Integument (Insects)

The integument is the external layer of tissue that covers the outer surface of insects and the surfaces of the foregut and hindgut. It is composed of the epidermis, which is a continuous single-layered epithelium, and an underlying thin basal lamina plus the extracellular cuticle that lies on top of the epidermis.

BASAL LAMINA

The basal lamina separates the epidermal cells from the hemo-lymph in the body cavity; it varies in thickness from 0.15 to 0.5 |im and is composed of structural proteins including collagens, glycopro-teins, and glycosaminoglycans. It is negatively charged and can act as a filter between the hemolymph and the epidermal cells, regulating which molecules gain access to the cells.

EPIDERMAL CELLS

The epidermal cells are attached to the basal lamina by hemidesmosomes, which anchor the cell membrane to collagen fibers in the basal lamina. Near their base, the cells are attached to each other by desmosomes; near their apical end, they are attached to each other by a narrow, impermeable zone (the adhering zonule), effectively separating the cuticular compartment from the lateral space between cells. Below the zonule are bands of septate desmo-somes, which may be adhesive, and gap junctions through which the cells can communicate chemically with each other by interchange of low-molecular-weight compounds.
The cuticular materials (chitin and proteins) are secreted from the apical surface of the epidermal cells into the subcuticular space, or deposition zone, where they are assembled into an intact cuticle. The apical surface is folded into shorter or longer microvilli, depending on the stage of the molting cycle and the secretory activity of the cells.


THE MOLTING PROCESS

The cuticle is a rather inextensible structure, and in order 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. The consecutive steps of the process are controlled by hormones.
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 previous epicuticle, and the epidermis together with the epicuticle is folded to be accommodated in the space available inside the old cuticle.
Deposition of new cuticle occurs simultaneously with the degradation of the old cuticle. An enzyme mixture, containing proteases, peptidases, chitinases, and glucosidases, is secreted into the space between the old cuticle and the new epicuticle, and the old cuticle is gradually degraded to free amino acids and N-acetylglucosamine. The degradation products are resorbed by the insect and reused for building proteins and chitin for the new procuticle. Chitin is synthesized by an enzyme complex (chitin-synthetase) located at the top of microvilli on the apical surface of the epidermal cells, and chitin microfibrils grow from here into the subcuticular space, the deposition zone. The cuticular proteins are synthesized intracellularly and transported via the Golgi complex to the apical plasma membrane, and are secreted into the subcuticular lumen by exocytosis. The chi-tin and protein molecules are in some way organized into a macro-molecular complex in the deposition zone between cells and cuticle, possibly by a process of self-assembly.
Soon after emerging from 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, while smaller and largerregions of the new cuticle are hardened (sclero-tized) by oxidative incorporation of phenolic compounds into the cuticular matrix. Both sclerotization and post-ecdysial deposition of endocuticle are governed by the neurohormone bursicon.
Secretion of material from the epidermal cells occurs not only at the apical surface. Some of the peptides and 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.

PORE CANALS

Most cuticles contain pore canals, minute ducts that traverse the cuticle from the apical surface of the epidermal cells to or close to the cuticular surface. When viewed with an electron microscope they may seem empty, but they often contain one or more cuticular filaments composed of wax and lipids. Cytoplasmic processes may extend into the ends of the pore canals during cuticle deposition. The pore canals are generally assumed to be a transport route for lipids and possibly also for sclerotizing agents and proteins to the epicuticle and outer exocuticle.


SENSE ORGANS

The integument of insects contains a large number of sensory cell types, involved in transferring information from the environment to the insect. The sensory cells in the epidermis are often connected to specific cuticular structures, forming sense organs of various types, such as contact chemoreceptors (taste), olfactory chemoreceptors (smell), and mechanoreceptors, which register any small distortion of the cuticle caused either by the movements of the animal or by influences from the environment.

OENOCYTES

Oenocytes, a special cell type, often are present between the basal region of the epidermal cells and the basal lamina, or they may adhere to the hemolymphal surface of the basal lamina. Electron microscope studies show that they have a highly developed smooth endoplasmic reticulum, characteristic of cells engaged in hydrocarbon synthesis. Oenocytes synthesize hydrocarbon waxes, which are transferred to the wax layer covering the epicuticle, presumably via the epidermal cells and the procuticular pore canals. It has been suggested that the oenocytes also provide lipids to the epicuticle and to the sclerotized regions of the exocuticle.

INTEGUMENTAL GLANDS

Epidermal glands are present in many types of integument; they often consist of a single cell surrounding a cavity, which functions as a product reservoir and is connected to the cuticular surface by a small duct. Glands of this type are assumed to be responsible for forming and maintaining the cement layer, which after ecdysis is spread as a protective layer on top of the epicuticular wax layer. Other integumental glands produce various chemical defense secretions, which may be forcibly ejected when the insect is disturbed.
Integumental glands may also consist of epidermal cells without a cavity and duct, but with direct contact to the inner surface of the cuticle, through which the secretion passes. Often the secretions of such glands function as pheromones, playing a part in communication between individuals.

COLORATION

The colors of most insects are the result of pigments located in cuticle and epidermis, or they may be physical colors caused by surface structures in the cuticle. Colored material in hemolymph or internal organs can also contribute to the insect’s color if the integument is transparent. Ommochromes, pteridines, carotenoids, bile pigments, melanins, and urates are the most widespread and important of the epidermal pigments. The colored light reflected from such epidermal pigments passes the overlying cuticle before it reaches the eye of the observer, and the observed color is influenced by the amount of colored material present in the cuticle.

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