What Makes Hair Curl?

While the presence of hair is a characteristic unique to mammals, we see great diversity in how curly hair is, both between and within species. Two theories have previously been proposed to explain the underlying cellular mechanisms for curvature in hairs. The ‘cell type distribution model’ suggests that the proportion and distribution of orthocortical and paracortical cells is responsible for curvature, whereas the ‘cell proliferation model’ suggests that asymmetry of cell division rates causes curls. 

In work published in the Journal of Experimental Biology, scientists from New Zealand and Japan tested the two theories using curly merino sheep wool. While they found that cell type does play a role in curvature, it is not dependent on the amount of each cell type, but rather the length of the orthocortical and paracortical cells. The study also found no supporting evidence that rate of cell proliferation determines curvature. 

We spoke to Dr Duane P Harland, Senior Scientist, AgResearch Limited, lead author of the paper, to learn more about the study and its wider implications.

Anna MacDonald (AM): Why was merino wool chosen as the hair type for this research?

Duane P Harland (DH): Because it is narrow diameter, non-pigmented and highly curled. This meant we could use the 3d (confocal) microscopy method to see through the entire fibre. Thicker fibres did not work well and neither did fibres with colour, which absorbed the light. We could have chosen other fine wool breeds, but merino is a well-studied wool and there is also an important industry in New Zealand and Australia, which produces high-tech sportswear from merino wool.

AM: How will the findings apply to other hair types such as human?

DH: Merino wool, and any wool, is very similar to human hair in its basic chemistry, structure and the process by which it is grown within the skin. Because the wool that we used was much lower diameter than human hair (approx. 20 micrometres compared to human hair which is between 60 and 150) there are fewer cells. Fewer components make it easier to understand, but the underlying mechanism of curvature is directly applicable to higher diameter hairs (including higher diameter wool). Wool and hair share a similar growth process within the skin and the result we got gives us really good clues about what parts of that shared mechanism to look at if we want to understand about what controls the process of growing curls. We also know from our work that every curl in merino wool is an individual with different cell lengths creating the curl. This is helpful for both understanding wool as a product as well as its formation, and human hair is the same.

AM: What function does hair curvature play in mammals? 

DH: For most mammals, hair curl is an important way of creating a coat which fills space in three dimensions, rather than just hanging lank against the skin. Most mammals, as diverse as cats, kangaroos and wild sheep, have coats in which long guard hairs stick out of an under coat of fine curly hairs. The combination gives physical protection as well as trapping air to form an insulating layer. Hair is an archetypal mammalian; which is why we can use wool to study human hair and vice versa. In domestic sheep, we used selective breeding over ten thousand years to get all hairs uniform and densely packed. However, curvature is still critical to keeping the wool lofted up to provide that vital insulation. Occasionally sheep are born with straight wool, and many of those die from exposure as lambs.

What curl does for humans is more debatable. It is clear that while scalp hair still protects our delicate brains from sun, cold and encounters with vegetation, it has perhaps taken on more of a social aspect in our species.

Dr Duane Harland was speaking to Anna MacDonald, Science Writer for Technology Networks.