clades.
  
  
  RESULTS
  Second-harmonic-generated images taken normal to the corneal surface showed that corneal collagen in all nonmammalian vertebrates was organized into sheets (fish and amphibians) or ribbons (reptiles and birds) extending from limbus to limbus that were oriented nearly orthogonal (ranging from 77.7°-88.2°) to their neighbors. The slight angular offset (2°-13°) created a rotational pattern that continued throughout the full thickness in fish and amphibians and to the very posterior layers in reptiles and birds. Interactions between lamellae were limited to "sutural" fibers in cartilaginous fish, and occasional lamellar branching in fish and amphibians. There was a marked increase in lamellar branching in higher vertebrates, such that birds ≫ reptiles > amphibians > fish. By contrast, mammalian corneas showed a nearly random collagen fiber organization with no orthogonal, chiral pattern.
  
  
  CONCLUSIONS
  Our data indicate that nonmammalian vertebrate corneas share a common orthogonal collagen structural organization that shows increased lamellar branching in higher vertebrate species. Importantly, mammalian corneas showed a different structural organization, suggesting a divergent evolutionary background.
authors.affiliation: Department of Biomedical Engineering, University of California-Irvine, Irvine, California, United States.
authors.forename: Moritz
authors.initials: M
authors.lastname: Winkler
authors.affiliation: Gavin Herbert Eye Institute, University of California-Irvine, Irvine, California, United States.
authors.forename: Golroxan
authors.initials: G
authors.lastname: Shoa
authors.affiliation: Gavin Herbert Eye Institute, University of California-Irvine, Irvine, California, United States.
authors.forename: Stephanie T
authors.initials: ST
authors.lastname: Tran
authors.affiliation: Gavin Herbert Eye Institute, University of California-Irvine, Irvine, California, United States.
authors.forename: Yilu
authors.initials: Y
authors.lastname: Xie
authors.affiliation: University of California-Davis, Davis, California, United States.
authors.forename: Sarah
authors.initials: S
authors.lastname: Thomasy
authors.affiliation: University of California-Davis, Davis, California, United States.
authors.forename: Vijay K
authors.initials: VK
authors.lastname: Raghunathan
authors.affiliation: University of California-Davis, Davis, California, United States.
authors.forename: Christopher
authors.initials: C
authors.lastname: Murphy
authors.affiliation: Department of Biomedical Engineering, University of California-Irvine, Irvine, California, United States 2Gavin Herbert Eye Institute, University of California-Irvine, Irvine, California, United States.
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Brown
authors.affiliation: Department of Biomedical Engineering, University of California-Irvine, Irvine, California, United States 2Gavin Herbert Eye Institute, University of California-Irvine, Irvine, California, United States.
authors.forename: James V
authors.initials: JV
authors.lastname: Jester
mesh_terms: D000663:Amphibians; D000717:Anatomy, Comparative; D000818:Animals; D005075:Biological Evolution; D001717:Birds; D003094:Collagen / Q000032:analysis*; D003315:Cornea / Q000033:anatomy & histology / Q000737:chemistry* / Q000166:cytology*; D005399:Fishes; D006801:Humans; D021621:Imaging, Three-Dimensional; D007908:Lens, Crystalline / Q000033:anatomy & histology*; D008322:Mammals; D018613:Microscopy, Confocal; D017711:Nonlinear Dynamics; D012104:Reptiles; D013045:Species Specificity; D013499:Surface Properties; D014714:Vertebrates / Q000033:anatomy & histology*
chemical_list: D003094:Collagen