Slime and fibers protect: hagfish
Glands of hagfish protect it from predators by secreting a complex material of fibers and rapid water-absorbing slime.
"Hagfishes are known for producing large volumes of slime when stressed. Their slime is believed to act as a defence mechanism against gill-breathing predators, as it has been shown to reduce water flow over the gills of fish. Hagfish slime is composed of two interacting components, slime thread skeins and mucin vesicles, which are both released from glands along the ventrolateral length of the animal. Each slime gland is surrounded by striated muscle and a connective tissue capsule, and contains large numbers of gland thread cells and gland mucous cells. Gland thread cells contain skeins of tightly coiled polymers rich in intermediate filaments, while gland mucous cells produce vesicles containing mucins, a class of glycoproteins. Both cell types rupture partially as they pass through the slime gland duct, causing each to lose its plasma membrane, and releasing both thread skeins and mucin vesicles into the external environment. The mucin vesicles are released by holocrine secretion rather than the more typical mechanism of mucus secretion through fusion of vesicles with the membrane of the mucous cell and release of mucin granules by exocytosis. In this way, the mucin vesicles remain intact until they come into contact with seawater in the external environment.
"The mature slime is formed when exudate released from the hagfish contacts convectively mixing seawater. Agitation during mixing causes the thread skeins to uncoil to lengths of 10–17 cm, providing a large surface area to which the mucins released from the ruptured vesicles can attach. The fully formed slime is a complex network capable of confining seawater to channels between the slime threads and ruptured mucins like a fine sieve. The interaction between the thread skeins and ruptured mucins is critical for the production of the mature slime." (Herr et al. 2010:1092; citations removed from quote)
"When 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 Ca2+ 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." (Herr et al. 2010:1092)
"Hagfishes 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 µm-long ellipsoid bundle to a thread that is 100x 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." (Winegard & Fudge 2010:1235)
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