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Scalp and hair

Histological features of the scalp and hair.

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Transcript

Hello everyone! It's Megan from Kenhub here, and welcome to today’s video on the histology of the scalp and hair. In this tutorial, we’ll get an idea of what hair looks like through the microscope. We’ll be discussing the scalp, the structure of hair, the parts of a hair fiber, and the accessory structures associated with hair fibers. We’ll also cover the types of hair we have on our body and the clinical scenario where knowledge of hair growth is important. But first let me give you an overview of hair.

Hair fibers are primarily made of cells and keratin strands. Keratin is a protein which also forms your nails and helps your skin be waterproof and resistant to damage. Hair is unsurprisingly found in hairy skin. This is the scientific name for the type of skin and is the opposite of glabrous or hairless skin. Hairy skin covers most of the body with glabrous skin normally only present on the palms of the hands, fingertips, and soles of the feet. So, on our man here, almost all of his body is, in fact, hairy. But why do we need hair?

Well, hair actually has many functions including regulating temperature , protecting the skin and other organs such as the eyes, ears and nose; providing a shield from UV radiation, and helping with our sense of touch. On this last point, the hair fibers act as levers to detect minute forces passing over the skin’s surface. This helps us to notice small flies and parasites on our skin. As a byproduct, it also lets us detect wind and the feeling of goose bumps.

If you say hair to someone, chances are they’ll think of the hair of the head, and that’s what we’ll be focusing on most today. Let’s begin by talking about the scalp.

In this section, we’ll take a look at the type of stain used on the histological sections we’ll see in this tutorial, that is, Ladewig’s stain, the boundaries of the scalp, the blood supply or the arteries and veins, as well as the lymphatics and innervation of the scalp.

This image is a histological section of the scalp with the epidermis at the top here and this being the dermis. As you can see, we’ve managed to cut longitudinally through a hair follicle. This section is stained with a special solution called Ladewig’s stain or Ladewig’s trichrome, which is used to visualize connective tissue. With this staining method, nuclei stain dark red/black, muscle and collagen stain blue, and cytoplasm stain red. We’ll be using this image and another Ladewig’s stain slide a lot in this tutorial, so familiarizing yourself with these colors should help you understand what you’re seeing, although I’ll try to describe what we’re looking at along the way.

The scalp includes all the layers covering the skull on the top, back and sides and has its boundaries at the border with the face at the front, above and behind the ears at the sides, and to the neck at the back and sides. Although we said that the scalp includes all the layers covering the skull, for the purposes of this tutorial, we’ll be discussing just the outer layer – the skin of the scalp – as this is where the hair grows from.

On the image, we have the epidermis here stained red due to the cell’s cytoplasms and the dermis here stained blue because of its collagen content. Note that the scalp skin is the same as other hairy skin histologically.

Let’s talk blood supply now starting with the arteries. This is a diagram showing a head with the top layers – that is, the skin and hypodermis removed – and we’re viewing it from the left-hand side. On each side, the scalp receives some of its blood supply from three arteries from the external carotid artery. These are the superficial temporal artery which supplies most of the frontal and parietal regions of the scalp, the occipital artery which supplies the posterior or occipital region, and the posterior auricular artery which supplies the scalp posterior and superior to the ear. It also receives blood from two arteries from the internal carotid artery. These are the supraorbital artery supplying the lateral forehead and frontal scalp and the supratrochlear artery supplying the middle section of the forehead and frontal scalp.

So how about veins? Now, we’ve layered on the vein shown in blue. We can see that from the scalp, blood drains via veins which follow the arteries. These veins are named the same as the arteries they trace. Those veins following arteries from the external carotid artery drain into the external jugular vein. These veins are namely the superficial temporal vein, the occipital vein, and the posterior auricular vein. The veins tracing the paths of the arteries from the internal carotid artery drain to the internal jugular vein. The veins in question here are the supraorbital vein and the supratrochlear vein.

