cuticle(redirected from primary cuticle)
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(1) In animals, a solid formation on the surface of cells of epithelial tissue. In invertebrates, a derivative of cells of the single-layered integumental epithelium, or hypodermis. The functions of the cuticle are chiefly protective and supportive. In worms and arthropods, it forms a tough body covering. The composition of cuticle includes chitin (which, together with mineral substances and proteins, provides mechanical strength) and lipids (which make the cuticle impervious to water).
In vertebrates, which have a multilayered integumental epithelium to perform the protective function, the cuticle is found mainly on the surface of the epithelial cells that line the viscera (the intestines, the air passages of the respiratory organs, parts of the uriniferous tubules of the kidneys, and the urinary tracts). Each cell has its own cuticle, which is represented completely by the microvilli (the surface of a single cell of the small intestine in man having as many as 2,000). The activity of certain enzymes (for example, alkaline phosphatase, invertases, and maltases) has been discovered in the cuticle, testifying to the active participation of the cuticle in the functional activities of the organs.
(2) In plants, the cuticle is the thin protective film covering the epidermis of the leaves and stems and consisting of cutin. The cuticle is an unstructured formation, lacking corpuscular or fibrillar elements. It is resistant to chemical influences. It is absent in the underwater organs of aquatic plants, and it is poorly developed in plants that live in shade or damp soil. It is especially well developed in plants that must limit transpiration. The smooth, shiny cuticle of the leaves of tropical plants reflects some of the sunlight striking the plants and serves as a shield against excessive insolation. In many xerophytes, the protective properties of the epidermis are reinforced beneath the cuticle by a cuticular layer, which consists of a mixture of polysaccharides, cutin, and waxes. In the majority of xerophytes, the cuticular layer holds pale yellow pigments, which help make the cell wall impenetrable to ultraviolet rays.
M. E. ASPIZ