It is found in tendons (joins muscles to bone or cartilage) and ligaments (bone to bone). The primary function of the collagen fibers is to provide tensile strength to tissues.
The fibroblasts that produced the fibers are relatively inactive in adults. They reside in close proximity to the collagen fibers and often only their flattened nuclei are visible. Their sparse cytoplasm is not visible largely because it blends in with the collagen fibers.