So that’s how blood leaves the scalp, but what about the interstitial fluid? A small amount will drain back into the veins but most will be directed into lymphatic channels. This is a diagram to illustrate the main lymphatic drainage roots from the head. Lymph from the posterior half of the scalp drains to the occipital lymph nodes which are on the back of the head somewhere around here and the posterior auricular lymph nodes which sit just behind the ear. The anterior half of the scalp drains to the parotid nodes which are these nodes here. Downstream from these nodes, the lymphatic drainage channels begin to interconnect and eventually drain to the submandibular and deep cervical lymph nodes.

So, we’ve cleared the lymph out, now what about sensation? This is the same diagram we used to show the arteries and veins except this time we’ve layered on the major nerves. Innervation from the scalp posteriorly up to the vertex or tip of the head derives from the C2 spinal root. This nerve supply is via the greater occipital nerves situated a couple of centimeters on either side of the midline and the lesser occipital nerves which are further around the scalp towards the pinnae of the ears.

Anteriorly, the scalp’s nerve supply comes from branches of the trigeminal nerve. The ophthalmic division of the trigeminal nerve gives off the supraorbital and supratrochlear nerves. The maxillary branch sends out the zygomaticotemporal nerve which supplies the hairless part of the temple, and the mandibular branch provides the auriculotemporal nerve. And you’ll be relieved to hear that that’s us finished learning about the scalp.

Now, it’s time to learn about the stuff that sprouts out of it – the hair.

Highlighted in green on this image are the hair fibers. Hair fiber is a broad term which refers to the entire strand of hair including the sections within and outside the skin. The hair follicle is the section within the skin, so that’s the bit highlighted here. The hair shaft is the section of hair fiber out with the skin which we’ve highlighted here. You may think, hold on, that’s beneath the skin surface, but we can see an area of no cells around it, meaning that it’s in the lumen. As such, it’s outside the epidermis, the top layer of skin, and is therefore out with the skin. Don’t you just love the technicalities?

Now that we know the two sections of a hair fiber, let’s look at the parts of one. To do this, we’ll look through the microscope at a cross-section of the hair fiber at the level of the hair follicle. The hair follicle contains the same layers as the hair shaft but has a few more parts to talk about, so we’ll start with the layers that are present in both by studying the shaft.

So here we have that cross-section of the hair follicle. We’ve highlighted the three layers which form the hair shaft, namely the medulla which is the inner core, the cortex which is the in-between layer and the cuticle on the outside. In the follicle as shown in this image, these areas stain red as the cells have cytoplasms and they’re alive. In the shafted cell, these areas do not stain brightly on Ladewig stain as they are biologically dead with the degraded cytoplasm. This is starting to happen here. Using both of these hairs as examples, we’ll start at the inside where we have the medulla.

The medulla is composed of large loosely connected medullary cells and amongst these cells are intercellular air spaces which keep the hair light but strong. The area can be identified in histology as being the medulla by the fact that it is the central core and is pale staining in the hair shaft as can be seen here. It’s pale staining due to the air spaces and the fact that the cells are biologically dead and so their cytoplasms no longer stain. The medulla highlighted here is within the hair bulb where the cells are very much alive and so the cytoplasms stain bright red.

The boundary between the medulla and cortex in the shaft becomes less visible as the hair strand cells die off. Although this is a cross-section of a hair shaft, the boundary can still be clearly seen here because even though the cells of the medulla have given upon on life, the cells of the cortex still have some cytoplasm. In the hair bulb, this slight change in shade seen here is the boundary.

Moving out, let’s talk about the cortex. The hair cortex which we can see in this image here is made up of fusiform cortical cells containing keratin fibrils, air spaces known as cortical fusi, and melanin pigment which gives hair its color. Fusiform simply means spindle-shaped, cortical means of the cortex, and the keratin fibrils are bundles of the keratin strands which run through the length of the hair fiber to give it high tensile strength – in other words, resistance against the pulling force.

