Cuticle

A cuticle is a tough but flexible outer covering found on many different organisms. It serves as a protective shield for various parts of a living thing. While the name is the same, different types of cuticles are non-homologous. This means they have different origins, structures, and chemical compositions. They do not all work the same way or come from the same biological building blocks.

In human anatomy, the term "cuticle" can refer to several distinct structures. It is often used to describe the eponychium, which is the thickened layer of skin surrounding the fingernails and toenails. It can also refer to the cuticula pili. This is the superficial layer of overlapping dead cells that covers a hair shaft. These cells work to lock the hair into its follicle. Additionally, some people use the word as a synonym for the epidermis, which is the outermost layer of human skin.

Many invertebrates rely on a multi-layered cuticle located outside their epidermis. In arthropods, this structure often forms an exoskeleton. The main structural component of an arthropod cuticle is chitin. Chitin is a polysaccharide made of N-acetylglucosamine units. This substance is combined with proteins and lipids. These proteins and the chitin are cross-linked to create the structure. The rigidity of the cuticle depends on the amount of chitin and the specific types of proteins used. It is believed that epidermal cells monitor the timing and the amount of protein to incorporate into the cuticle.

Other invertebrates, such as nematodes or roundworms, have a different chemical makeup. Their cuticle consists of proteins and highly cross-linked collagens. They also contain specialized insoluble proteins called cuticlins, along with glycoproteins and lipids. Some arthropods also show structural coloration within their cuticle. This color is produced by nanostructures on the surface. In the mealworm beetle, known as Tenebrio molitor, the color of the cuticle may be important for survival. Darker individuals may be more resistant to pathogens than those that are more tan in color.

In the world of botany, plants produce cuticles to protect their aerial organs. These include leaves and young shoots. The cuticle is a waxy, hydrophobic covering created by epidermal cells. Hydrophobic means the surface repels water. The main components of a plant cuticle are unique polymers called cutin or cutan, which are impregnated with wax. These layers act as permeability barriers. They prevent water-soluble materials from moving through the surface and help prevent the plant from drying out.

Plants have evolved different cuticle thicknesses to survive in specific environments. Xerophytic plants, such as cacti, possess very thick cuticles to survive in arid climates. Plants located near the ocean may also have thicker cuticles to protect against the toxic effects of salt spray. Some plants in damp or aquatic environments have an extreme resistance to wetting. A famous example is the sacred lotus. This resistance is caused by both the chemical waxy coating and the microscopic shape of the surface. When a surface is sculpted into microscopic, regular, elevated areas, it creates the "Lotus effect." This is also called ultrahydrophobicity. Because the water cannot flow into the tiny spaces between these microscopic plateaus, the contact area is reduced. As a result, water beads up and rolls off, carrying dust and contamination away.

In mycology, the term cuticle is used to describe the outer layer of tissue on a mushroom's fruit body, or basidiocarp. A more technical term for this is the pileipellis, which comes from Latin words meaning the "skin" of a "cap." The pileipellis is a distinct layer. It is different from the trama, which is the inner fleshy tissue. It is also different from the hymenium, which is the spore-bearing tissue layer. This is the part of the mushroom that is removed when someone is "peeling" mushrooms.