Hair is a keratinous (say: ker-EAT-in) filament that grows from a follicle in the dermis. Each strand of hair gets its nutrients from a blood vessel running underneath it. When a follicle is active, cells form, die and harden, which gives hair its strength.

Each follicle is innervated by sensory and autonomic nerves. Nerve signals from the sympathetic nervous system cause a muscle called the arrector pili to contract. This makes the external hairs “stand up,” as if to add insulation and intimidate rivals or predators. Click the Hairicc to learn more.

The cuticle is the hair’s outermost layer, a multi-laminate structure of flattened overlapping cells that give your hair its lustrous shine. It is a protective covering that covers the cortex and medulla of the hair strand and shields them from external stressors like the sun, water, brushes, and heat tools.

The individual keratin scales of the cuticle are flat, have smooth edges and neatly overlap one another to form a tough sheath around the core of the strand. It is an incredible work of nature that is able to provide the ultimate balance between strength, rigidity and permeability, by limiting movement of materials in and out of the underlying cortex. When the cuticle is healthy, it has a very smooth surface with a slight sheen and the ability to stretch before breaking.

You can feel the shape of a hair’s cuticle by pinching a single strand between your fingers and running them along its length, starting at the root. Does it feel smooth and slick, or rough and kind of squeaky? The difference is caused by the direction of the cuticle layers. When they are healthy and unaltered by harsh chemical services, heat treatments or abrasive mechanical processes, the cuticle layers move in the same direction as your hair is growing.

It is only at the point when you change the direction of those layers that damage begins to occur. The first sign of this is when the tips of your hair start to split, which is because they have been exposed to excessive heat or harsh chemicals and are moving against the flow of their surrounding cuticle cells. The cuticle is not able to heal itself when this happens, and the only way to restore it to its smooth, healthy condition is to cut off the affected hair strand.

You can help keep the cuticle in tip top condition by encouraging your clients to gently cleanse their hair with a good salon shampoo and to avoid excessive use of heating and styling tools. It is also important to keep your clients hydrated to promote the health of their cuticles and to help minimise the effects of environmental and chemical stresses on the cuticle and underlying cortex.

The Cortex

The cortex forms the main bulk and pigment (colour) of your hair and consists of long keratin filaments. The health of your hair depends largely on the cortex being protected by the cuticle. If your cuticle is damaged by harsh treatment like over brushing or hot water, the keratin filaments in your hair can start to fray and break down. This is what causes split ends.

Wide angle x-ray scattering, a technique which allows us to measure the structure of a single hair with a beam incident on it, revealed that the cortex has a dense packing of a-keratin based intermediate filaments (IFs). The IFs are coiled and oriented along the hair’s axis and are fluid-like in two dimensions. However, the IFs in the new region found close to the cortex/cuticle boundary are still orientated in-plane and the sharp 5.2 A peak characteristic of a keratin coiled coil is absent from this region. In the cuticle, on the other hand, we found a broad isotropic peak at 4.6 A that correlates with a random network of nearest-neighbour correlations between IFs.

We also found that the cell membrane complex in the hair is a mixture of lipids and proteins with an overall structure of one 15 nm protein delta layer sandwiched between two 5 nm lipid beta layers. This general cell membrane composition is very consistent between individuals with only a few standard deviations in the underlying molecular dimensions.

The characteristic SAXS pattern for the medulla region, shown in Fig. 1C, shows a monotonically decreasing intensity with momentum transfer which is consistent with the real-space electron microscopy image in Fig. 1G. The empty porous region in this picture is a result of the keratin matrix liquid that normally fills this part of the medulla having evaporated during preparation of the sample.

Lastly, the new region of the cortex near the cuticle/cuticle boundary reveals that IFs there are a mix of a and b keratin. This is confirmed by comparison of the WAXS patterns shown in Fig. 4A -C, where the line projections of the integrated intensities for the peak 5.2 A and the peak 4.6 A of the cuticle and cortex respectively are shown. This demonstrates that the 5.2 A peak associated with the a keratin in the cortex is present but missing from the cuticle and the broad 4.6 A feature is slightly weaker in the cuticle, which corresponds to a transition from a to b keratin.

The Medulla

The cortex and medulla make up the hair. The medulla is the part of the follicle that contains the pigment (color). Hairs that are red, blonde or light brown have a darker medulla than those that are black. The medulla is also responsible for the sensation of the hair.

The medulla of the human hair is very similar to those of other mammals and birds. It is made of a layer of keratin cells that is surrounded by blood vessels and other nerves.

It is the brainstem’s most important relay center and conduit for ascending sensory information from the periphery. This part of the brainstem controls many vital bodily functions, including movement of the heart, lungs and stomach.

It contains the nuclei of cranial nerves IX, X, XI and XII. In addition, it houses important nerve centers that control movement of the head and facial musculature.

This part of the brainstem, along with the pons and spinal cord, makes up the neuroaxis. The medulla is the lowest portion of this structure, and it is responsible for a number of unconscious activities.

The medulla consists of two paired swellings referred to as the pyramids and olives. These are located on the anterior (“basal”) surface of the medulla and are rostrally continuous with the basilar pons. They contain the cranial nerve nuclei affiliated with the medulla and is a major relay center for ascending sensory information from the periphery, including the autonomic nervous system.

There are also important paired nuclei in the medulla called reticular formation and solitary nuclei that regulate many autonomic functions. Examples of reticular nuclei in the medulla are the raphe nuclei, gigantocellular (magnocellular) nucleus, perihypoglossal nucleus and lateral reticular nucleus. They are involved in a variety of functions, including pain modulation, sleep/wake cycles, and thermoregulation. They are also associated with ocular movements. This is because the ipsilateral somatosensory areas in these nuclei are connected to the cerebellum via the lateral spinothalamic tract. This allows ipsilateral input to influence motor responses in the medulla. In the case of a drooping eyelid, this is manifested as a lack of movement in that part of the face or body.

The Root

A root hair is a special epidermal cell that possesses unique properties, and it represents the principal means by which water and minerals are absorbed from soil. Its length and asymmetrical shape allow it to penetrate between soil particles, while its large surface area allows it to trap mucilage and microbes for uptake. Its absence of chloroplasts enables it to avoid uptake of harmful bacteria.

The development of a root hair can be split into four stages: cell specification/bulge site selection, initiation, tip growth and maturation. In the bulge site selection stage, potential hair-forming cells are distinguished from other epidermal cells by the presence of a dense actin structure surrounded by perinuclear bundles, similar to the actin architecture seen in pollen grains prior to germination [1].

Once a potential hair cell has been identified, the morphogen Rop binds to the future tip region of the cell. This triggers a series of events that cause the cell to become wider, longer and deeper by diffuse growth. The hair cell reaches its characteristic shape when the expansion becomes localized to a central disc-shaped region of the outer wall.

The cell is now able to generate turgor pressure and swell, which enables the elongation of the cell tip. During this process, the cell also releases its internal mucilage and nutrient storage materials. The internal vacuole enables the production of a steady stream of growth-promoting materials that promotes the polar organization of the actin cytoskeleton in the non-expanding part of the cell, while the tip-focused G-actin disappears and a dense array of fine F-actin takes its place.