Why can hair grow curly or straight?

Curls, waves, wavy or straight, each has its own typical shape hairbut where does all this diversity come from? There are different theories of how a hair is born curly like a fusilli or smooth like a piece of spaghetti. It has been observed that it partly depends on the shape of the follicle (i.e. the structure in which the hair originates), regularly round for straight hair, curved, oval or elliptical for curly hair. But diversity also comes into play cell distribution that produce keratinocytes (the cells that make up the hair shaft), the synchrony or asynchrony with which these cells work and finally, a different distribution of keratin. All these mechanisms influence each other, but if for the chicken and the egg we could have an answer, in the case of curly hair, it is not yet clear which mechanism is preponderant over the other.

The hair is born curly right from the follicle

Whether curly, straight or somewhere in between, a hair always comes from a hair folliclea structure immersed in the dermis and composed of a complex of cells and compartments, each with a specific function that leads to the formation of the hair shaft. A hair arises from the innermost part, the bulband then lengthens thanks to rapid division (via a process called mitosis) of keratinocytes, the cells that will make up the hair. Curly hair is born curly already at this initial level, before it even leaves the hair follicle!

The determination of the shape of the hair has a strong genetic component and the follicular mechanisms that are involved in the curvature can be different: studying the various components of the follicle, from the simple shape, to the angle of the hair bulb (the innermost structure of the follicle), up to the follicular substructures responsible for the production of keratin by the keratinocytes (the building blocks that form hair).

The shape of the follicle and curly hair

First of all, it has been observed that straight hair grows from round shaped follicles, elliptical and oval follicles are responsible for wavy and curly hair. These curved follicles when dissected actually show a double curvature S-shaped.

A different shape is associated with a different one angle of the hair bulb with respect to the skin: a round follicle is like a real cylinder perpendicular to the epidermis, while the more curved the follicle, the more angled the bulb is and hair growth will follow this curvature.

From a more technical and structural point of view, the curly hair shaft has a shape helical and asymmetric, with a difference between the inner and outer cells of the cortex. In curly hair the hair cortex is mainly made up of ortho and paracortical cells, while in straight hair there are also mesocortical cells, which disappear as the curvature of the hair increases.

The asymmetry of curly hair

It has been suggested and observed during experiments on how the hair of sheep (yes, the fleece of sheep grows in a very similar way to our hair) that the curved follicles present a sort of asymmetry in the proliferative compartment of the hair. This asymmetry seems to reside in the activity and distribution of the cells of the dermal papilla and the bulb (the areas in which the hair actually grows), which they don’t work at the same time, leading to an asymmetric proliferation rate of keratinocytes.

In practice, in curved follicles the cells from which these bricks derive do not work in unison, but in a manner asynchronouscausing the hair itself to bend. In contrast, in straight hair keratinocytes are produced synchronously, so that the hair shaft can grow straight on its way. It’s like saying that in straight hair, all the cells follow the same hair production rhythm, while in curly hair the cells “dance” at different rhythms.

She was observed asymmetry also in the distribution of a specific keratin of curly hair compared to straight hair. Straight and curly are composed of the same structures and proteins, but one protein in particular belonging to the keratin family is distributed differently. In technical terms, this protein is called keratin hHa8which in curly hair accumulates in the concave loops of the curl. In contrast, in straight hair all the keratins and structures of the hair shaft are distributed evenly. However, it is not clear whether it is a problem with the production of this keratin that creates the curvature, or whether the asymmetry in the production of keratinocytes also causes the asymmetry of this protein.