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A method of determining cell types in wool fibres by methylene-blue staining has been extended to allow relationships between cortical cell type and cortical diameter to be studied in wools from individual sheep. Application of the method to wools from 12 sheep from six breeds showed that orthocortical cells were the predominant cell type produced. The percentage area occupied by orthocortical cells in a fibre cross-section increased with increasing cortical diameter in either a curvilinear (log-linear) or a linear manner. Nutritional stress or season may have affected the relationship in some sheep.

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Hair curvature underpins structural diversity and function in mammalian coats, but what causes curl in keratin hair fibres? To obtain structural data to determine one aspect of this question, we used confocal microscopy to provide measurements of the two cell types that make up the cortex of merino wool fibres, which waschosen as a well-characterised model system representative of narrow diameter hairs, such as underhairs. We measured orthocortical and paracortical cross-sectional areas, and cortical cell lengths, within individual fibre snippets of defined uniplanar curvature. This allowed a direct test of two long-standing theories of the mechanism of curvature in hairs. We found evidence contradicting the theory that curvature results from there being more cells on the side of the fibre closest to the outside, or convex edge, of curvature. In all cases, the orthocortical cells close to the outside of curvature were longer than paracortical cells close to the inside of the curvature, which supports the theory that curvature is underpinned by differences in cell type length. However, the latter theory also implies that, for all fibres, curvature should correlate with the proportions of orthocortical and paracortical cells, and we found no evidence for this. In merino wool, it appears that the absolute length of cells of each type and proportion of cells varies from fibre to fibre, and only the difference between the length of the two cell types is important. Implications for curvature in higher diameter hairs, such as guard hairs and those on the human scalp, are discussed.