Characterization of the zebrafish cornea structure using FIB-SEM volumetric imaging
Following Nature’s principle of minimum inventory for maximal diversity, the cornea is made of the same protein as dermis, tendon and the organic component of bone – collagen type I. We developed a volumetric FIB-SEM imaging protocol for the zebrafish cornea, spanning the full corneal stroma thickness of 6-10 õm, with a voxel size of 4 nm. The fibrils are about 20 nm thick and form orthogonal layers. The distances between adjacent collagen fibrils are also uniform, about 18-20 nm, attributable to their proteoglycan-mediated hydration shells. Our custom 3D centroid tracking analysis demonstrated remarkable continuity of the fibrils, that are gently twisted within orthogonal layers. The twist results in geometric frustration that is apparently resolved by layer branching with a strikingly repetitive angle 12ð. This branching makes the orthogonal layers of the cornea bicontinuous. Within one layer, the fibrils demonstrate hexatic packing with uniform spacing, consistent with the position of proteoglycans. The hexatic packing, helicity, anastomotic branching and orthogonality can be theoretically explained by inanimate physical interactions and hierarchical self-assembly of collagen fibrils in a crowded milieu, rather than by obligate cell-mediated control.
Bio: Natalie Reznikov is an associate professor at the Department of Bioengineering at McGill University, Montreal. She studies biomineralization (structure-function relationships, formation and biomechanics of bone, teeth, eggshells, and mollusk shells) and the architecting role of structural water in natural materials. She uses a wide variety of 3D imaging from cone-beam computed X-ray tomography, microcomputed X-ray tomography, to multiscale electron microscopy (Focused ion beam – scanning electron microscopy, scanning transmission electron tomography). Advanced 3D image processing and analysis methods are at the core of each project conducted in her lab.