The cortical fusi in the same way as the air spaces in the medulla keep the hair light and act as splints to increase the volume of the hair strand and keep it resistant to compression. Think of it like bubble wrap. To recognize the cortex in histology, we can show that it’s a stratified layer of cells meaning there are multiple layers of cells stacked on top of each other within the cortex. This is shown here although the cells are very thin. Look for the presence of pigment granules which can be seen here contained within the soon-to-be cortical cells, and look for its borders with the medulla and the cuticle.

In the hair shaft, the boundary with the medulla is usually indistinct as neither layer stains with Ladewig’s trichrome. In the follicle, there’s a subtle difference in shade and just above the follicle, the medulla doesn’t stain but the cortex still does as is displayed here. The boundary where the cuticle is more difficult and really comes down to identifying the different cell type. That different cell type is the cuticle cell like the one we see in this image.

The cuticle is a single layer of keratinizing cuticle cells. These began life as cuboidal cells but as they die and lose their cytoplasm, they start to collapse a bit like this. They flatten over the cell distal to them on the hair fiber to create a roof tile-like outer layer. This makes the hair more slippery, more waterproof, and protects the cortex and medulla.

In histology, the cuticle and the follicle looks like this. It stains red on Ladewig’s trichrome due to the presence of cytoplasm and has this flower petal appearance as the cells are starting to flatten. In the shaft, the cells do not stain having lost their cytoplasms. They have also flattened out so the cuticle ends up appearing as a gray ring of overlapping flattened cells around the hair fiber.

So that’s the three layers of the shaft and how to identify them in the shaft and follicle. But what about the extra layers in the follicle – the internal root sheath, the external root sheath, the glassy membrane, and the dermal sheath? Three sheaths and a membrane sounds like a pretty bad in demand to me but if it helps you remember the additional layers, then it could be helpful. Let’s look at each added layer in more detail.

First stop is the internal root sheath. Composed of cells which keratinized and die similar to those of the epidermis, the internal root sheath is responsible for protecting and anchoring the hair follicle in the skin. The internal root sheath itself can be split into three layers. The innermost layer is the cuticle which is continuous with but not the same as the cuticle of the shaft. The cuticle of the follicle interlocks with the cuticle of the shaft to anchor the hair within the skin.

The second layer is Huxley’s layer which is two to four cells thick. In the hair follicle, some of the cells penetrate through the third layer to connect with the external root sheath. Through these bridges, nutrients and energy are passed from the external root sheath to the hair follicle. As the cells are pushed further from the root, all the cells become keratinized and die and these bridges break. The third and final layer is Henle’s layer. This is a single layer of keratinized cells with clear flattened nuclei. Keratinization allows it to slide with the hair follicle over the stationary external root sheath.

Surrounding the internal root sheath is the external root sheath. Through the microscope, we can see that the external root sheath is made up of stratified columnar cells. The external root sheath is continuous with the epidermis of the skin. This makes sense when we consider that the stratified columnar cells are very similar in appearance to those of the stratum basale and stratum spinosum.

In this sheath, the number of cell layers varies depending on the level. The number of layers is maximal at a region known as the bulge. The bulge contains additional stem cells for hair growth and is where the hair muscles attach. These stem cells differentiate as they are pushed further from the hair root. The area highlighted in the image is probably the bulge of this hair follicle due to the number of cell layers.

The glassy membrane seen here is a hyaline connective tissue layer that surrounds and supports the external root sheath. While the external root sheath is continuous with the epidermis of the skin, the glassy membrane is continuous with the basement membrane. It separates the epidermis from the dermis and similarly separates the dermal papilla or hair root which is derived from the dermis from the external root sheath which is derived from the epidermis.

This is the metaphorical seabed that the hair is anchored to by the root sheaths. Outside the glassy membrane is the dermal sheath. This layer blends into the dermis and its function is to be the start of the transition from skin to hair follicle. The dermal sheaths supports the dermal papilla and its function of hair production and has been proposed to have a role in wound healing. This is suspected due to the presence of myofibroblasts.

