Epicuticle

Of several methods described for isolation of epicuticle, one by Langer-malm and Philip [62] involves dissolving the bulk of the fiber from the membrane material with dilute sodium sulfide. Another method, that of Lindberg et al. [71], involves treatment of intact fibers with chlorine water or bromine water followed by neutralization and shaking. Neither of these procedures produces pure epicuticle, but they probably provide a portion of the cell membranes, including part or all of the epicuticle. 

Cell membrane complex is the material that binds or holds the cells together 

The cell membrane complex and the endoctitide are the most vulnerable to the attack by shampoos, to build-up and even to stretching and cracking. If build-up gets beneath the scales, it can cause scale lifting a type of scale damage that makes the hair feel dry, coarse and look dull 

EndocuUde is the innermost layer of each cuticle cell It consists of amorphous proteins 

After removal of the surface lipids, wool fiber still undergoes the All-worden reaction [79]. This fact confirms that these surface lipids are only part of the cell membranes of the cuticle. Feeder et al. [80] have also provided evidence that the cell membrane lipids of wool fiber do not consist of phospholipids that normally form bilayers in living tissue. 

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In fine wool such as that obtained from merino sheep, the cuticle is normally one cell thick (20 x 30 x 0.5 mm, approximate dimensions) and usually constitutes about 10% by weight of the total fiber. Sections of cuticle cells show an internal series of laminations (Figs. 1 and 2) comprising outer sulfur-rich bands known as the exocuticle and inner regions of lower sulfur content called the endocuticle (13). On the exposed surface of cuticle cells, a membrane-like proteinaceous band (epicuticle) and a unique hpid component form a hydrophobic resistant barrier (14). These hpid and protein components are the functional moieties of the fiber surface and are important in fiber protection and textile processing (15). 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

Page, A.P., Rudin, W., Fluri, E., Blaxter, M.L. and Maizels, R.M. (1992b) Toxocara canis. a labile antigenic coat overlying the epicuticle of infective larvae. Experimental Parasitology 75, 72-86. 

Penetration into insects is greatly influenced by the manner in which the compound is presented. Classic studies by Treherne (19) indicated that when the toxicant is supplied to detached insect cuticle in aqueous solution penetration decreased with increasing polarity, explicable on the assumption that partition into and passage through the lipoidal epicuticle is the rate determining step. In contrast several studies (20 - 22) have shown that when the toxicant is dissolved... 

The morphological structure of the fiber determines the pathway that dyes take during dyeing and is critical for the rate and extent of dye uptake. In some way, the dye has to penetrate the more or less hydrophobic layer on the fiber surface, formed by the epicuticle and the exocuticle. The strong swelling capacity of the intercellular cement is important for the penetration of dyes into the fiber. Only then are the sulfur-rich keratins also penetrated by the dye molecules. In general terms, Fick s law can be applied to the diffusion phenomena [46],... 

Young H. P., Bachmann J. A. S. and Schal C. (1999a) Food intake in Blattella germanica (L.) nymphs affects hydrocarbon synthesis and its allocation in adults between epicuticle and reproduction. Arch. Insect Biochem. Physiol. 41, 214—224. 

Figure 5.3 Schematic drawing showing transport of hydrocarbons (and other lipids) from site of synthesis (oenocytes) to cuticle surface (epicuticle) and various tissues and glands. Arrows represent hypothetical transport of hydrocarbons (and/or precursors) [legends e epicuticule p procuticule h hydrocarbons (and/or precursors) d epidermal cell c canal issuing from an epidermal cell o oenocytes 1 lipophorins fm microsome fraction (reticulum endoplasmic of oenocytes, site of hydrocarbon biosynthesis) hi hemolymph pg pheromone glands ot other tissues (ovaries)] (updated from Bagnhres, 1996).

Wool is a natural protein fiber characterized by the scaly structure of its external surface-cuticle (Fig. 1). This structure, i.e. the stiffness of cuticle and smoothness of the epicuticle as well as the ability of wool to contract, causes the shrinking of wool fabric during mechanical washing processes. 

External hair of animals, generally called wool, was spun into yam and woven into fabrics. Like silk, wool is essentially protein it is composed of various amino acids, a majority of which are keratin. (Unfortunately, the keratin contains sulfur, which attracts certain insects that thrive on wool and contribute to the scarcity of historic woolen fabrics.) The outstanding morphological characteristic of wool fiber is its external scales that overlap in one direction toward the tip of the fiber. The scales can be chemically, mechanically, and temporally damaged and can disappear as the wool deteriorates. Outside of the scales is a membranous layer, the epicuticle inside them is the bulk of the wool fiber, the cortex, which consists of millions of double-pointed, needle-like cells neatly laid... 

