Curly or straight: What determines hair shape? – Medical News Today

Straight and curly hair
Scientists know that the shape of the hair follicle and the behavior of follicle cells during hair growth contribute to hair shape.
Whether it is straight, curly, or wavy, your hair is unique to you. But what biology is at play? Curvy or straight hair follicles set the look of each hair throughout your life.

Hair has several biological and cultural functions. It protects the scalp from sunlight and keeps it warm in cold weather. It is also important in social communication.


The hair follicle is a complex, multi-cellular compartment, buried deep inside your skin. Straight hair follicles produce straight hair fibers, while curly hair grows from curved follicles. But why are some hair follicles curvy, and how does a hair get its curl?


Many types of hair


Curly hair includes a wide range of hair fiber shapes, ranging from twists and crimps, to waves and kinks. Scientists from the Center for Skin Sciences at the University of Bradford and Unilever R&D in Bedfordshire, both in the United Kingdom, explain what is known about curly hair in a recent article published in the journal Experimental Dermatology.


The shape of each hair follicle is thought to be determined during embryonic development, but scientists do not know exactly how this happens. Curvy hair follicles are “S-shaped,” meaning that they have two bends. This shape is set for life, although the hair follicle naturally undergoes structural changes every few years.


Each hair grows for around 3 to 5 years before undergoing a resting phase of several months, and then, eventually, it falls out.


During this resting phase, the hair follicle changes in structure, essentially shriveling up in the skin. To start a new cycle of hair growth, the follicle grows back to its original shape.


Curvy follicles always grow back curvy. But what regulates this remarkable re-establishment of curvature in the follicle remains a mystery.



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What makes a hair curl?


In addition to the shape of the hair follicle, scientists believe that differences in cell behavior during hair fiber production contribute to hair shape.


In straight hair, all the cells in the follicle act in a coordinated fashion, leading to even hair growth from the straight follicle. These hairs are round.


However, in curly hair, the way that the cells divide and produce certain proteins is asymmetrical and correlates with the bends in the curved follicle. This results in a hair fiber that has an elliptical shape, which allows it to curl.


How exactly the different types of twists, kinks, and waves are created from this elliptical shape has eluded scientists so far. But what controls these processes?



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Genes control curly hair


Scientists know that our genes control hair shape. So far, the main driver to have been identified is a gene that produces a protein called trichohyalin (TCHH), which strengthens the growing hair.


The TCHH gene contains distinct variants called small nuclear polymorphisms, which are associated with different hair shapes in people across the globe. For example, in individuals of East Asian decent, variations in the gene for the receptor EDAR are associated with straight, coarse hair.


There are a handful of additional genes that have recently been identified and might play a role in regulating hair shape.


Interestingly, some of these are speculated to be under epigenetic control, which means that how they are expressed can be inherited from your parents or even be affected by your environment.


So, the next time you look at your hair and wish that it were a different shape, rest assured that scientists are working on figuring out the precise mechanisms that determine your locks.