Teleost fish scales: A unique biological model for the fabrication of materials for corneal stroma regeneration

The corneal stroma is composed of multiple lamellae, each containing closely packed collagen fibrils. The orientation of fibrils in a lamella is parallel, but those in different lamellae are orthogonal. As a result, the corneal stroma has a characteristic orthogonal plywood-like structure. Such a highly-regulated three-dimensional arrangement of collagen fibrils gives strength and transparency to the corneal stroma, but it also presents a challenge in the fabrication of materials to replace it. A bioinspired technology is required to process such materials, but the regulatory mechanism of collagen-fibril orientation is still unknown. The low regenerating activity of the corneal stroma seems to be a major factor preventing progress in this field of research. A similarly highly-ordered arrangement of collagen fibrils can be seen in the basal plates of teleost fish scales. Moreover, the scales have high regenerating ability. When a scale is mechanically lost, a new scale is rapidly regenerated. The cells that produce the basal plates are extremely activated; thus, production of the highly-ordered collagen fibrils is very rapid. Therefore, the regenerating scales should be a uniquely helpful biological model for studying the regulatory mechanism of collagen-fibril orientation. Fish-scale collagen has another advantage for use as a biomaterial: the low probability of zoonotic infection. Therefore, scale collagen is a most promising biomaterial for fabricating three-dimensionally arranged collagen fibers to substitute for the corneal stroma. Three tasks that must be clarified for the bioinspired production of a corneal substitute from fish scale collagen are proposed.