Sheep's wool

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Introduction

Figure 1. A hairy and woolly coated lamb from the Romane breed, a hybrid of Berrichon du Cher (a single coated woolly breed) and Romanov (a double coated, hair producing breed)[1]

Sheep (Ovis aries) have been selectively bred to continuously produce single coated wool fleece rather than coats composed of an outer hair layer and an inner wool layer.[2] True wool, as opposed to hair, is characterised by its high follicle density in the skin, small diameter, and high crimp [3]
The single woolly coat is recessive trait caused by the insertion of an antisense EIF2S2 retrogene[4] into the 3′ untranslated region of the IRF2BP2 gene.[1] This gene mutation creates a chimeric IRF2BP2/asEIF2S2 RNA transcript that targets the genuine sense EIF2S2 mRNA and creates EIF2S2 dsRNA that regulates the production of EIF2S2 protein. [1] Differences in EIF2S2 expression are visible in lamb's wool: a woolly lamb has visible curls or ringlets while a hairy lamb has more of a wave pattern (Figure 1).

Wool structure and composition

Wool is a keratin fiber containing 18 amino acids, many of which have hydrophilic amino and carboxyl groups that allow wool to absorb high amounts of water.[5] Each individual fiber is composed of an cuticle layer of overlapping cells wrapped around a cortex. Coarse wools and many animal fibers also contain a medulla consisting of empty vacuoles.[6]
Wool has a cuticle layer that is only one cell thick, while human hair, for example, has a cuticle layer up to 10 cells thick. Wool cuticle cells also have a wedge-shaped shaped cross-section as opposed to rectangular, so the exposed edge height of wool cuticle cells is about 1 um as opposed to < 0.5 um in other animal fibers.[6] The cuticle layer is why wool felts – the fibers can only move smoothly over each other in one direction. If going the other direction, the exposed edges of the cuticle cells catch the gaps between edges and lock together, similar to a zipper.[5]

Figure 2. The cross-section of a wool fiber showing the arrangement of para- and ortho-cortical cells. [7]

Cortical cells are helix shaped and generally categorised into para- and ortho- cortical cells, with meso- cortical cells having characteristics in between the para- and ortho- categories. Para-cortical cells are loosely packed and have large amounts of cystine, while ortho-cortical cells contain discrete macrofibrils and are high in tyrosine.[8] It was previously believed that wool’s crimp was caused by the distribution of para- and ortho-cortical cells because wool fibers show well defined bilateral segmentation in the cortex.[6][8] However, further research has disproved the association between para and ortho-cuticle cell distribution. [9][10] The most recent hypothesis is that crimp is caused by the differences in length between para- and ortho-cuticle cells.[10] However, It is generally accepted that crimp is caused by differences in para- and ortho-cortical cell division rates.[11][9]

Apart from the fibers themselves, wool can contain up to 40% or more contaminants by weight; the most common are lanolin (analogous to sebum in humans), suint (sweat), and environmental contaminants such as dirt and vegetable matter.[12]

Microbial interactions with wool

There are many microbes naturally present in sheep’s wool with >95% of bacteria found on the outer ends of the fleece and relatively few on the sheep’s skin and innermost parts of the fleece.[13]

Overpopulation of certain bacteria such as Pseudomonas aeroginosa is associated with one of wool farming’s primary concerns – blowfly infestation (also called flystrike).[14] In prolonged moisture, the protective waxy layer of sheep skin breaks down and allows P. aeroginosa and other opportunistic bacteria to multiply. This causes fleece rot – matted wool, staining or discoloration (green in the case of P. aerifinosa[14]), and skin lesions. Flystrike is not only encouraged by the damp conditions and vulnerable skin associated with fleece rot, the bacterial odors can attract female blowflies and stimulate oviposition. [15]

Microbial products have potential industrial applications, such as proteases being used for subtractive superwash treatment (degrading the cuticle cells so that wool can be machine washed and dried without felting)[16] Studies[16] are being done on finding proteases to use in the superwash process by studying microbes naturally present on wool. This is important research for the sustainability of the wool industry because the most common supperwash method, the chlorine-Hercosett process, discharges carcinogens into the environment and kills aquatic organisms.[17]

Another potential application of microbial products is the use of biosurfectants to remove lanolin from wool (scouring). [18] Research into new scouring methods is important because current methods use detergent in 8-10 liters of water per kilogram of wool. The resulting solution is considered one of the most polluted industrial wastewaters.[19]

