Epicuticle structure

Figure 1-26. Schematic diagram of the epicuticle structure by Negri and Cornell [75].

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],... 

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. 

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... 

An important component of cuticle is 18 - methyl - eicosanoic acid [40]. Fatty acid is bound to a protein matrix, forming a layer in the epicuticle [41,42], and this layer is referred to as F - layer [43]. The F - layer can be removed by treatment with alcoholic alkaline chlorine solution in order to enhance wettability. The cuticle and epicuticle control the rate of diffusion of dyes and other molecules onto the fibre [44]. The cortex, however, controls the bulk properties of wool and has a bilateral structure composed of two types of cells referred to as ortho and para [45,46]. The cortical cells of both are enclosed by membranes of at least three distinct layers within which the microfibrils fit. Cells of intermediate appearance and reactivity designated meso - cortical have also been reported [47]. Cortical cells on the ortho side are denti-cuticle and thin, those on the para side are polygonal and thick [47]. Fig. 1-7 illustrates the bilateral structure which is responsible for the crimp of the... 

A portion of the undermembrane of Figure 1-21 is also epicuticle. The cystine-rich proteins of the cuticle belong to the group of proteins called keratin-associated proteins. Although structurally different, keratin-associated proteins are also found in the matrix of the cortex. See the section that discusses keratin-associated proteins in Chapter 2. For more details of the intercellular structures, see Figure 1-23. Thus, the cuticle of human hair is a laminar structure similar to the cuticle of wool liber, and the different layers of the cuticle have been described for merino wool [64] and for human hair [58, 65, 66, 67], Figures 1-23 and 1-24 illustrate the... 

Feeder et al. [74] have defined the epicuticle as a chemically resistant proteinaceous membrane that remains on keratin fiber surfaces after strongly bound lipids have been removed with potassium t-butoxide in anhydrous butanol. Thus, the epicuticle is a proteinaceous layer about 50 angstroms thick covered by strongly bound structural lipid that Feeder calls the F-layer. The F layer represents the outermost fiber surface of Figure 1-21. 

The structure and degradation of the cell membrane complex including the structure of the epicuticle. 

Leeder and Rippon [85] have analyzed the lipid composition of wool fibers after removing surface grease. Continued extraction with solvent removed the beta layers evidenced by electron microscopy however, the extract contained free cholesterol and free fatty acid and triglycerides but negligible quantities of phospholipid normally associated with biological membrane lipids. Koch [86], in his work with internal lipid of human hair, did not report significant quantities of phospholipid. These lipid-protein layers of hair are most likely related structurally to those of the epicuticle. 

The dissolution or the removal of structural lipids or proteinaceous matter from hair-probably from the cell membrane complex or epicuticle by either shampoos, surfactant solutions, or other cosmetic treatments-has been demonstrated by several different scientists. For example, Marshall and Ley [153] demonstrated the extraction of proteinaceous components from the cuticle of wool fiber by surfactant solutions of sodium dodecyl sulfate, cetrimonium bromide, and triton X-100. Zahn and co-workers [154, 155] and Mahrle [156] independently showed that part of the lipid components of the cell membrane complex were removed from hair by permanent waving. 

FIGURE 5.6 Model structure proposed for the fiber cuticle surface membrane. The exterior surface of the cuticle consists of a monolayer comprising C21 branched-chain fatty acids linked covalently via thioester bonds to the proteins comprising the Allworden-induced resistant membrane (epicuticle). The epicuticle consists of an inert protein matrix containing isopeptide cross-links and is linked via an unknown mechanism to the underlying a-layer of the exocuticle. 

The most important part of the cuticle from the point of view of polymer deposition is, of course, its outermost surface, i.e., the epicuticle. The epicuticle has been defined by Lindberg (11) as the membrane that contains sacs or bubbles raised upon immersion of wool fibers in chlorine water, i.e., the so-called Allwoerden reaction. It has been shown, however, that the removal of strongly bound surface lipids does not affect this phenomenon (12). It is assumed therefore, that the epicuticle, which is about 25 A thick, is a residue of the cuticle cell membrane and is at least partly proteinaceous in nature and, together with surface-bound lipids (9), forms the F-layer. A recent model of the epicuticle membrane proposed by Negri and co-workers (13) incorporates new evidence and defines the epicuticle in terms of a membrane consisting of 25% lipids and 75% protein, the latter having an ordered, possibly P-pleated sheet structure with 12% cystine, as shown in Figure 4. 

Wool fibers contain two types of cells, viz. cuticle cells and cortical cells. The cuticle cells consist of external epicuticle, exocuticle, and endocuticle. The cortical cells are divided into two different types of cells termed as orthocortical and paracortical cells which occupy about 90% of the wool fibers. They are separated from one another by a cell membrane complex with three layer structure. The cortex structure is constituted from the crystalline microfibril of the a-helical aggregate embedded in a matrix of high sulfur content. Wool fiber is thus a composite material with a variety of function on mechanical, chemical, and physical properties. 

The complex morphology of hair essentially consists of four components of different functionality (i) The cortex that gives the hair its mechanical properties consists of elongated, spin-shaped cells aligned in the direction of the fiber axis. The keratinized protein in the form of microfibrils resides in these cells, (ii) The medulla is located in the center of some thicker fibers and it consists of a loosely packed porous cellular structure (it does not contribute to the mechanical properties of the hair), (iii) Cell membrane complex which cements the various cells of the cuticula and the cortex and it consists of several layers, (iv) Cuticle, a multilayered structure which consists of flat cuticle cells and the most outer layer, i.e. the epicuticle (which is about 2.5 nm thick) is the most important part for deposition of surfactants and polymers in the shampoo formulation. This consists of 25 % lipids and 75 % protein, the latter having an ordered possibly p-pleated sheet structure with 12% cystine. The cystine groups are acylated by fatty acids which form the hydrophobic surface region. A schematic representation of the epicuticle is shown in Fig. 1.46. 

Ivanova, E.P., Nguyen, S.H., Webb, H.K., Hasan, J., Truong, V.K., Lamb, R.N., Duan, X., Tobin, M.J., Mahon, RJ., Crawford, R.J., 2013b. Molecular organization of the nanoscale surface structures of the dragonfly Hemianax papuensis wing epicuticle. PLoS One 8, e67893. 

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