Cuticle

A cuticle /ˈkjuːtɪkəl/, or cuticula, 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. A cuticle /ˈkjuːtɪkəl/, or cuticula, 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. In human anatomy, 'cuticle' can refer to several structures, but it is used in general parlance and even by medical professionals when speaking with patients to refer to the thickened layer of skin surrounding fingernails and toenails (the eponychium) and to refer to the superficial layer of overlapping cells covering the hair shaft (cuticula pili) that locks the hair into its follicle, consisting of dead cells. It can also be used as a synonym for the epidermis, the outer layer of skin. The cuticle of fingernails and toenails protects the skin underneath from moisture and environmental germs. In zoology, the invertebrate cuticle or cuticula is a multi-layered structure outside the epidermis of many invertebrates, notably roundworms and arthropods, in which it forms an exoskeleton (see arthropod exoskeleton). The main structural components of the nematode cuticle are proteins, highly cross-linked collagens and specialised insoluble proteins known as 'cuticlins', together with glycoproteins and lipids. The main structural component of arthropod cuticle is chitin, a polysaccharide composed of N-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 monitors the timing and amount of protein to be incorporated into the cuticle. Often, in the cuticle of arthropods, structural coloration-producing nanostructures are observed. In botany, 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 of plant cuticles are the unique polymers cutin or cutan, impregnated with wax.