In the cortex there are various types of cells. These ?cells? are made up of smaller fibrous units ? macrofibrils, which are made of smaller units, matrix (proteins) and those of ever smaller units calledmicrofibrilswhich ultimately are made from the "bare naked" alpha helical structure of keratin proteins, coiled together. Coil alpha helical proteins together and you have a fiber. Coil a bunch of those fibers together and you have "hair." It's a lot like making "plied" yarn or rope if you have ever seen or done that.?Science-y Hair Blog 2013

You may recall that proteins get their ?recipe? from your DNA. A particular sequence of DNA may contain instructions for ?making keratin? and then ?making individual keratin microfibrils? and ?making macrofibrils? and so on. Your genes, the code your DNA bears, determines what proteins will be made and how. These are set when you are born, but then environmental factors can change them ? things like hormones at puberty, pregnancy, menopause, or as part of chemotherapy. You can call these "things that change how your genes are expressed" by the term "epigenetics" and impress your friends!

In straight versus curly hair, one main difference is what sorts of cells are in the cortex of the hair and how they are arranged. An article published in the Journal of Structural Biology (Bryson et al., 2009) identified 4 types of cells in human hair which have different characteristics and, when arranged in different patterns in the cortex of hair, can cause hair to grow in a curved manner. Three of these cell types are most common in all hair ? A, B and C. This was determined using scanning electron microscopy, staining of hairs and slicing them in thin sections, then again using extremely high magnification and dye detection.?Science-y Hair Blog 2013

In the past, it had been thought that the curvature of hair was dictated by the shape of the hair shaft. This has proven to be inconsistent. There is a correlation ? curlier hair tends to have flatter hair shafts ? but correlation does not mean a cause. For example, If I slip on the ice, tear my green trousers and skin my knee one day (correlation), that does not mean the next time I wear my green trousers I will slip on the ice again. Green trousers do not cause slips and falls on ice. It was also suggested that hair may grow at different speeds on different sides of the hair shaft, causing one side to bend more than the other and this hypothesis is still viable and even fits with what Bryson et al. described.

In straight hair, the arrangement of the types of hair cells is more-or-less evenly distributed with A, B, and C cells around the medulla or center-most part of the hair. But in curving hair, the hair shaft seems to be divided in half, with one half having a distribution of cell types similar to straight hair (evenly distributed) and the other half heavily concentrated with ?Type C? cells ? we?ll think of them as ?C? for ?curvature.?

Type C cells are high in cystine (a sulfur-rich protein), often have their macrofibrils fused rather than separated (like in the other common cell types, ?A and B?) and tend to have their macrofibrils arranged in whorls, their microfibrils are helical. These Type C cells are clustered in the concave half of the curve of hair (the inside or short side of the bend) and the A and B cells (and a few C cells) are scattered more evenly on the outside or longer side of the bend. To make it simple, think of the C cells as having an inherent ?twisting? nature.

What does this mean for hair curvature? That half of curved hair which makes it curve, does so because it is a rather different material than the other half. The ?ingredients? may be the similar, but the way it is assembled (the recipe) is different. It is hypothesized that the more ?tightly wound? (helical microfibrils within whorl-like or otherwise twisting macrofibrils) half of curving hair contracts more than the other half when dry, causing the hair to bend and twist when dry but stretch out somewhat (or a lot) when wet.

If your hair has loose, big waves, the asymmetry (the 2 different halves) of your hair may not be as great as illustrated here; for example, the layer of Type C cells in the concave half of the hair may not be very thick. If your hair has tighter waves or curls, there may be a greater difference between the two halves of your hair shafts - a thicker layer of Type C cells. Most studies of how hair curls have been done on wool, which has a very small (tight) curl diameter and extreme difference between the cell types on either side of the curve. So it stands to reason that the greater the difference between cell-type (protein) composition and distribution on each half or side of your hair shafts, the smaller the curl diameter. This sort of protein fiber and cell-type based explanation also makes it easier to understand how hormones or illnesses or medications can change how our genes are expressed to change a not-curly hair to a curly one or vice versa.?Science-y Hair Blog 2013

Updated January, 2015