The Molting Process

The cuticle is a rather inextensible structure, and to grow, insects need to shed their old cuticle at intervals after having produced a new one with a larger surface area. The whole process, from breaking the connections between the epidermal cells and the cuticle (apolysis) to emerging from the remnants of the old cuticle (ecdysis), is called molting. A molt is initiated by a brief increase in concentration of the molting hormone, ecdysterone. Whether the new cuticle will become a larval, a pupal, or an adult cuticle depends on the concentration of juvenile hormone during the ecdysterone surge.

The first microscopically visible sign of molting is apolysis, which is the formation of a narrow space between the apical membrane of the epidermal cells and the inner surface of the cuticle. The cell membrane is folded into rather low microvilli, which carry small, dense plaques at the tip. Patches of a new epicuticle form at the top of the plaques, and gradually these patches merge to form a thin continuous layer, called the outer epicuticle or the cuticulin layer, that effectively separates the old cuticle from the space in which new cuticle is deposited.

The epidermal cells undergo mitotic divisions at the onset of molting, resulting in an increase of cell number and total epidermal surface. To allow the animal to increase in size at molting, the epicuticle deposited above the apical cell surface has a larger area than the former epicuticle, and the epidermis together with the epicuticle is folded to be accommodated in the space available inside the old cuticle.

The epidermal cells secrete a mixture of hydrolytic enzymes (the molting fluid) into the space below the old cuticle. The enzymes (proteases, peptidases, chitinase, and glucosidases) degrade the endocuticular chitin and proteins in the old cuticle to free A-acetylglucosamine and amino acids to be resorbed by the insect and reused for building the new cuticle.

While the old cuticle is being digested, the new inner epicuticle is deposited beneath the outer epicuticle and the new procuticle starts forming. Chitin is synthesized by an enzyme complex (chitin—synthetase) located at the tip of the microvilli, and chitin microfibrils grow from here into the subcuticular space, the deposition zone. The cuticular proteins are synthesized intracellularly and transported via secretory vesicles from the Golgi complex to the apical plasma membrane, where by exocytosis they are secreted into the subcuticular lumen. The chitin and protein molecules are in some way organized into a macromolecular complex in the deposition zone between cells and cuticle, possibly by a process of self-assembly.

The procuticle grows steadily in thickness until ecdysis, when the partially digested old cuticle ruptures and the insect emerges. Free of the old cuticle (exuvium), the insect expands the new cuticle to a predetermined size, often dependent on the area of the epicuticle. Cuticular deposition of chitin and proteins is resumed and continues for several days after ecdysis, and extended regions of the new cuticle are hardened

(sclerotized) by oxidative incorporation of phenolic compounds into the cuticular matrix. In many insects, sclerotization and deposition of endocuticle after ecdysis are governed by the neurohormone bursicon.

Secretion of material from the epidermal cells occurs not only at the apical surface. Some of the proteins synthesized by the cells are exported to the hemocoel via the basolateral membrane system, and others, such as arylphorins, are synthesized in the fat body and secreted into the hemolymph to be taken up by the epidermal cells and incorporated into the cuticle.

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