1. Which side of a sheep’s wool has longer cells?
Hair is made of elongated cells arranged in rows. An article introducing a 2018 study on Merino sheep suggests that hair curves because the cells on one side are longer than those on the other [3]. The outer curve consists of longer cells (orthocortex), while the inner curve has shorter cells (paracortex). The study reported a tendency for outer cells to be longer than inner ones within a single hair strand [3].
There was an idea that hair curves because there are fewer cells on the inside, but this study did not support that view [3]. Think of ribbon curling when you run it over the back of a spoon: one side stretches more, creating the curve. Hair might be similar, but this is just an analogy to make it easier to understand; the cells inside hair do not move in the same way as ribbon.
However, these results are from sheep wool. The article uses cautious language, noting "at least in sheep" [3]. So, at what point does hair first get its curve?
2. Is curly hair already bent inside the follicle?
Hair is made in a bag-like organ under the skin called the follicle. In people with curly hair, this follicle often bends like an arch, and the deepest part that makes the hair (the bulb) may also be curved [1]. General explanations suggest that stronger bending leads to stronger curls [1].
A review of hair science research states that curly hair in humans (regardless of race) and sheep typically comes from a curved bulb [5]. This suggests that the shape of the follicle is thought to be related to the curl, rather than the hair twisting after it grows.
This connects to the sheep study. If hair coming from a bent follicle has longer cells on one side, then "follicle bend" and "cell length difference" might be two views of the same phenomenon: outside and inside. However, within the range of materials consulted for this article, no source concluded that they are definitely the same thing.
So, if we look inside human hair, do we see the same clear split between long and short cells as in sheep?
3. Does human hair have the same long-and-short cell split as sheep wool?
In sheep wool, long and short cells were neatly separated on opposite sides. However, reports say that cross-section images of human hair typically do not show such a clear left-right division of cells [4]. Human hair does have distribution differences by cell type, but it is not as distinct as in wool, and the mechanism for why human hair bends is not yet confirmed [4].
Here is a clue from measurements. Researchers from a cosmetics company used X-rays at the large synchrotron facility SPring-8 to examine internal structure by looking at scattering patterns [2]. They reported that straight hair showed round patterns, while curly hair showed elliptical patterns with structures aligned in a certain direction [2].
This report suggests a link between internal structure and curl shape [2]. While this is different from the "cell length" view in sheep, both show that hair inside is not uniform. But whether this causes the bend or results from it is unclear from these materials.
If we look at genes, can they explain curly hair?
4. Can gene analysis explain curly hair?
Genetic research has progressed. A 2009 study of people of European descent reported that variants near the TCHH gene, which works in the inner sheath of the follicle, are involved in differences in hair shape (straight, wavy, curly) [6].
In 2018, an analysis of three European groups totaling 16,763 people added 8 new regions. Even combining 14 variants, the accuracy for predicting hair shape (AUC) was 0.66 [7]. Since 0.5 is random guess and 1 is perfect prediction, 0.66 falls in between.
These studies show that many genes are involved, not just one, and genes alone cannot explain all hair shape. These results are from European groups and may not apply to everyone. How follicle shape or internal hair biases connect to genes is also unclear from these sources.
Why does the same person’s hair wave on some days?
5. Why does hair wave on humid days?
Regarding why hair waves on rainy days, researchers at beauty company Milbon commented that when humidity increases internal moisture, uneven swelling occurs due to differences in the internal state, causing waves [8]. They also stated that the distribution of SS bonds (sulfur links) inside the hair differs between curly and straight hair [8].
This connects to the idea from the previous section that "hair inside is not uniform." However, this is a comment by company researchers, not backed by public institutions or academic papers. This explanation should be treated cautiously.
6. How can you observe a fallen hair strand with moisture?
Place one fallen hair from your brush on white paper. First, observe if it is straight or slightly curved, and draw a simple sketch of it.
Next, put one drop of water on your fingertip and lightly wet the hair. Compare its shape before wetting, immediately after wetting, and after drying. Whether it changes depends on the hair. If the shape does not change, that is not a failure.
Drawing the same hair before and after wetting and placing them side by side is one way to investigate. Do not jump to conclusions like "it works like sheep" or "it is humidity." Just record what you see. Researchers used wool studies and X-ray measurements to examine features of hair [3][2].
SAFETY NOTE: Use a small amount of water and do not pull the hair. Since you are handling wet hair, wipe up any water left on the floor or desk afterward to prevent slips.