Through the microscope, the dermal sheath is indistinguishable from the dermis other than by its location around the hair fiber. Looking at the sheath on the image, their appearance is noticeably similar to that of the dermis.

As a quick aside, the root hair plexus forms within the dermal sheath. This is a network of free nerve endings which fire or change their firing rate in response to movements of the hair. Its function is to detect tiny movements such as flies or insects landing on or crawling over the skin or wind passing over your body.

So that concludes how to look at hair fibers on a Ladewig stained histology section.

Before we rush on, let’s take a minute to summarize what we’ve learned but apply it to a slightly different image. This image is stained with hematoxylin and eosin, also known as H&E, which is one of the most widely used stains in histology. In this image, we’ve again caught the hair in cross-section and are looking down into the follicle. The skin surface is up here and this is the dermis. Hair has a few distinguishing features. Let’s summarize them.

Firstly, it simply looks very different to the surrounding tissue. Starting at the edge of the region is a well-demarcated area cordoned off by a surrounding ring of cells and connective tissue. The ring of cells is the external root sheath and the connective tissue ring is the glassy membrane. Now to deal with the strange structure in the middle. This is a hair follicle identifiable by the pale core surrounded by pigmented cells with a smeared nuclei. The pigment is melanin and the elongated or smeared nuclei represent the fusiform cortical cells. Good. Now you should be able to identify hair on both Ladewig’s trichrome and standard H&E sections of skin.

The next aspect to cover is the parts of a hair fiber as it grows. To do this, we’ll follow the direction in which the hair grows from the root outwards. This will take us through the dermal papilla at the base of the hair fiber, the hair follicle nestled in the skin and consisting of the hair matrix, the birthing pool for hair cells, the hair bulb, the metaphorical preschool for hair cells, and the hair root where the cells prepare for life outside the skin.

The hair follicle will then become the hair shaft which is mature hardened and ready for life on the other side of the skin. The first structure to talk about is the base of the hair – the dermal papilla. A dermal papilla is made of connective tissue to form its structure, mesenchymal stem cells to produce that connective tissue, nerve endings to provide innervation to the region, and capillaries to supply nutrients and remove waste from these other cells and the hair matrix.

The dermal papilla itself does not produce hair and instead is the firm structure to anchor hair in the skin. That being said, for hair growth to occur, the trichocytes – the cells which hair cells come from – must receive chemical signals from the mesenchymal stem cells of the dermal papilla. Outwards from the dermal papilla is the hair follicle which is attached to the dermal papilla. We already mentioned this but it’s worth mentioning again. The hair follicle is the part of the hair fiber within the skin and is where new hair is formed and matured.

Going in order of growth, the first stop is the hair matrix. This region is the birthplace of the hair fiber. The hair matrix contains trichocytes which are the stem cells for hair production. The matrix forms around the dermal papilla because it needs a good blood supply to maintain the high rate of cellular division. It also needs those all-important go signals from the stem cells in the dermal papilla.

Trichocytes are specialized epithelial stem cells which produce the keratin for hair and nails. In the hair follicle, they also differentiate into inner root sheath cells. Cells migrate from the matrix and form the hair bulb. Highlighted here, the bulb is the area just above the matrix where the hair is first visibly arranged into its layers. The bulb is softer and whiter than the hair fiber. It’s softer because the cells still have their cytoplasms who are more spongy and is whiter due to a lack of melanin pigment. Although melanin is produced by melanocytes in the bulb, it’s passed through long fingerlike projections called dendrites to be inserted into cortical cells in the area above the hair bulb. Therefore, no melanin collects in the hair bulb. Instead, it starts to be deposited in cortical cells in the next level.