It is important to understand the structure of insect cuticle before we study the cuticu-lar penetration of insecticides. Figure 6.1 shows the structure of insect integument. The integument is the outer layer of the insect, comprising the epidermis and the cuticle. The epicuticle is generally about 1 micron in thickness. It can be composed of as many as four sublayers the cement layer (outermost), the wax layer, the polyphenol layer, and the cuticu-lin layer. The epicuticle, which makes up about 5% of the total thickness of the cuticle, contains lipids, lipoprotein, and protein and, therefore, it is lipophilic. Beneath the epicuticle lies the procuticle, which comprises the exocuticle and the endocuticle. This is essentially a hydrophilic chitin-protein complex containing considerable quantities of water. The endocuticle is soft and is the major constituent of larvae and soft-bodied insects. It is composed of microfibers of chitin and protein, which may impart elasticity to the cuticle. Above this section, the exocuticle is predominant in hard-bodied insects and forms most of the cuticle in adult beetles. It is present only as a thin layer in many larvae and in the hard parts of... 

Important solvent properties are volatility, viscosity, surface tension, and lipid solubility. The first three determine the area over which a given volume of solvent spreads the larger the area of contact between insecticide and outer cuticle layers, the larger its total penetration rate will be. Acetone does not spread very far from the site of application, because it is so volatile. Lipid solubility affects the dissolution of the wax components of the epicuticle. By disrupting this layer, e.g., depositing a drop of acetone, the insecticide could bypass the epicuticular barrier. All these effects together may explain why an optimal balance of solvent properties is necessary to obtain maximal penetration rates (Welling and Patterson, 1985). 

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

Table 6.3 shows penetration rates of four insecticides dimethoate, paraoxon, dieldrin, and DDT, through cockroach cuticle. It is seen that the rates of penetration are inversely related to their partition coefficient in the olive oil-water system. In other words, the compound with the best solubility in water, as indicated by its partition coefficient, moved through the cuticle most rapidly. In this experiment, the insecticides were applied to the cuticle as acetone solutions, and it was suggested by the authors that this may have neutralized or canceled any barrier presented by the epicuticle. Thus, the data indicate the... 

Bd M = body weight d = day(s) EPICU = epicuticle F = female Gn Pig = guinea pig hr = hour(s) LDsj = lethal dose, 50% kill LOAEL = lowest-obsen/able-adverse-effect level M = male min = minute(s) NOAEL = no-observable-adverse-effect level NS = not specified occup = occupational wk = week(s) x = times... 

The fiber surface is bounded by a thin membrane 100 A thick called the epicuticle. The cuticle is a scaly, tubular layer and consists of flattened cells which overlap to give a rachet-like profile to the fiber. Each scale cell contains two distinct layers, a keratinous outer layer termed the exocuticle and a nonkeratinous inner layer that appears to be derived from cytoplasmic debris and is termed the endocuticle. There is some evidence that the exocuticle itself is complex with an outer cystine-rich layer termed exocuticle a. In coarse fibers the cuticle may be many scale-cells thick and where the cells overlap they are separated by an intercellular layer formed during biosynthesis by the deposition of nonkeratinous protein between the cell membranes. This layer is sometimes referred to as intercellular cement. 

The crustacean cuticle is made of chitin and protein and has an outer tanned epicuticle within which lies a tanned and calcified exocuticle. The thickest layer is the inner endocuticle which is heavily calcified but contains no tanned problems. The inside of the exoskeleton is formed by a membranous layer which is neither tanned nor mineralized but which covers the cellular epidermis. This layer contains a variety of cell types including melanophores and epidermal cells but the secretion of the exoskeleton is mainly performed by the cuticle-secreting cells and the associated intra-epidermal or reserve cells. The exoskeleton is penetrated by tegumental ducts and by pore canals. The first of these carry secretions from glands through the thickness of the cuticle while the latter may consist of 50—90 fine protoplasmic extensions of each cuticle-secreting cell penetrating the exoskeleton at a density of about 4 X 10 pore canals mm". ... 

The synthesis of the new epicuticle and exocuticle commences during premoult, requiring the resorption of the old exoskeleton across its surface. In fact, high rates of calcium flux occur across the pre-exuvial cuticle-epidermis complex in vitro (R. Roer, personal communication). The exocuticle is not mineralized until after moult and then it is calcified simultaneously with the formation and mineralization of the endocuticle. Thus, the exocuticle exists before it is mineralized but the endocuticle is secreted and calcified simultaneously (Travis, 1965). 

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