Six things you may not know about hair

Hair follicles all form before birth during a tightly coordinated process in early human development. They arise from small, specialised regions in the embryonic skin called epithelial placodes, which are clusters of cells that signal to the surrounding tissue to begin building a follicle. Through a series of tightly regulated interactions between these placodes and underlying cells, the structure progressively invaginates (folds in) and matures into a fully formed hair follicle.

Earliest stages of hair follicle development (known as placodes). Image credit: Elias Farr and April Rose Foster / Wellcome Sanger Institute

By birth, the entire repertoire of follicles is already established, and no new follicles are generally made afterwards. This means that all later changes in hair reflect the cycling and activity of these pre-existing developmental units, which are then influenced by a range of genetic, hormonal and environmental factors.^1,2

How is it possible that hair, teeth, glands and even feathers are linked? The answer lies in how these structures are built during early development. In the embryo, the body forms from three primary layers – ectoderm, mesoderm and endoderm – and the skin is derived from the ectoderm. From this outer layer, the specialised epithelial placodes give rise to a wide range of structures that, despite their very different appearances, follow a similar developmental strategy.

They thicken and then fold or invaginate into the underlying tissue, forming structures like follicles, buds and glands. This allows epithelial cells to maximise their surface area and specialise for different functions, from secretion to protection. Whether forming hair, teeth, glands or feathers, these structures are built using variations of the same fundamental blueprint, showing how seemingly unrelated features are, at their core, deeply connected.³

Development of ectodermal organs: teeth, hair, mammary glands and feathers all form through interactions between adjacent epithelial and mesenchymal tissues that determine organ shape. Adapted from DOI: 10.1016/S0012-1606(03)00325-7

As humans, our environment is the remarkable planet Earth. But for some creatures, the environment is… quite a bit closer to home – inside human hair itself. Yes, really. Demodex folliculorum are microscopic mites that live in the vast majority of adult human hair follicles and sebaceous glands (skin glands that secrete sebum, an oily substance that protects the skin), particularly on the face and scalp, where they feed on sebum.

False colour scanning electron microscope image of Demodex folliculorum hair mite. Image credit: DOI: 10.1186/1471-2164-15-1124 / Wikimedia Commons

These mites are usually harmless and go completely unnoticed, but in some cases, overgrowth can contribute to itching, dandruff-like scaling and inflammation. Nearly everyone carries them, and they seem especially fond of eyebrows, eyelashes, the nose, forehead and cheeks. Who else feels the need to go and wash their face?⁴

We are all familiar with losing baby teeth before our adult ones come in – but hair follows a surprisingly similar pattern. From around 9-weeks post conception, the body begins to develop a very fine, soft and unpigmented layer of hair known as lanugo. This hair covers the entire foetus, helping with insulation and aiding vernix caseosa – a waxy, protective layer that coats the baby’s skin in the womb – adhere to skin. Lanugo hair is usually shed before birth, although it can still be seen in some newborns – particularly those born prematurely – where it typically disappears on its own within a few weeks.^1,5

Lanugo hair follicle highlighting the arrector pili muscle (orange) that contracts in goose-bumps. Image credit: Elias Farr and April Rose Foster / Wellcome Sanger Institute

Lanugo hair is then replaced by vellus hair (often called ‘peach fuzz’), which is short, fine and lightly pigmented, covering most of the body. In contrast, terminal hair is thicker, longer and more pigmented, and is found on areas such as the scalp, beard, underarms and genital regions.^1,5

In rare cases, this process does not follow the usual pattern. Congenital hypertrichosis lanuginosa is an extremely rare genetic disorder – fewer than 50 cases have been reported in medical literature – in which lanugo hair persists and continues to grow after birth.⁶ Interestingly, lanugo-like hair can also reappear later in life in response to severe malnutrition, most notably in anorexia nervosa, where it is thought to be a physiological response that may help reduce heat loss.⁷

Even if you do not dye your hair a funky colour – hair is never fixed. It is a dynamic structure that reflects what is happening in your body over time, which is why it can look and feel quite different at different stages of life.

Hair texture and colour can change throughout life due to a combination of genetics, hormones and environmental influences. At the centre of this, is the hair follicle itself. The shape of the follicle is one of the key factors that determine texture: round follicles tend to produce straight hair, while more oval or asymmetrical follicles generate wavy or curly hair, as the hair shaft bends unevenly during growth. This shape is largely established during embryonic development, when tightly coordinated signalling between different cell types sets up the structure, orientation and behaviour of the follicle. Genetics strongly influence these processes, which is why hair texture often runs in families.^8,9

Work at the Sanger Institute is beginning to explore how these signalling events unfold in humans. Using spatial transcriptomics, Muzz’s group is investigating pathways like fibroblast growth factor (FGF), wingless-related integration site (Wnt) and sonic hedgehog (SHH) signalling that are all involved in early hair follicle formation. By improving our understanding of these processes, this research aims to uncover the cues that drive hair development and improve the representativeness of our skin organoid models. This in turn will help strengthen disease modelling and accelerate therapeutic innovation for many skin- and hair-related conditions.

Although follicle structure is largely set before birth, it is not completely fixed. Hormonal changes at different life stages – such as puberty, pregnancy and ageing – can alter follicle activity and subtly reshape how the hair is produced, leading to changes in curl pattern, thickness or overall texture. At the same time, hair colour is controlled by pigment-producing cells within the follicle – called melanocytes – which gradually lose function with age, resulting in greying or whitening. External factors, including heat styling, chemical treatments and overall health, can further affect how hair looks and feels.¹⁰

Have you ever thought about why your eyebrows are not the same length as the hair on your scalp? The answer lies in the built-in rhythm of the hair growth cycle. Each hair follicle cycles through phases of growth (anagen), regression (catagen) and rest (telogen). The key difference between body sites is how long each hair stays in the growth phase. Scalp hair can remain in anagen for years, allowing it to grow long, while eyebrow and eyelash follicles switch out of this phase much more quickly, limiting how far those hairs can extend. This timing varies across different regions of the body and between individuals, shaped by a combination of genetics, hormones and age.¹

This is also highlighted in rare genetic conditions. For example, in Loose Anagen Syndrome, hair struggles to remain anchored in the growth phase and is shed prematurely, meaning it fails to grow to normal lengths.¹¹ At the molecular level, genes such as FGF5 help regulate how long follicles stay in anagen, acting as one of several key regulators that help control the timing of when growth stops. When this balance is altered, hair length can change dramatically across different regions of the body and between individuals.¹²

“One of the most exciting aspects of studying hair follicles is how rapidly they form complex structures from two seemingly simple layers of cells. With spatial transcriptomics, we can capture this process from beginning to end and uncover the signals that shape it. This has the potential to transform our pre-clinical models and support the development of new therapies addressing a wide spectrum of hair-related disorders.”

Elias Farr
PhD Student, Wellcome Sanger Institute

Elias Farr looking at spatial data of the hair follicle. Image credit: Joseph McWilliam

As you can see, hair is far more than a superficial feature – it is a complex biological system built before birth and maintained through tightly regulated cycles throughout life. Each hair reflects the behaviour of a follicle, a miniature organ shaped by precise genetic and signalling programmes that determine when and how it grows. Work at the Sanger Institute is helping to uncover these mechanisms by mapping human hair follicle development at cellular resolution, revealing just how much coordination is required to build something we usually take for granted.

So next time you are having a bad hair day, it might be worth remembering: it is not just hair – it is development, timing and biology all playing out on your body (along with a creature or two).