top of page
merino sheep swimming in the ocean.jpg

Structure of wool 

Basic wool stuff

Really basic

This blog section only includes cuticular cells, cortex cells, and keratin. If you are looking for other information, such as breeds, fire resistance, dyeing and finishing, or other related information, please wait for further updates! You can also learn more from the Woolmark Company or other fascinating research from different scholars. 

You can also take a look at:

Swimming with wool

merino wool sheep.jpg

Wool

The fibre in the undercoat of sheep is hydrophobic and hydrophilic simultaneously. 

Structure of wool

cuticle.jpg

 Cuticle 

cortex.jpg

 Cortex 

image_edited_edited.jpg

 Keratin 

image.png

Cuticle

Outer shell layer of wool fibre. 

Envelops the cortical by forming a sheath of overlapping scales 4

Each cuticle cell in the wool fibre includes a layered structure in mainly five layers 5

  • F-layer

  • Epicuticle

  • Exocuticle A Layer

  • Exocuticle B Layer

  • Endocuticle 

The F-layer and epicuticle are usually marked as the same layer, called the ‘proteolipid layer’, ‘resistant membrane’ or the ‘fibre cuticle surface membrane’ (FCUSM 6

F-layer consists of a lipid layer that provides a hydrophobic feature to the fibre

F-layer

and

epicuticle

Without this layer, e.g. delipidated wool with felt-resist treatment, water can be rapidly spread across the fibre surface and wet the fibre 6

The epicuticle layer is made of protein and resists oxidation by chlorine 1

In the general wool process, this membrane is usually avoided to be damage, except some chemical finishes are designed to break down this layer 6

The exocuticle layer is firm and highly cross-linked 1

Acts as a protective shell and resists the wool fibre swells in water. 

Exocuticle

Overlapping scale structure creates small spaces between the scales, which allow water vapour enters the fibre slowly  8

The exocuticle has two layers: the harder A-layer and the B-layer  4

The exocuticle A-layer: is mainly made by cystine residues and is highly crosslinked 6

This large amount of disulphide cross-links: Hydrophobic9

Structure of the cuticle 

Endocuticle

The weakest part of the cuticle and is less cross-linked

It has holes which could let the water vapour pass 7

Swells more in the water when compared with other parts of the cuticle
Primary responsibility is to swell the cuticle cells in water and lift the scale edges

Lipid layer

and Intercellular layer& Cell membrane complex

Lipid layerhydrophobic

Cell membrane complex (CMC)

Found between cortical cells and cuticle cells 10 

Cross-linked by isopeptide bonds 10 11 

Specific enzyme is needed 

Cortex

Wool fibre's internal cells layer, surrounded by the cuticle cells 1

There are three types of cortical cells in wool fibres 12

  • Ortho-cortical

  • Para-cortical 

  • Mesocortical 

Differed by their crystallinity, sulphur content and rate of water uptake 12

image.png

Ortho-cortical

Para-cortical

Cotrical cell

(Source: Woolmark Learning Centre.)
https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-3the-cortical-cells-of-the-wool-fibre/

Ortho-cortical

Higher crystalline with less sulphur and a lower water take-up rate 12

Para-cortical

Less crystalline and contain more sulphur  12

Have a more uniform keratin structure and are more resistant to chemical and mechanical attacks than the ortho-cortex  13

Higher water-update rate and swell at different rates and degrees making the wool fibre bend and create crimps  13

Mesocortical : around 4% of the wool fibre and has less impact on water absorption 12

Macrofibril

Matrix

Microfibril

Long filaments, bundles of microfibrils  14

Consists of high sulfur proteins which attract H2O  14

  • hydrophilicabsorb and retain liquid and solvent

  • Tends not to create static electricity: anti-static 15

High glycine-tyrosine proteins 10

  • Contributes to the mechanical strength and shape of hair 16

Give strength and flexibility 14

Contain pairs of left-hand curled protofibril with right-hand curled molecular chains

Keratin

Cross-links in Keratin groups 14

A significant component of wool fibre  2

A tough fibrous protein

Structure:

Molecular structure of folded polypeptide chains that situate parallelly to each other along the axis of the fibre. 

