Eight pterygia alone, eight pterygia combined with pingueculae, and three pingueculae were studied by light and electron microscopy to evaluate the changes in collagen and elastic tissue that occur in these conditions. Hyaline degeneration in these growths results from an increase in the amount of a substance composed of degenerating collagen fibers, altered ground substance, abnormal conjunctival cells, and areas of increased electron density. Elastoid fibers, which are prominent in all growths of this type, appear to arise within bundles of preexisting collagen that has undergone degenerative changes. Early changes consist of transformation of collagen into a granular ground substance. This is followed by deposition of elastoid tissue. An eosinophilic granular material, unrelated to preexisting collagen, was found in some growths. It failed to stain positively for amyloid with Congo red or for acid mucopolysaccharide with the Abul-Haj-Rinehart stain.
Hogan MJ, Alvarado J. Pterygium and Pinguecula: Electron Microscopic Study. Arch Ophthalmol. 1967;78(2):174-186. doi:10.1001/archopht.1967.00980030176010