Summary The defensive slime of hagfishes contains thousands of intermediate filament protein threads1 that are manufactured within specialized gland thread cells2–4. The outstanding material properties of these threads, which rival spider dragline silks, make them an ideal model for biomimetic efforts to produce sustainable protein materials5. The gland thread cell is remarkable because of the strength of the thread it produces, but also because of the thread’s impressive length (~150 mm)1, its exquisite packaging within the cytoplasm3,4, and its ability to deploy rapidly in seawater without tangling6. The thread bundle (or “skein”) is organized into staggered loops that spiral around the long axis of the cell. In mature cells, these highly organized loops fill most of the cell volume. Although the exact site of thread assembly is unknown, the thinnest regions of the thread are found adjacent to the nucleus7,8. Intermediate filaments and microtubules are known to interact during early stages of thread assembly, but the mature thread is electron dense with no discernable ultrastructure7,8. Until now, we have lacked information about gland thread cell development, including high power images of very young cells that could provide insight into how the thread is coiled and how it matures. Here we show (1) how changes in nuclear morphology, size, and position can explain the three-dimensional pattern of thread coiling in gland thread cells, and (2) how the ultrastructure of the thread changes from very young thread cells up to large cells with fully mature skeins7–9. Our model provides an explanation for the complex process of thread assembly and organization that has fascinated and perplexed biologists for over a century10, and provides valuable insights for the quest to manufacture high-performance biomimetic protein materials.

SUMMARYHagfishes are benthic marine protovertebrates that secrete copious quantities of slime when threatened. The slime originates as a two-component glandular exudate comprised of coiled bundles of cytoskeletal intermediate filaments (thread skeins) and mucin vesicles. Holocrine secretion of the slime into seawater results in the rapid deployment of both fibrous and mucin components, resulting in about a liter of dilute slime. Deployment of the thread skeins involves their unraveling in a fraction of a second from a 150 mm-long ellipsoid bundle to a thread that is 100ϫ longer. We hypothesized that thread skein deployment requires both vigorous hydrodynamic mixing and the presence of mucin vesicles, both of which are required for whole slime deployment. Here we provide evidence that mixing and mucin vesicles are indeed crucial for skein unraveling. Specifically, we show that mucin vesicles mixed into seawater swell and elongate into high-aspect ratio mucin strands that attach to the thread skeins, transmit hydrodynamic forces to them and effect their unraveling by loading them in tension. Our discovery of mucin strands in hagfish slime not only provides a mechanism for the rapid deployment of thread skeins in vivo, it also helps explain how hagfish slime is able to trap such impressive volumes of seawater via viscous entrainment. We believe that the deployment of thread skeins via their interaction with shear-elongated mucins represents a unique mechanism in biology and may lead to novel technologies for transmitting hydrodynamic forces to microscale particles that would typically be immune to such forces. Supplementary material available online at

SUMMARYWhen agitated, Atlantic hagfish (Myxine glutinosa) produce large quantities of slime that consists of hydrated bundles of protein filaments and membrane-bound mucin vesicles from numerous slime glands. When the slime exudate contacts seawater, the thread bundles unravel and the mucin vesicles swell and rupture. Little is known about the mechanisms of vesicle rupture in seawater and stabilization within the gland, although it is believed that the vesicle membrane is permeable to most ions except polyvalent anions. We hypothesized that the most abundant compounds within the slime gland exudate have a stabilizing effect on the mucin vesicles. To test this hypothesis, we measured the chemical composition of the fluid component of hagfish slime exudate and conducted functional assays with these solutes to test their ability to keep the vesicles in a condensed state. We found K + concentrations that were elevated relative to plasma, and Na + , Cl -and Ca 2+ concentrations that were considerably lower. Our analysis also revealed high levels of methylamines such as trimethylamine oxide (TMAO), betaine and dimethylglycine, which had a combined concentration of 388 mmol l -1 in the glandular fluid. In vitro rupture assays demonstrated that both TMAO and betaine had a significant effect on rupture, but neither was capable of completely abolishing mucin swelling and rupture, even at high concentrations. This suggests that some other mechanism such as the chemical microenvironment within gland mucous cells, or hydrostatic pressure is responsible for stabilization of the vesicles within the gland.

No abstract

Intermediate filaments are filaments 10 nm in diameter that make up an important component of the cytoskeleton in most metazoan taxa. They are most familiar for their role as the fibrous component of α-keratins such as skin, hair, nail, and horn but are also abundant within living cells. Although they are almost exclusively intracellular in their distribution, in the case of the defensive slime produced by hagfishes, they are secreted. This article surveys the impressive diversity of biomaterials that animals construct from intermediate filaments and will focus on the mechanisms by which the mechanical properties of these materials are achieved. Hagfish slime is a dilute network of hydrated mucus and compliant intermediate filament bundles with ultrasoft material properties. Within the cytoplasm of living cells, networks of intermediate filaments form soft gels whose elasticity arises via entropic mechanisms. Single intermediate filaments or bundles are also elastic, but substantially stiffer, exhibiting modulus values similar to that of rubber. Hard α-keratins like wool are stiffer still, an effect that is likely achieved via dehydration of the intermediate filaments in these epidermal appendages. The diverse mechanisms described here have been employed by animals to generate materials with stiffness values that span an impressive eleven orders of magnitude.