Epidermal threads reveal the origin of hagfish slime.

When attacked, hagfishes produce a soft, fibrous defensive slime within a fraction of a second by ejecting mucus and threads into seawater. The rapid setup and remarkable expansion of the slime make it a highly effective and unique form of defense. How this biomaterial evolved is unknown, although circumstantial evidence points to the epidermis as the origin of the thread- and mucus-producing cells in the slime glands.

Evolution of a remarkable intracellular polymer and extreme cell allometry in hagfishes.

The size of animal cells rarely scales with body size, likely due to biophysical and physiological constraints. In hagfishes, gland thread cells (GTCs) each produce a silk-like proteinaceous fiber called a slime thread. The slime threads impart strength to a hagfish's defensive slime and thus are potentially subject to selection on their function outside of the body.