That next level is the hair root. Keratinization of hair cells begins here. This is a process by which the various layers of hair cells harden and die to form the mature hair shaft. This process is what gives hair its water and damage resistance. It’s also in this area that the shaft cuticles starts to detach from the inner root sheath cuticle. The cuticle cells in the shaft subsequently flatten onto each other to form the root tile effect.

From the hair root, mature hair cells form the hair shaft. This image shows a different hair fiber on the same section of scalp. This hair fiber has been sliced at a slightly level to give us the hair shaft rather than the follicle. The hair shaft is the mature fiber which leaves the skin and is ready to face the world. The hair will keep being pushed out from the base up. As the section of hair gets further and further from the dermal papilla, the cells of the cuticle become less and less stuck together and eventually start to peel apart. This compromises the cuticle’s ability to protect and waterproof the cortex.

The cortical fusi release their air and the hair fiber loses volume and strength and you might even get some split ends. Within the skin, however, there is a structure to help the cuticle last longer. We’ll talk about this now along with two other accessory structures. These structures are sebaceous glands found in all hairy skin, apocrine sweat glands only present in some areas, and arrector pili muscles, again found in all hairy skin. We’ll talk about each briefly in turn but first we’ll need a new image.

So here’s another H&E stained image to help us. We’ve highlighted the lumen of the hair follicle – in other words, where the hair fiber sits – but the hair itself has been removed. Let’s begin with the sebaceous glands.

Each hair fiber has at least one associated sebaceous gland which secretes sebum. Sebum is an oily, waxy secretion which coats the hair shaft. This is to help maintain the cuticle and assists with protection and waterproofing of the hair fiber and the skin. Basically, it stops your hair from drying out. So, what is sebum made of and where can you get some? Well, hold up, we need to take a look at the sebaceous gland first.

A sebaceous gland is made of secretory lobules shown in green here and connective tissue to separate the lobules like this. A secretory lobule is a big birthing pool for sebaceous gland cells. These sebaceous gland cells produces and stores fats and wax esters. They keep filling up until they burst and their contents are released. The burst cells and their contents are sebum. So, maybe, don’t put in that order for a kilo of sebum for your dry hair just yet. As the cells fill themselves and burst, they passed along the pilosebaceous canals towards the lumen of the hair follicle. These canals have been caught in cross-section and are highlighted on the image here.

Another secretory associated with hair are the apocrine glands. This is a different H&E stained image with the hair fiber here and an apocrine gland highlighted in green. Apocrine glands are sweat glands found in the axillae or armpits, external genitalia, and around the anus. They secrete an oily liquid of proteins, fats, and steroids which mixes with sebum to assist with its functions. Pheromones are a type of steroid which are part of apocrine sections and may have a role in nonverbal communication including attraction in mammals though their role in human behavior has not yet been proven.

The last structure to discuss is the arrector pili muscles. Here’s another H&E stained image, this time with an arrector pili muscle highlighted but note that there’s no hair fiber visible. We know this is an arrector pili muscle as there is no other structure in the dermis with this arrangement and structure. We can tell it’s muscle because of the color – muscle stains a much richer pink than the surrounding tissue – the elongated nuclei of the cells, and the parallel arrangement of the cells.

In this diagrammatical representation, the skin surface is here, the hair fiber is here, and there’s a sebaceous gland here along with many other structures of the skin. The arrector pili muscles are highlighted in green. Each hair fiber has one associated arrector pili muscle. An arrector pili muscle originates at the basement membrane of the skin up here and inserts on the bulge of the external root sheath. The muscle pulls in the same direction as the hair fiber comes out of the skin.

When the muscle contracts, the bottom of the hair follicle is pulled towards the skin below the skin surface, which causes the hair shaft to be pulled erect above the skin surface. The muscle’s contraction is stimulated by the sympathetic nervous system and so hairs are pulled up in the cold and during times of stress. Ever had goose bumps and felt your hair stand on end? Well, this is why. The primary function of this mechanism is to create an air pocket around us to insulate us in the cold.

At the start of this tutorial, we mentioned that hairy skin is found all over the body. So why is our head hair so prominent?

