Wool fibre is produced from follicles embedded 500-600 |xm below the skin surface. The follicular tissue in the skin of Merino sheep amounts to about 50 g, or 0.1% of live weight (Williams, 1995). The follicle has three regions: the bulb is the end most deeply embedded in the dermis and is where cell proliferation occurs; cells migrate into the keratogenous zone where protein synthesis occurs and cell volume increases 10- to 20-fold. Further up, the cell reaches the zone of hardening where the hard disulphide linkages of the keratin are formed. Some cells from the bulb differentiate into the inner or outer root sheath (Black, 1988). The turnover time of the bulb (total number of bulb cells/new germinated cells per hour) ranges from 19 to 42 h (Wilson and Short, 1979). The inverse of turnover time (i.e. 0.024-0.053 h"1, or 0.57-1.26 day"1) is the turnover rate of the bulb (in terms of a proportion of new germinated cells to the total number of the bulb). This value indicates that the follicle has one of the highest turnover rates of all the tissues in the body.
The wool fibre is primarily protein with as little as 0.5% lipids and minerals (Williams,
© CAB International 2003. Amino Acids in Animal Nutrition, 2nd edition (ed. J.P.F. D'Mello)
1995). Wool protein is grouped into three main types: the low sulphur proteins making up about 60-70% of total protein, the high-sulphur proteins that account for up to 20-40% of total protein and contain very high levels of Cys but no Met, and the high-tyrosine (Tyr) proteins that make up about 1-12% of the total protein and are characterized by high levels of Tyr and no Met, lysine (Lys), isoleucine (lie), histidine (His) or glutamate (Glu) (Table 17.1; Reis, 1979). The keratin wool protein is synthesized just above the bulb, then assembled into mibrofibrils and matrix, and finally hardened by formation of disulphide bonds within and between protein chains (Williams, 1995).
Cys accounts for about 10% (8.6-13.1%) of the amino acids in wool protein (Reis, 1979) compared with 1.3% in the whole body (MacRae et al., 1993) (Table 17.1). The concentration of Met in wool is low, about half that in the whole body. Met is contained in the filament proteins, but not in the matrix protein. Ser concentrations are also high in wool protein, approximately double that in the whole body. The function of Ser in the fibre is not clear. Possibly, the hydroxyl group of Ser forms hydrogen bonds that help to consolidate the fibre structure.
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