References

  1. ↑ 1.0 1.1 1.2 Demars J, Cano M, Drouilhet L, et al. Genome-Wide Identification of the Mutation Underlying Fleece Variation and Discriminating Ancestral Hairy Species from Modern Woolly Sheep. Mol Biol Evol. 2017;34(7):1722-1729. doi:10.1093/molbev/msx114
  2. ↑ [Ryder M. A survey of European primitive breeds of sheep. Ann Genet Sel Anim. 1981;13(4):381-418. doi:10.1186/1297-9686-13-4-38]
  3. ↑ Doyle EK, Preston JWV, McGregor BA, Hynd PI. The science behind the wool industry. The importance and value of wool production from sheep. Anim Front. 2021;11(2):15-23. Published 2021 May 17. doi:10.1093/af/vfab005
  4. ↑ Staszak K, Makałowska I. Cancer, Retrogenes, and Evolution. Life (Basel). 2021;11(1):72. Published 2021 Jan 19. doi:10.3390/life11010072
  5. ↑ 5.0 5.1 Hassan MM, Carr CM. A review of the sustainable methods in imparting shrink resistance to wool fabrics. Journal of Advanced Research. 2019 18:39-60. doi:10.1016/j.jare.2019.01.014
  6. ↑ 6.0 6.1 6.2 Wortmann, F.-J. (2009). The structure and properties of wool and hair fibres. Handbook of Textile Fibre Structure, 108–145. doi:10.1533/9781845697310.1.108
  7. ↑ Science Learning Hub – Pokapū Akoranga Pūtaiao. (2010). Cross-section of wool fibre. The University of Waikato Te Whare Wānanga o Waikato. Available from: https://www.sciencelearn.org.nz/images/984-cross-section-of-wool-fibre
  8. ↑ 8.0 8.1 Marshall RC, Orwin DF, Gillespie JM. Structure and biochemistry of mammalian hard keratin. Electron Microsc Rev. 1991;4(1):47-83. doi:10.1016/0892-0354(91)90016-6
  9. ↑ 9.0 9.1 Hynd PI, Edwards NM, Hebart M, McDowall M, Clark S. Wool fibre crimp is determined by mitotic asymmetry and position of final keratinisation and not ortho- and para-cortical cell segmentation. Animal. 2009;3(6):838-843. doi:10.1017/S1751731109003966
  10. ↑ 10.0 10.1 Harland DP, Vernon JA, Woods JL, et al. Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells. J Exp Biol. 2018;221(Pt 6):jeb172312. Published 2018 Mar 22. doi:10.1242/jeb.172312
  11. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ”doyle”
  12. ↑ Shi C, Wang Q, Li D, Zeng B, Liu Q, Cui Y, et al. Inorganic composite coagulant for wool scouring wastewater treatment: Performance, kinetics and coagulation mechanism. Separation and Purification Technology. 2023 May 15;313:123482. doi: 10.1016/j.seppur.2023.123482
  13. ↑ Jackson, T. A. et al. (2002) ‘Abundance and distribution of microbial populations in sheep fleece’, New Zealand Journal of Agricultural Research, 45(1), pp. 49–55. doi: 10.1080/00288233.2002.9513492.
  14. ↑ 14.0 14.1 Norris BJ, Colditz IG, Dixon TJ. Fleece rot and dermatophilosis in sheep. Vet Microbiol. 2008;128(3-4):217-230. doi:10.1016/j.vetmic.2007.10.024
  15. ↑ Emmens RL, Murray MD. The role of bacterial odours in oviposition by Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae), the Australian sheep blowfly. Bulletin of Entomological Research. 1982;72(3):367-375. doi:10.1017/S0007485300013547
  16. ↑ 16.0 16.1 Queiroga AC et al. (2007). Novel microbial-mediated modifications of wool. Enzyme and Microbial Technology. doi: 10.1016/j.enzmictec.2006.10.037
  17. ↑ Abou-Taleb M and El-Sayed H (2024) ‘Durable Machine-Washable Wool via AOX-Free Plasma-Mediated Coating with Keratin’, Journal of Natural Fibers, 21(1). doi: 10.1080/15440478.2024.2408626.
  18. ↑ Jibia SA et al. (2017) ‘Biodegradation of Wool by Bacteria and Fungi and Enhancement of Wool Quality by Biosurfactant Washing’, Journal of Natural Fibers, 15(2), pp. 287–295. doi: 10.1080/15440478.2017.1325430
  19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named “shi”



Edited by Isaac Yu, student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.

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