A is a light micrograph in which collagen fibers stain red and cell nuclei stain blue. bv is a blood vessel. B is an electron micrograph showing a bundle of collagen fibers, one of which is indicated by an arrow, in longitudinal section. An arrowhead indicates a collagen fiber in transverse section. Collagen fibrils have a distinct banding pattern in electron micrographs that is due to the way the tropocollagen molecules overlap with one another.