That’s because we actually have two types of hair. These two types of hair are terminal hairs and vellus hairs. Terminal hairs are the thicker, coarser pigmented hairs. They offer more protection from trauma, UV radiation, and friction than their vellus counterparts, and so are found on the scalp, in the axillae or armpits and around the external genitalia. Terminal hairs extend through the dermis so that their dermal bulb sits at the level of the hypodermis. So, this hair on the diagram is a terminal hair.

Vellus hairs are thinner and usually unpigmented. They originate in the reticular dermis and lack a medulla which allows us to differentiate between the two hair types histologically. Lanugo hair is another special type of hair. It’s a very fine usually unpigmented hair type found on the fetus, but it may also be found on severely malnourished adults. Its job on the fetus is to help protect the delicate fetal skin from the amniotic fluid and it may still be present on some newborns.

So that’s us done with the hair, but before we finish, let’s have a brief look at a clinical scenario which involves the hair namely the condition of alopecia areata.

Alopecia areata has been around for a long time as is evident by the pictures here. Alopecia means hair loss and areata is one type of alopecia. Alopecia areata is hair loss occurring in patches. These patches are usually round in shape. Hair in the affected area falls out but is subsequently able to regrow as only the hair shaft is lost. This makes the disease episodic, meaning that it comes and goes. It’s most noticeable on the scalp due to the density of hair, however, it can occur in any area of the skin. It’s thought to be an autoimmune disease and because of this, people with alopecia areata have an increased risk of having other autoimmune conditions.

On histology sections, immune cells gather around the dermal papilla in a swarm-of-bees appearance which is characteristic of alopecia. While alopecia is not a physically damaging condition, well apart from to the hair follicles, it can understandably be very distressing for patients. Treatment is generally not required as hair will grow back in time, however, an ongoing episode or recurrent episodes may be managed with locally-applied steroids, biologic drugs and ultraviolet light therapies.

So, well done, you’ve made it through the tutorial on the histology of the scalp and hair. Let’s recap what we’ve covered.

We started with an overview of hair, covering what hair is made of, where it’s found, and what its functions are. This was followed by a look at the scalp through the microscope and discussion of its blood supply through branches from the internal and external carotid arteries and drainage via veins draining into internal and external jugular veins. We then talked about the lymphatic drainage to the occipital, posterior auricular, and parotid lymph nodes as well as the innervation with the C2 spinal roots supplying the posterior scalp and branches of the trigeminal nerve supplying the anterior scalp.

We then used the histological section to check out the layers of the hair fiber and the hair shaft and hair follicle. From the center of the hair shaft outwards, the layers of the hair shaft are the medulla which is the inner core, the cortex which is the in-between layer, and the cuticle which is the outermost layer of the shaft. The hair follicle consists of those of the hair shaft as well as four extra layers – the internal root sheath, the external root sheath, the glassy membrane, and the dermal sheath. We also mentioned the root hair plexus responsible for innervating the hair follicle.

A change of image let us see the parts of the hair fiber as we moved up the fiber in the same direction as hair growth. These are the dermal papilla at the base of the hair fiber, the hair follicle situated in the skin and consisting of the hair matrix or the birthing pool for hair cells, the hair bulb or the preschool for hair cells, and the hair root where the cells prepare for life outside the skin. The hair follicle will then become the hair shaft which is mature and ready to break through the skin surface.

The accessory structures associated with hair came next, namely the sebaceous glands one of which is shown here in green, the apocrine glands responsible for sweat production, and the arrector pili muscles which erect our hair fibers. We then looked at the two types of hair in adults – terminal or coarse hair and vellus or fine hair as well as lanugo hair, the type of hair found on a fetus. We ended by touching on the condition of alopecia areata to put our knowledge of hair into a clinical context.

So that concludes our tutorial on histology of the scalp and hair. I hope you found it useful. Thanks for watching and see you next time!

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