The cells of fibrous connective tissue are mostly fibroblasts, irregular, branching cells that secrete strong fibrous proteins as an extracellular matrix. The most commonly secreted protein is collagen which represents one-fourth of all vertebrate protein. Collagen is tough and flexible and gives strength to tissue. Elastin fibres are thinner than collagen fibres and are also secreted by fibroblasts. These protein fibres have longer cross-links than collagen fibres, which gives elastin fibres great elasticity.