The oligomers can aggregate together and further stabilize to make amyloid fibrils. IAPP is capable of forming amyloid fibrils "in vitro". Collagen has a hierarchical structure, forming triple helices, fibrils, and fibers. These synthesise collagen within the cell, and then secrete collagen fibrils. Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Then the osteoblasts create an extracellular matrix containing Type-I collagen fibrils, which is osteoid. Chitin forms crystals that make fibrils that become surrounded by proteins. Cellulose consists of fibrils with crystalline and amorphous regions. They originally nucleate at the gaps between collagen fibrils. C-proteinases, the tropocollagen molecules spontaneously assemble into insoluble fibrils. Other sources say they have a tapetum lucidum composed of collagen fibrils. Fibronectin was also observed to be closely associated with the newly deposited collagen fibrils. Additional assembly of fibrils is guided by fibroblasts, which deposit fully formed fibrils from fibripositors. I collagen fibrils, creating light scatter. Alterations in the organization or polarity of the collagen fibrils can be signs of pathology.