Curly, wavy or straight? The shape of our hair is one of our defining features.
Yet, despite decades of research, it has been hard to pin down what causes kinks in individual hairs.
The answer to this curly question may lie with an animal known for its super-springy locks: the merino sheep, according to a team of researchers.
They went to painstaking lengths to put wool from New Zealand’s finest sheep under the microscope to test long-held beliefs about the structure of hair.
As it turns out it’s not the number, but the length of individual cells that is critical to curls in wool fibres, they report in the Journal of Experimental Biology.
While merino wool is about five times finer than human hair, the basic biology is similar, so it’s a good hair model, said the study’s lead author Duane Harland from AgResearch in New Zealand.
“The [fibres] are different sizes but what we learn from animals does have direct relevance in hair,” Dr Harland said.
And it is much easier for microscopes to penetrate through the finer fibres to see individual cells.
“If you’ve got to try to understand something that’s made of cells, and you’re counting cells, it’s best to have the minimum number of cells to understand the problem,” he said.
“The other good thing about merino wool is that it doesn’t have any pigment in it, so it’s fairly transparent.”
Hair, which is made from a protein called keratin, grows from follicles in the skin.
At the core of every hair or wool fibre lie paracortical cells (containing parallel strands of the protein keratin) and orthocortical cells (containing twisted keratin fibres).
Dr Harland and colleagues set out to test hypotheses about how these cells create curls.
One idea is that longer orthocortical cells line the outside of the hair fibre, while paracortical cells line the inner edge.
Or all cells could be the same length and there are simply more cells on the outside of the curled fibre than the inside, similar to the design of an archway.
“We went in to test those two theories because as it turns out no-one has been able to directly measure them before,” he said.
And with good reason.
“It was very fiddly because the pieces of wool you’re dealing with are incredibly small,” Dr Harland said.
Precision and patience needed to tease out answer
The team clipped wool from the winter coats of six sheep that had been kept under identical conditions for six months.
They then selected 700 individual fibres — each no more than five micrometres across and half a centimetre in length.
The fibres were washed to remove lanolin coating and any artificial curvature caused by the wool being squashed as it grew, to reveal the natural state of the fibre.
Then the fibres were dried as they hovered above a vibrating surface, which ensured they wouldn’t be kinked by sticking to anything else.
After that, the fibres were chopped up into uniform pieces and manoeuvred onto a microscope slide using electrostatic forces.
From there the team were able to measure the average curvature of each fibre and then zoom into the cell structure at the exact point of the curve.
They looked to see whether the location of the two different types of cells was related to the curvature or whether there were simply more cells of any type on the outside.
What they found was that neither hypothesis was right.
While orthocortical cells on the top of the fibre were indeed longer than paracortical cells at specific locations on the fibre, the team found the curve in each hair was unique because the length of orthocortical and paracortical cells can differ.
“It’s not just one size fits all, it’s not just like all orthocortical cells are longer than all paracortical cells [in every hair fibre],” Dr Harland said.
“So there are still things to discover out there.”
It’s still unclear whether this relationship seen at a specific point on the fibre applies to the whole hair.
And more research is needed to tease out whether the lengthened cell structures seen in merino wool also occur in fibres with higher diameters such as human hair.
“Our current methods make it difficult to measure the details of the cells in very thick fibres,” Dr Harland said.
The next step for his research, he said, is to understand how genes influence the development of keratin in the hair follicle.
“The cells start out as balls and end up as long spikes,” Dr Harland said.
“We don’t know enough to understand how genes would control processes [inside the follicle] to program [cells] in a particular part of the curvature.”
Dr Harland said research in this field could help the wool industry, especially in the production of super-fine fibres used in high-performance sportswear.
“Wool has a lot of good properties, but we still don’t know how to make it slightly stronger or slightly uniform,” he said.
It could also be used by cosmetics companies such as Kao Corporation, which part-funded the research, for the development of products to tame or enhance curly tresses.