Procuticle

The region of the cuticle, located between epicuticle and the epidermal cell layer, is called procuticle; it constitutes the main part of the total cuticle. Histologically, the sclerotized regions (sclerites) are often subdivided into layers with different staining properties: (1) the outermost layer, the exocuticle, may be dark colored because of sclerotization, but is refractory to staining; (2) the innermost, uncolored layer, the endocuticle, stains blue; and (3) in between one often observes a layer of mesocuticle, staining red with Mallory triple stain. The flexible cuticle (arthrodial membranes), which connects the sclerites, stains blue with Mallory throughout most of its thickness. Exocuticle may correspond to the part of the procuticle deposited before ecdysis, stabilized by sclerotization. Mesocuticle plus endocuticle often correspond to the post-ecdysially deposited procuticle, and if these layers are sclerotized at all, it is only slightly.

The procuticle consists mainly of chitin and proteins; water is an essential component, and other materials, such as lipids, phenolic compunds, salts, pigments, and uric acid may be present. Chitin (poly 1,4-P-A^-acetylglucosamine) is a polysaccharide, present as long and nearly straight microfibrils, usually about 2.8 nm in diameter and of indeterminate length. The filaments tend to run parallel to the cuticular surface, but columns of chitin filaments running perpendicular to the surface have been described for some types of cuticle (lepidopteran larval cuticle). The function of such chitinous columns remains uncertain.

The chitin microfibrils are organized in various patterns, and the organization seems to have importance for the mechanical properties of the cuticle. The most commonly observed patterns are the heliocoidal pattern, where the microfibril direction changes by a small, constant angle between neighboring layers; the preferred, unidirectional orientation, where the fibrils run in the same direction in all layers, and the pseudo-orthogonal orientation, where unidirectional layers of chitin microfibrils alternate with layers running at nearly right angles to each other. In certain cuticles the pattern of chitin microfibrils depends on a daily rhythm: in locust tibiae, heliocoidal cuticle is deposited during the night and unidirectional cuticle is deposited during the day, making it possible to determine the number of days since ecdysis.

The chitin microfibrils are embedded in a protein matrix; the protein content tends to equal the chitin content in flexible cuticles and is usually three to four times higher than the chitin content in hard cuticles. The number of different proteins present in a given type of cuticle can vary from about 10 to 100. Different types of protein are present in flexible and hard cuticles; the proteins are species specific, and some of them are also specific for certain cuticular regions. A characteristic amino acid sequence region, common to a large number of cuticular proteins, is supposed to have a function in the linking of proteins to the chitin microfibrils. The proteins are often extractable immediately after deposition. In many cuticular regions, however, they are later rendered inextractable by sclerotization, whereby low molecular weight phenolic compounds are covalently incorporated into the cuticular matrix, cross-linking the proteins, and making the cuticle harder and stiffer, and more difficult to digest with enzymes.

Sclerotization may occur soon after a molt when the insect has expanded its new cuticle to a larger size, but the regions that are not enlarged may have been sclerotized in the pharate stage, which is the stage that is present before emergence from the exuvium, or old cuticle. The elastic protein, resilin, present in rubberlike cuticular regions, is cross-linked as soon as it is deposited extracellularly. The cross-

linking process is different from that in sclerotized cuticle because no low molecular weight compounds are involved, but tyrosine residues in the protein chains are oxidatively coupled to each other, forming di- and trityrosine residues.

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