Insect integument

By: Reem Alajmi

What is integument

• The integument is the protective outer covering of the body

• It is the outer layer of the insect, comprising the epidermis and the cuticle.

Epidermis• It is the outer cell layer of the insect. It is one cell thick,

but the cell densities and cell depth changes during development.

• The apical plasma membrane of an epidermal cell forms a series of short projection or ridges, flattened at inside. This specialized regions are known as plasma membrane plaques and they are the sites of secretion of the outer epicuticle and of chitin fibers. During just after molting, the epidermal cells have cytoplasmic process on the outside extending into pore canal of the cuticle, but these process maybe disappear after maturation.

Integument• The epidermal cells are held together near their

apices by zonula adherens and lower down by septate junctions. But at greater distance from the cuticle, the cells are not tightly bound to each other and the spacer between them known as lateral lymph spaces which is isolate from the haemolymph by desmosomes. Gap junction between epidermal cells provides a pathway for the movement of low molecular weight substance, and this enhance the coordination between cells.

Integument

• The basal plasma membrane is commonly flat and attached to the basal lamina.

• Epidermal cells have extensive rough endoplasmic reticulum and golgi complexes, and they often contain membrane bounded pigment granules.

Integument

• All epidermal cells are glandular in the sense of they secret cuticle and the enzyme concerned in its production and its digestion at the time of molting.

• Some epidermal cells have additional specialized glandular functions.

Basal lamina

• The epidermal cells stand on a basal lamina or basmenet membrane. The primary components of it are fibrous protein, collagen, glycoprotien and glycosaminoglycans. The basal lamina acts as a molecular sieve. It forms a contenious sheet where muscles are attached. It maybe produced by the epidermal cells, but plasmatocytes also contribute to it.

Basic structure of cuticle

• The cuticle is secreted by the epidermis and covers the whole of the outside of the body as well as lining ectodermal invagination such as stomodum and proctodum and the trachea.

• It is differentiate into two main regions: 1- Inner region, characterized by the presence

of chitin and forming the block of the cuticle.2- Outer thin epicuticel, which not contain

chitin.

Chitinous cuticle

• Chitin is a characteristic constituent of insect procuticle, commonly comprising 20-50% of its dry weight. Cuticle is always bound to protein. As it is first secreted it is known as procuticle. Subsequently, the outer part become hard and rigid to form exocuticle, while the inner undifferentiated part is called endocuticle. Between of the two there maybe a region of hardened, but not fully darkened cuticle (which contain stain) is called mesocuticle.

Chitin• Is a polysaccharide made up largely of N-

acetylglucosamine residues, but it is also probably contain some glucoseamine. The sugar residues are linked by 1-4 β linkage, so that they form a chain in which all the residues are oriented in the same direction. Adjacent chitin chains are held together by hydrogen bonds to form microfibrils. Neighboring chains run in opposite directions. The chitin microfibrils are embedded as protein matrix, their orientation is often different in successive levels.

Chemical structure of poly-N-acetyl-d-Glucosamine (chitin) linked to proteins in insect cuticles through catecholamines and histidine moieties (modified after Schaefer et al. 1987)

Protein

• Proteins are the major constituents of insect cuticle. Different part of insect cuticle contain differernt mixture of proteins.

• Different proteins give the cuticel different physical properties in different part of the cuticle, independently of the hardning process.

• Hardnning of cuticle is primarly a consequence of cross-links between protein molecules so that they form a rigid matrix. This process is called tanning or sclerotization, and the cuticle is said to be scelrotized.

Lipid

• Lipids are present in procuticle and in some insect they impregnate the walls of pore canals.

Epicuticle

• Is made of several layers, the thickest layer is the inner epicuticle and a thin outer epicuticle. Outside is a wax of variable thickness. Some insects have a thin cement layer outside the wax.

Inner epicuticle

• This layer is chemically complex and its known to consist primarily of tanned lipoprotiens, phenolic substance and phenoloxidase are also present which are concerened with tanning the protein. Phenoloxidase persists as an extracellular enzyme in mature cuticle producing further tanning if the epicuticle is damaged.

Outer epicuticle

• It is a very thin layer, highly polyerized lipid, and probably also has a protein components. It is the first formed layer of new cuticle produced at each molt, protecting the new procuticle from the molting enzyme.

Wax

• The epicuticle wax layer contain many different compounds. Hydrocarbons are universally present and many comprise over 90% of the wax.

• The wax is important in water proofing the cuticle and in some insects is the source of chemical signals important in inter and intra specific signalling. It is synthesized by the oenocytes.

Cement

• It is a very thin layer outside most of the wax. • It is consists of mucopolysaccharide which

become closely associated with lipid. • It may serve to protect the underlaying wax.• It is secreted by gland cell in the epicuticle.

Pore canals and epicuticular filaments

• Running through the cuticle at right angle to the surface are very fine pore canals. Thus extends from the peiderms to the inner epicuticle. The canal maybe filled with chitin and protein and the filament have lipid-filled lumen and these filaments are concerned with the transport of lipids from the epidermal cells to the surface of the cuticle.

• In some part of the cuticle, it have different type of metals (Zinc, manganese, iron, calcium, calcium carbonate) to increase hardening of the cuticle (mandibles, claws, ovipositors)

Cuticular extensions• Insects are well endowed with cuticular extensions, varying

from fine and hair-like to robust and spine-like. Four basic types of protuberance , all with sclerotized cuticle, can be recognized on morphological, functional, and developmental grounds:

•1. Spines are multicellular with undifferentiated epidermal cells.

2. Setae, also called hairs, macrotrichia, or trichoid sensilla, are multicellular with specialized cells;

• 3. Acanthae are unicellular in origin.

4. microtrichia are subcellular, with several to many extensions per cell

Cuticular extensions Con…

Setae sense much of the insect’s tactile environment. Large setae may be called bristles or chaetae, with the most modified being scales, the flattened setae found on butterflies and moths (Lepidoptera) and sporadically elsewhere. Three separate cells form each seta, one for hair formation (trichogen cell), one for socket formation (tormogen cell), and one sensory cell (Fig. 4.1).

Cuticular extensions Con…

• There is no such cellular differentiation in multicellular spines, unicellular acanthae, and subcellular microtrichia. The functions of these types of protuberances are diverse and sometimes debatable, but their sensory function appears limited. The production of pattern, including color, may be significant for some of the microscopic projections. Spines are immovable, but if they are articulated, then they are called spurs. Both spines and spurs may bear unicellular or subcellular processes

Fig. 2 The four basic types of cuticular protuberances: (a) a multicellular spine; (b) a seta, or trichoid sensillum; (c) acanthae; and (d) microtrichia.