Main cross-links that secure the structure

  • Hydrogen bonding (N-H…O=C)

  • Disulphide bonds

  • Salt linkages

A large number of highly polar peptide bonds in keratin increase the number of hydrogen bonds 14

Disulphide bond: 

Critical cross-link on keratin to prevent enzymatic digestion. 

The high quantities of amino acid cysteine in keratin contain sulphur

  • Disulphide linkages link these cysteines together and comprise cystine molecules 17 18

Salt linkages:

Formed to connect the free amino and acidic carboxyl groups 14

These two cross-links help keratin resist enzymatic degradation.

I have only mentioned the rudimentary knowledge of wool structure, and I highly recommend the free e-learning platform from Woolmark, which offers all-round information.

What are your thoughts?

merino wool sheep.jpg

References

1 Lakshmanan, A. (2022). Physical and chemical properties of wool fibers. In Wool Fiber Reinforced Polymer Composites (pp. 49–71). Elsevier. https://doi.org/10.1016/b978-0-12-824056-4.00011-x

2 Eslahi, N., Dadashian, F., & Nejad, N. H. (2013). An investigation on keratin extraction from wool and feather waste by enzymatic hydrolysis. Preparative Biochemistry and Biotechnology, 43(7), 624–648. https://doi.org/10.1080/10826068.2013.763826

3 Qiu, J., Wilkens, C., Barrett, K., & Meyer, A. S. (2020). Microbial enzymes catalyzing keratin degradation: Classification, structure, function. In Biotechnology Advances (Vol. 44). Elsevier Inc. https://doi.org/10.1016/j.biotechadv.2020.107607

4 Ammayappan, L. (2013). Eco-friendly Surface Modifications of Wool Fiber for its Improved Functionality: An Overview. Asian Journal of Textile, 3, 15–28. https://scialert.net/fulltext/?doi=ajt.2013.15.28

5 Woolmark Learning Centre. https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-2-the-cuticle-cells-of-the-wool-fibre/

6 The Woolmark Cpompany. THE CUTICLE CELLS OF THE WOOL FIBRE. Wookmark Learning Centre. Retrieved November 12, 2022, from https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-2-the-cuticle-cells-of-the-wool-fibre/

7 Scanavez, C., Joekes, I., & Zahn, H. (2004). Extractable substances from human hair: A discussion about the origin of the holes. Colloids and Surfaces B: Biointerfaces, 39(1–2), 39–43. https://doi.org/10.1016/j.colsurfb.2004.08.021

8 Wang, L., & Wang, X. (2009). Effect of structure–property relationships on fatigue failure in natural fibres. Fatigue Failure of Textile Fibres, 95–132. https://doi.org/10.1533/9781845695729.2.95.

9 Das, D., & Das, S. (2022). Wool structure and morphology. In Wool Fiber Reinforced Polymer Composites (pp. 13–32). Elsevier. https://doi.org/10.1016/b978-0-12-824056-4.00013-3

10 Jones, L., & Rogers, G. (2009). 2. Structure and Composition of Wool. Wool Biology and Metrology. 

11 Rice, R.H., V.J. Wong, and K.E. Pinkerton. 1994. Ultrastructural visualization of cross-linked protein features in epidermal appendages. Journal of Cell Science.:1985-1992.

12 Woolmark.. THE CORTICAL CELLS OF THE WOOL FIBRE. Woolmark Learning Centre. Retrieved November 12, 2022, from https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-3the-cortical-cells-of-the-wool-fibre/

13 Jones, D., & Brischke, C. (2017). Nonwood bio-based materials.

14 Mathison (1964), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.

15 Woolmark. The Properties of wool. https://www.learnaboutwool.com/globalassets/law/resources/factsheets/secondary/gd3270-secondary-fact-sheet_2019_j.pdf

16 Matsunaga, R., Abe, R., Ishii, D., Watanabe, S. I., Kiyoshi, M., Nöcker, B., Tsuchiya, M., & Tsumoto, K. (2013). Bidirectional binding property of high glycine–tyrosine keratin-associated protein contributes to the mechanical strength and shape of hair. Journal of structural biology, 183(3), 484-494.

17 Hearle & Peters (1963), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.

18 Martin (1990), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.

Modified in 15th March, 2024.

bottom of page