Acuticle (/ˈkjuːtɪkəl/), orcuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homologous, differing in their origin, structure, function, and chemical composition.
Inhuman anatomy, "cuticle" can refer to several structures, but it is used in general parlance, and even by medical professionals, to refer to the thickened layer of skin surrounding fingernails and toenails (theeponychium), and to refer to the superficial layer of overlapping cells covering thehair shaft (cuticula pili), consisting of dead cells, that locks the hair into itsfollicle.[1] It can also be used as a synonym for theepidermis,[2] the outer layer of skin.
Inzoology, theinvertebrate cuticle or cuticula is a multi-layered structure outside theepidermis of many invertebrates, notablyarthropods androundworms, in which it forms anexoskeleton (seearthropod exoskeleton).
The main structural components of the nematode cuticle areproteins, highly cross-linkedcollagens and specialised insoluble proteins known as "cuticlins", together withglycoproteins andlipids.[3]
The main structural component of arthropod cuticle ischitin, apolysaccharide composed ofN-acetylglucosamine units, together with proteins and lipids. The proteins and chitin are cross-linked. The rigidity is a function of the types of proteins and the quantity of chitin. It is believed that the epidermal cells produce protein and also monitor the timing and amount of protein to be incorporated into the cuticle.[4]
Often, in the cuticle ofarthropods,structural coloration is observed, produced by nanostructures.[5] In the mealworm beetle,Tenebrio molitor, cuticular color may suggest pathogen resistance in that darker individuals are more resistant to pathogens compared to more tan individuals.[6]
Inbotany,plant cuticles are protective, hydrophobic,waxy coverings produced by the epidermal cells of leaves, young shoots and all other aerial plant organs. Cuticles minimize water loss and effectively reduce pathogen entry due to their waxy secretion. The main structural components ofplant cuticles are the uniquepolymerscutin orcutan, impregnated withwax. Plant cuticles function as permeability barriers for water and water-soluble materials. They prevent plant surfaces from becomingwet and also help to prevent plants from drying out.Xerophytic plants such ascacti have very thick cuticles to help them survive in their arid climates. Plants that live in range of sea's spray also may have thicker cuticles that protect them from the toxic effects ofsalt.
Some plants, particularly those adapted to life in damp or aquatic environments, have an extreme resistance to wetting. A well-known example is thesacred lotus.[7] This adaptation is not purely the physical and chemical effect of a waxy coating but depends largely on the microscopic shape of the surface. When a hydrophobic surface is sculpted intomicroscopic, regular, elevated areas, sometimes infractal patterns, too high and too closely spaced for the surface tension of the liquid to permit any flow into the space between the plateaus, then the area of contact between liquid and solid surfaces may be reduced to a small fraction of what a smooth surface might permit.[8] The effect is to reduce wetting of the surface substantially.[9]
Structural coloration is also observed in the cuticles of plants (see, as an example, the so-called "marble berry",Pollia condensata.[10]
"Cuticle" is one term used for the outer layer oftissue of amushroom'sbasidiocarp, or "fruit body". The alternative term "pileipellis", Latin for "skin" of a "cap" (meaning "mushroom"[11]) might be technically preferable, but is perhaps too cumbersome for popular use. It is the part removed in "peeling" mushrooms. On the other hand, somemorphological terminology in mycology makes finer distinctions, such as described in the article on the "pileipellis". Be that as it may, the pileipellis (or "peel") is distinct from thetrama, the inner fleshy tissue of a mushroom or similar fruiting body, and also from thespore-bearing tissue layer, thehymenium.
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