Importe efectivo a liquidar.

One of the primary functions of fellmongering is the removal of wool. Knowledge of the structure of the wool fibre and wool root is therefore important. A growing wool follicle can be divided into two zones. Firstly the transient zone which involves the growth of connective tissue and fibroblasts, and secondly the permanent zone which contains the mature hair shaft that extends out of the follicle mouth, the erector pili muscle, and the sebaceous gland (Montagna & Parakkal, 1974). The transient zone consists of the matrix, cell growth and differentiation, and keratogenous zones. The hair starts at the matrix where a mass of cells begin growth that then differentiates into the different cell types of the different layers of the hair (Straile, 1965).

2.5.4.1 Hair shaft structure

Like a spring onion (or scallion) a hair has annular layers extending from a common base. Moving from the centre of the generic hair outwards the layers are called medulla, cortex, and cuticle. Surrounding the hair’s outer cuticle layer inside the follicle mouth are further layers or root sheaths. The next layer (still moving from inside out) is the inner root sheath consisting of Huxley’s layer and Henle’s layer. Next is the companion layer. Finally is the outer root sheath (Cantera, 2001a).

Cortex/medulla Cuticle Root cuticle Huxley’s layer Henle’s layer Companion layer

Outer root sheath (ORS)

Outer root sheath

Figure 6: Hair structure (coloured for sake of visualisation)

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2.5.4.1.1 The medulla

The medulla is present in the wool of only some of the sheep breeds in New Zealand and then only in an intermediary form in a small portion of some of the wool fibres (Maddocks & Jackson, 1988). The quantity of medullated hairs and the size of the medulla depend on the sheep breed. Specifically the proportion of medullated fibres depends on a range of environmental factors including breed, diet, and the age of the animal. The portion of medullated fibres in the wool from the New Zealand Romney ranges between 1 and 5 % (Scobie et al., 1998).

When present, the medulla exists at the centre of the wool shaft. In merino sheep for example, the medulla consists of an unbroken column along the axis of the shaft that is symmetrically shaped in cross section with a fibrous amorphous infill material. An inner framework supports a shell surrounding the air spaces (Blažej et al., 1989).

2.5.4.1.2 The cortex

The cortex makes up the majority of the fibre. The wool cortex can be differentiated into two parts; the orthocortex and the paracortex. The two types can be observed by differential staining. The cortical cells are about 95 um long and 5 um in diameter and contain a small portion of cytoplasmic debris. Cortical cells consist mostly of keratin that is organised in a structural hierarchy Keratin molecules are packed into microfibrils, which are in turn packed into macrofibrils that make up the bulk of the cortical cells (Maclaren & Milligan, 1981). Under transmission electron microscopy the intercellular spaces appear as a region of protein deposited during keratinisation that is sandwiched between two layers of covalently bound lipids. The central protein layer is known as the δ-layer, and the intermediary lipid layer as the β-layer (Negri et al., 1996). It is quite possible that this material is degraded by so called “keratinases” resulting in the release of cortical cells.

2.5.4.1.3 The cuticle

Surrounding and encapsulating the cortex is the cuticle. The cuticle is made up of scale like cuticle cells which act as a protective barrier. The cuticle cells have two parts; the exo-cuticle and the endo-cuticle and is surrounded by a hydrophobic membrane complex containing covalently bound fatty acids that cements the cuticle together. These lipids differ from those present in the cortical cell membrane matrix in that they

can be removed using methanolic KOH unlike those of the cortical membrane. (Negri et al., 1996).

2.5.4.2 Hair root structure

2.5.4.2.1 Inner root sheath (IRS)

The inner root sheath consists of three layers; firstly the root cuticle that is in direct contact with the hair cuticle, and is followed by Huxley’s layer and then by Henle’s layer as illustrated in Figure 6.

The root cuticle consists of a single layer of cells that are partially nested into the scale formation of the hair cuticle cells, and helps to hold the hair in place. This layer may also be involved in the formation of the shape of the hair cuticle. Huxley’s layer may be more than one cell thick and differences in thickness appear to be associated with changes in the hair diameter. Huxley’s layer may also be involved in the transport of nutrients to the hair cells (Straile, 1965).

