fibril -
Any fine, filamentous structure in animals or plants.
Wikipedia
Fibril is a fine fiber approximately 1 nm in diameter. Cytoplasmic fibrils are observed on the protoplasmic cylinders found in most spirochetal species, although no function of the cytoplasmic fibrils has been ascribed. Polysaccharides, the union of several linked monosaccharides, sometimes serve as a structural compound.