Corneal stroma microfibrils

Abstract

Elastic tissue was first described well over a hundred years ago and has since been identified in nearly every part of the body. In this review, we examine elastic tissue in the corneal stroma with some mention of other ocular structures which have been more thoroughly described in the past. True elastic fibers consist of an elastin core surrounded by fibrillin microfibrils. However, the presence of elastin fibers is not a requirement and some elastic tissue is comprised of non-elastin-containing bundles of microfibrils. Fibers containing a higher relative amount of elastin are associated with greater elasticity and those without elastin, with structural support. Recently it has been shown that the microfibrils, not only serve mechanical roles, but are also involved in cell signaling through force transduction and the release of TGF-β. A well characterized example of elastin-free microfibril bundles (EFMBs) is found in the ciliary zonules which suspend the crystalline lens in the eye. Through contraction of the ciliary muscle they exert enough force to reshape the lens and thereby change its focal point. It is believed that the molecules comprising these fibers do not turn-over and yet retain their tensile strength for the life of the animal. The mechanical properties of the cornea (strength, elasticity, resiliency) would suggest that EFMBs are present there as well. However, many authors have reported that, although present during embryonic and early postnatal development, EFMBs are generally not present in adults. Serial-block-face imaging with a scanning electron microscope enabled 3D reconstruction of elements in murine corneas. Among these elements were found fibers that formed an extensive network throughout the cornea. In single sections these fibers appeared as electron dense patches. Transmission electron microscopy provided additional detail of these patches and showed them to be composed of fibrils (∼10nm diameter). Immunogold evidence clearly identified these fibrils as fibrillin EFMBs and EFMBs were also observed with TEM (without immunogold) in adult mammals of several species. Evidence of the presence of EFMBs in adult corneas will hopefully pique an interest in further studies that will ultimately improve our understanding of the cornea's biomechanical properties and its capacity to repair.

Elastin Content and Distribution in Endothelial Keratoplasty Tissue Determines Direction of Scrolling

“…We initially proposed the potential implications of the elastic fibre system in glaucoma 2 and we have now revealed that the posterior elastic fibres in the corneal stroma are indeed linked with the TM. 6 The results in the current article fit nicely with our results as the authors show that these fibres contain a high concentration of elastin that is continuous with the TM. Interestingly, it is known that full thickness keratoplasty has been shown to cause high incidence of glaucoma when compared to partial thickness deep anterior keratoplasty.…”

“…TEM morphological observations in the same study also revealed that true elastic fibres containing fibrillin sheaths and amorphous elastin cores were restricted to the corneal peripheral region, limbus and trabecular meshwork (TM) while thinner, predominantly fibrillin-1 fibres, previously described by Hanlon et al 5 , were only present in low densities in the central posterior cornea. Since our previous studies, we have now characterised the human elastic fibre system using a range of antibodies including elastin and fibrillin-1 6 and we have clarified the association and distribution of elastin-and fibrillin 1-containing fibres within the corneal elastic fibre system.…”

Stromal striae: a new insight into corneal physiology and mechanics

“…On the one hand, cornea is devoid of elastin fibrils which are major features of elastic tissues. Elastin-free microfibril bundles are present during embryonic and early postnatal development and could be also found in adult corneas 36 . They could support corneal elasticity at least partly.…”

Structuring of Hydrogels across Multiple Length Scales for Biomedical Applications

“…In the anterior region, the collagen does not exhibit any preferred orientation, whereas the mid‐ and posterior regions demonstrate preferred orthogonal arrangement of collagen along the nasal‐temporal and superinferior directions . The key structural factors of the cornea, leading to high optical transparency, are the uniformity of the collagen fibril diameters and tight regulation of the distances between adjacent collagen fibrils . The fourth layer of the cornea, Descemet's membrane, is composed of a thin layer of collagen type IV and is designed to support the corneal endothelium layer that makes up the posterior surface of the cornea.…”