Henle’s layer lies between Huxley’s layer and the companion layer to the outer root sheath. It may be involved in the movement of the hair up through the hair pocket and may be involved with nutrient transport to the hair.

2.5.4.2.2 Outer root sheath (ORS)

The outer root sheath is continuous with the epidermis being one to two cells thick around the bulb. The cellular structure is therefore very similar to that of the epidermis. 2.5.4.2.3 Companion layer

The cell layer closest to the inner root sheath becomes differentiated from the rest of the outer root sheath after four or five cells distant from the bulb. These cells become flattened and are known as the companion layer (Orwin, 1979). It is likely that the cells of this layer are involved in the movement of the inner root sheath up the hair pocket and the eventual destruction of the inner root sheath.

2.5.4.3 Hair growth

Cell growth and differentiation begins in the bulb, at the centre of which is the dermal papilla, which supplies nutrients to the growing hair cells. The papilla controls the type

of hair produced. For example a small papilla results in a small hair. Surrounding the dermal papilla is a region of mitotically active cells known as the matrix where cell growth begins. As cells are pushed up past the dermal papilla cell differentiation begins. During growth and differentiation of the cortical cells the intercellular spaces decrease, cell membranes increase in size and the laminar structure of the plasma membrane becomes indistinguishable until it can no longer be detected. Surface glycoproteins which are detectable in the lowest zone disappear as the cells adopt their final shapes and are undetectable in the mature hair. Cell adhesion appears to be required only at early stages of differentiation and is not needed in the mature cells (Orwin, 1979). Specifically targeting this cell adhesion may help to weaken the base of the fibre and assist in depilation processes without damaging the mature wool fibre product.

2.5.4.3.1 Keratinisation

Prekeratin is formed in the differentiating cells of keratinising tissues (Matoltsy, 1965). Prekeratin can be extracted from prekeratinaceous tissue by 6M urea solution which partially breaks down the prekeratin units. Sulfur is present in the keratin extracts in the S-H form. The molecular weight of pre-keratin is 60 – 100 kDa (Matoltsy, 1965) and its amino acid content has been reported by Matoltsy et al. (1964) pg 302. Prekeratin is formed into microfilaments that are about 60-80 Å thick made up from molecules that are around 1050Å long.

As the cells move further up the hair pocket keratinisation begins with cells of the inner root sheath becoming keratinised first. The process begins in Henle’s layer followed by the root cuticle followed by Huxley’s layer (Orwin, 1979). Keratinisation begins as filaments form in the cell cytoplasm with associated trichohyalin granules. As cells move further up the hair pocket the filaments begin to fill the cell cytoplasm and other cellular components are destroyed or eliminated in order to make room for these growing microfibrils (Montagna & Parakkal, 1974). It has been suggested that the organelles in the inner root sheath (IRS) become incorporated into the hardened IRS (Orwin, 1979). As cells mature, keratin in the macrofibrils acquires disulfide cross links between cysteine groups. Within the cortex, the macromolecular form of the keratin can vary according to the cell type. Macrofibrils are discrete and circular in the orthocortex, but form an amorphous mass in the para-cortical cells (Orwin, 1979). The final

macrofibrilar structure is made up from a number of microfibrils within a sulfur-rich matrix forming macrofibrils.

As the inner root sheath reaches the top of the hair pocket it becomes corrugated, and protrudes from Huxley’s layer into the ORS, the protrusions increasing in number as the cells move up until the cells finally compact, break up and are eliminated inside the pilosebaceous canal (Maclaren & Milligan, 1981).

2.5.4.4 Connective tissue sheath (follicular basement membrane)

Surrounding the hair follicle is the basement membrane and surrounding this is the connective tissue sheath. The basement membrane contains mostly sulfated glycosaminoglycans, while the connective tissue sheath is composed of collagen fibres arranged in two layers. The inner layer of collagen fibres lies parallel to the hair shaft whereas the outer layer has fibres lying at right angles to the hair shaft (Orwin, 1979). The connective tissue sheath and the dermal papilla are continuous with each other and share some traits (Couchman et al., 1991). Basement membrane components are found throughout the papilla matrix; for example an observable basement membrane was found between the hair follicle epithelium and the dermal papilla (Couchman et al., 1991).