Cuticle

Overview
In biology, the term cuticle or cuticula is given to a variety of tough but flexible, non-mineral outer coverings of an organism, or part of an organism, that provide protection. They are non-homologous, differing in their origin, structure and chemical composition.

In human anatomy
In human anatomy, cuticle refers to the dead layers of epidermal cells or keratinocytes that produce the horn protein keratin, to the strip of dead skin cells at the base and sides of the fingernail, the eponychium and also to the superficial layer of overlapping cells covering the hair shaft (cuticula pili) that locks the hair into its follicle.

In invertebrate zoology
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. 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 a polysaccharide, chitin, composed of N-acetylglucosamine units, together with proteins and lipids.

In botany
In botany, plant cuticles are protective waxy coverings produced by the epidermal cells of leaves, young shoots and all other aerial plant organs.

The main structural components of plant cuticles are the unique polymers cutin and/or cutan, impregnated with wax.

The cuticles of plants function as permeability barriers for water and water-soluble materials. The cuticle both prevents plant surfaces from becoming wet and helps to prevent plants from drying out. Xerophytic plants such as Cactus have very thick cuticles to help them survive in their arid climates. Plants that live in range of sea's spray also tend to have thicker cuticles, to protect them from the toxic effects of salt.