Understanding Hair Texture: Curly vs. Straight

Research Insights from Merino Sheep

A recent study published in the Journal of Experimental Biology investigates the reasons behind the varying textures of hair, specifically why some individuals have curly hair while others have straight hair. The study tested two prominent theories using wool samples from Merino sheep.

Theories on Curly Hair Formation

Over the years, two contrasting theories have been proposed to explain the curly wool of sheep. The first theory suggests that differential cell division on one side of the hair strand leads to a denser cluster of cells on that side, causing the hair to bend towards the less crowded side. The second theory posits that the presence of two distinct types of cells results in curls. According to this theory, hair curls towards the shorter paracortical cells that line one side of the hair strand, while the longer orthocortical cells on the opposite side exhibit a more chaotic structure due to their keratin fibers being oriented away from the hair fiber axis. Despite their longstanding presence in scientific discussions, neither theory has fully clarified the mechanics of curly hair.

Experimental Methodology

To explore these theories further, a research team in New Zealand conducted experiments using wool from six adult Merino sheep, as detailed in the Journal of Experimental Biology. Merino sheep are renowned for their tightly curled, fine wool, which resembles the underhair of many mammals. The researchers maintained a controlled environment for six months before collecting full-length wool samples from the sheep’s sides. Through a meticulous cleaning and rinsing process, they isolated individual wool fibers, trimmed them, and air-dried them on a vibrating platform to avoid introducing artificial curls.

To facilitate analysis, the team cut each wool sample into 717 uniform snippets. Rather than using forceps, which could damage the fibers, they employed electrostatic forces to place snippets onto microscope slides, ensuring that only those with consistent curves and no torsion were selected. Confocal microscopy was then utilized to measure the quantities and lengths of paracortical and orthocortical cells, along with assessing the curvature of each fiber.

Revising Established Theories

Upon analyzing five samples from each sheep, the researchers discovered that the expected differences in cell numbers on each side of the curl were not present, contradicting the first theory. Instead, their findings aligned more closely with the second theory, indicating that the orthocortical cells on the curve’s exterior were consistently longer than the paracortical cells on the interior. However, the team recognized that this theory was overly simplistic. They found that the absolute lengths of both cell types varied, even within individual fibers. Notably, the lengths of both cell types increased with fiber diameter, irrespective of whether the fiber was curly or straight. Crucially, the researchers identified that the degree of curvature was defined by the relative lengths of the two cell types, where a greater difference in length resulted in tighter curls.

Implications for Human Hair

Although the structural characteristics of hair are similar across mammals, the researchers could not conclusively determine whether their findings apply to humans. Human hair, for example, has a greater diameter, and the differentiation of cell types is less pronounced than in Merino wool. Despite these differences, the research findings could have significant implications for the hair-care industry and may pave the way for future studies on the science behind curly hair in humans.

References

1. Harland DP, et al. Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells. J Exp Biol. 2018; 221, jeb172312. doi: 10.1242/jeb.172312.
2. Knight K. Curl secret in cells. J Exp Biol. 2018; 221; jeb178418. doi: 10.1242/jeb.178418.