Abstract
Fibers are ubiquitous in biology, and include tensile materials produced by specialized glands (such as silks), extracellular fibrils that reinforce exoskeletons and connective tissues (such as chitin and collagen), as well as intracellular filaments that make up the metazoan cytoskeleton (such as F-actin, microtubules, and intermediate filaments). Hagfish gland thread cells are unique in that they produce a high aspect ratio fiber from cytoskeletal building blocks within the confines of their cytoplasm. These threads are elaborately coiled into structures that readily unravel when they are ejected into seawater from the slime glands. In this review we summarize what is currently known about the structure and function of gland thread cells and we speculate about the mechanism that these cells use to produce a mechanically robust fiber that is almost one hundred thousand times longer than it is wide. We propose that a key feature of this mechanism involves the unidirectional rotation of the cell’s nucleus, which would serve to twist disorganized filaments into a coherent thread and impart a torsional stress on the thread that would both facilitate coiling and drive energetic unravelling in seawater.
Highlights
Fibers are ubiquitous in biology, and include tensile materials produced by specialized glands, extracellular fibrils that reinforce exoskeletons and connective tissues, as well as intracellular filaments that make up the metazoan cytoskeleton
The best known structural fibers are the three main classes of cytoskeletal elements: F-actin, microtubules (MTs), and intermediate filaments (IFs), which participate in myriad cell functions, including cell division, cell motility, vesicular transport, and structural reinforcement
The regular pattern of staggered thread loops and spiraling conical loop arrangements is suggestive of a spinning mechanism within the cell [24]. Is it possible that the thread is formed via a process similar to a spinning wheel, in which disorganized textile fibers are twisted into a coherent and mechanically robust yarn? Such a wheel-like structure may exist in the cytoplasm near the apical side of the nucleus, but it seems unlikely that the several groups that have examined gland thread cells (GTCs) ultrastructure would have missed it [20,24,31]
Summary
Fibrous structures, which are characterized by a high aspect ratio (i.e., length/diameter), are ubiquitous in both engineering and biology. Fibers are used for similar functions, such as spider draglines [1,2], and are commonly found in fiber-reinforced biomaterials such as tendon and insect cuticle [3,4]. In these extracellular materials, stiff fibers (i.e., collagen and chitin) are embedded in elastomeric matrices to form tough fiber-reinforced composites. The cocoon silk fiber spun by the silkworm moth larva (Bombyx mori) has a typical diameter of about 20 μm and a length of 1 km [5], for an impressive aspect ratio of 5 ˆ 107. Examples of fibers in biology, from lowest to highest aspect ratio
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