Histology of the scalp and the hair
Covering the surface of your head, the scalp, extends from the top of your forehead across to the epicranial aponeurosis of the head. Laterally, it reaches down to the external auditory meatus and zygomatic arch (cheekbone of the skull). The scalp consists of 5 distincts layers:
- the skin
- connective tissue
- the epicranial aponeurosis
- loose areolar tissue
- the pericranium
These layers can easily be remembered using the handy mnemonic SCALP. The upper layers of the scalp can slide over the connective tissue beneath them, which is why the skin on your head has some ‘movement’. The scalp is obviously hairy and has many sebaceous glands (oil glands) scattered across it. This density at which these glands are found means that the scalp can commonly be affected by sebaceous cysts. Scalp hairs protrude from structures known as hair follicles, which are situated in the dermis of the scalp. The hair follicle is comprised of layers of cells that work together to support and form the hair shaft. Hair becomes erect through the action of tiny smooth muscles known as erector pili muscles.
Histology of the scalp
The epidermis of the scalp contains multiple layers. These include the:
- stratum corneum
- stratum spinosum
- stratum basale
- stratum granulosum
The details of these elements and further information regarding the epidermis can be found here.
Beneath the epidermis the dermis is found. This is the portion of skin that contains structures such as the hair follicles, sweat glands, and dermal papillae. The dermal papillae is stromal and comprised of connective tissue. There is also a thicker layer of connective tissue beneath known as the reticular layer. This extends to the subcutaneous layer (hypodermis), which is positioned above the fascia. Within the subcutaneous layer, the basal portion of of sweat glands can be found. There are many hair follicles in the skin of the scalp. They are densely packed together and often oriented at an angle from the scalp.
The skin of the scalp is highly innervated with blood vessels and sensory receptors known as Pacinian corpuscles. The corpuscles are egg-shaped and comprise many concentric rings of tissue layers. They are innervated with a free nerve ending and therefore work as deep pressure receptors to external stimuli.
Also referred to as the superficial fascia, the connective tissue of the scalp is a fibrofatty layer. This layer forms the bridge between the skin and the epicranial aponeurosis by connecting the two together. The tissue is also innervated with blood vessels and nerve endings. The fact that the blood vessels are attached to the connective tissue is limiting to survival. This is because vasospasm (constriction to reduce bleeding) cannot occur if the blood vessels become severed. This results in excessive bleeding should the scalp become injured.
Known also as the epicranial aponeurosis or the galea aponeurotica, this is an important structure within the scalp. It is a thin but tough layer of fibrous tendinous tissue and is the site at which the occipitofrontalis muscle inserts into the tissue of the scalp. The occipital belly gives rise to it, whereas the epicranial aponeurosis inserts into the frontal belly of the occipitofrontalis. Posterolaterally, it extends between the superior nuchal line and the superior temporal line of the occipital bone.
Loose areolar tissue
As its name might suggest, this type of tissue forms a loose connection between the epicranial aponeurosis and the pericranium. This allows the other layers of the scalp to slide of over the pericranium. Loose areolar tissue comprises a network of reticular fibers, elastic tissue, and collagen. Since this is a loose connective tissue, cell types vary beyond fibrocytes and can include plasma cells, mast cells, and adipocytes.
The pericranium, or periosteum, is the final layer of the scalp. It is a fine membrane which covers the outer surface of the skull. It is made up of dense irregular connective tissue. It has 2 distinct layers; the fibrous layer (outermost) and the cambium layer, which is the innermost layer. The fibrous layer of the pericranium contains fibroblasts. Meanwhile, the cambium layer contains progenitor cells which later develop and form osteoblasts.
Histology of the hair
The histology of your hair can vary slightly depending your ethnicity. When a cross section of a hair is made, its shape differs depending on the characteristics of your hair determined by your race, and the genes you get from your parents. For example, the straight hair of many Asian people gives a perfectly round cross section. Meanwhile, the wavey hair of European people gives an oval shaped cross section, and the curly hair of black people has a kidney-shaped cross section. Hair follicles (the sheath of cells that surround the base of each hair) head tend to be long and straight, but curly hair is often produced from curved hair follicles. Despite the unique differences between hair around the world, the basic histology of hair is universal.
There are many hair follicles within the scalp. They are tubular and are formed from multiple layers of epithelial cells. The base of the follicle bulges, forming a hair bulb which surrounds the hair papilla. The bulb is invaginated by connective tissue known as dermal papilla. The dermal papilla contains many tiny blood vessels and nerve projections. This becomes the hair papilla once it invaginates into the hair bulb. The 3 innermost layers of epithelial cells within the hair follicle keratinize to produce the hair shaft. The outer epithelial layers form the hair’s epithelial sheath. The mass of cells from which the hair shaft is produced is referred to as the hair matrix.
When a hair is actually growing, epithelial cells around the dermal papilla multiply. This then forms 4 inner layers of the hair follicle. There is another outer layer, and this is known as the stratum basale of the scalp’s epithelial surface. The mass of epithelial cells found around the dermal papilla whilst the hair is growing is known as the hair root.
The innermost layer of the hair follicle contains cells which become keratinised to an extent, forming the medulla. The medulla is the very core of the shaft of hair. Surrounding the medulla, there is a keratinised layer of cells called the cortex. The cortex makes up the main body of the hair. The third cell layer of the hair follicle is also keratinised, forming a cuticle which is thin but hard. The cuticle is made up of keratin plates. These overlap creating a structure that supposedly prevents the hair from becoming matted.
The fourth cell layer of the hair follicle comprises the internal root sheath. This is a layer of cells that become mildly keratinised. They break down at the ducts of sebaceous glands. This is to leave a space into which sebum can be secreted around the hair.
The outermost layer of cells is known as the external root sheath. This is not involved in the formation of hair and is separated from surrounding connective tissue by the glassy membrane, a form of specialised basement membrane.
Arrector pili muscles are a type of smooth muscle found at an angle to hair follicles. They are attached to the papillary layer of the scalp’s dermis and connective tissue. When these muscles contract, the hair shaft becomes erect.
Large melanocytes occur within the hair follicle, allowing melanin to be incorporated into the hair shaft. Melanin can be black, brown, or yellow, and varying combinations and quantities of each type gives us each our own unique hair color.