Figure 2.

Short-range order (SRO) and transparency. An electron micrograph of a cross section of collagen fibrils in the corneal stroma (22) is compared with three simulations of fibril packing order. The concentration of collagen fibrils in the cornea accounts for the high refractive index (n ∼ 1.37), and the fibril packing order accounts for transparency. Random order of fibrils produces opacity. Long-range “crystalline” order results in transparency, but the arrangement of the fibrils has a periodic repeat. In the cornea, fibrils are arranged in SRO, where their spatial positions are variable but nonrandom. Proteoglycans (PGs) maintain the spatial organization of the collagen fibrils that allows transparency (see the text). Hydration of the cornea is carefully controlled to maintain SRO and corneal transmittance. Similarly, lens cell transparency results from SRO in the packing arrangement of the crystallins (bar represents 0.1 micron).