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What is skin?

Look at you!

You look so cute

in your brand-new birthday suit.

This is how we all begin:

small and happy in our skin.

Fran Manushkin and Lauren Tobia, Happy in Our Skin¹

Boredom, that suspended state of time-stopping ennui, is a feature of childhood. My own childhood unfurled long before the endless distraction of ubiquitous screens. Children back then endured more enforced tedium than now, adapting to the schedules and demands of others, not the other way around. Waiting, waiting, always waiting for the talking, talking, always talking adults to finish.

Grown-ups organised pupils into rows in the school playground with instructions to be silent. There we sat cross-legged on asphalt that imprinted the skin under our uniforms with its rough texture. Adults glared at us to quell our fidgeting and whispering as we sat-knelt-stood-sat and sat some more during Mass. They commanded us to keep our hands to ourselves while we were squeezed along the back seat of the family station wagon. Forced sleep upon us during afternoon naptime. I can remember rubbing my hands together under my woollen blanket as I lay in my tiny preschool cot, fingers and palms dancing in mimicry of the teacher’s motions as she applied hand cream. A daily ritual of self-care for her as she watched over a room of children, all sleeping except for one. Sometimes, bless her, she would quietly pass me a book.

Intense study of one’s own skin – its injuries and infelicities – was one antidote to childhood restlessness, a way to be present but elsewhere. I recall meticulously examining the freckles on my arms as if I were a (very) junior dermatologist. Trained with imaginative rather than clinical skills, I identified a shape that was indisputably an ant with two legs and wished my little freckle might grow another four and become a proper insect. I searched for other distinguished (as opposed to distinguishing) moles in the same way we looked for animal shapes hidden in clouds. Scabby knees could really help pass the time, because the miracle of wound-healing produced a crust to be picked at. But only after the application – and trepidatious removal – of the medical wonder that is a bandaid. Like a certificate of achievement, a sticking plaster revealed to the world the injury suffered, bravery displayed and recovery underway.

Parents are astonished at how babies suck on their own fists. Before long, infants learn to wave their little arms in the air, gazing at them as they flail seemingly independently of their bodies. Children achieve another milestone when one hand grabs the other or finds a foot – always just within reach, somehow – floating in space. Quite the synapse-driven feedback loop, watching your hand grab something you didn’t know was attached to you and feeling it at the same time. Humans can’t tickle ourselves, but babies find comfort in touching their skin, discovering that they are themselves. The world around an infant is so enormous and overwhelming that it seems they must focus intensely on what is at hand, which, more often than not, is their hand.

Children’s fascination with the biological wonders of the world begins with the mysteries of their own little bodies, and those of others. The body’s emissions and extrusions are part of that, but skin is the main game. Skin that protects us and exposes us. That can change before our very eyes. Skin that makes us who we are. That intense urge to examine it, to pay attention, may come and go, but for many of us it never leaves.

‘What is the human body’s biggest organ?’ asks a quizmaster. ‘Skin!’ answers the contestant in the hot seat. The audience might suspect it is a trick question, but skin is the correct, winning answer. A 70-kilogram individual has skin that covers two square metres and weighs five kilograms.² Reminding us that knowledge is never set in stone, Professor Nina Jablonski, a biological anthropologist at Pennsylvania State University who appears often in this book, reveals in a footnote to her book Skin: A natural history that some experts challenge skin’s status as our biggest organ. They argue that the mucosal lining of the intestines is larger than the surface area of the skin. It’s hard for laypeople, or perhaps anyone who is not a gastroenterologist, to conceive of this obscure but crucial anatomical feature, against which skin is so, well, obvious. But most medical experts are still on the side of skin as the largest organ, as am I. I prosecute the case for it being the most versatile and remarkable, too.

There are many popular books devoted to the intricacies and importance of various organs of the human body. Sitting on the bookshelves near my desk are three engaging accounts of this genre, all written by specialists: one called Heart, the second called Gut and the third called The Brain.³ (It is a coincidence that the three books focus on the deficiencies of the companions Dorothy meets along the yellow brick road to Oz: the Tin Man in need of a heart, the Cowardly Lion in need of courage, or guts, and the Scarecrow in need of a brain.) These are the star contenders, perhaps, but there can be no gold medallist in the hypothetical ‘most-important organ’ Olympics, given the interconnectedness of the workings of our bodies. And consider those crucial organs forever destined to be character actors in the shadow of the top-billed stars. The spleen and the pancreas, for example, might not be sexy enough to inspire a popular book devoted to them. But skin, the most visible organ of them all, so often taken for granted and misunderstood, deserves to be up on the podium.

The subject of skin can turn usually sober academic or medical writers lyrical. Here is Nina Jablonksi writing beautifully about skin as a border and a receptor: ‘Far from being an impervious barrier, however, the skin is a selectively permeable sheath. It is constantly at work as a watchful sentinel, letting some things in and others out. The skin is also home to hundreds of millions of microorganisms, which feed on its scales and secretions. But our skin is more than a defensive shield, a gatekeeper, and a personal zoo. The pores and nerve endings of our skin unite us with our surroundings. Skin is the interface through which we touch one another and sense much of our environment.’⁴

Science and wonder are never mutually exclusive, but understanding the former, particularly the taxonomies and pathologies of skin, feeds the latter. DermNet NZ, an online resource with the tagline ‘All about the skin’, opens its much-visited ‘Principles of dermatological practice’ page with a list of the components found within a square centimetre of skin. Every bullet point provokes an admiring nod, and their cumulative effect draws the reader in to skin’s cheer squad. The list is:

6 million cells

5000 sense end organs

400 centimetres of nerve fibres

200 pain sensors

100 centimetres of blood vessels

100 sweat glands

15 sebum glands

12 cold receptors

5 hairs

2 heat receptors.⁵

Skin is part of the cutaneous sensory system or integumentary system, a term I admit I was unfamiliar with before I started this project. So conditioned was I into thinking that skin was all surface, I hadn’t understood that skin is one of eleven systems that allow the body to function. (Some of the others – digestive, nervous, reproductive – are more commonly known.) The human integumentary system consists of skin, hair, nails, and some glands and nerves. For animals, it includes scales, feathers, whiskers and hooves. The integumentary system acts as a barrier, protecting us from the vicissitudes of our environment and the nasty microbes, viruses and fungi on a mission to infect us. It protects our internal organs, keeps us hydrated, regulates our body temperature, receives sensations and touch – both loving and painful – provides a storage system for water and fat, and eliminates waste products. In short, we could not live without it.

Skin is laminated, and this concept of layering helps to make sense of how skin functions. The epidermis is the technical yet familiar word that describes the top layer of skin. Epi means ‘on’, so the epidermis sits on the dermis, the second layer. The third layer is called the hypodermis, or subcutaneous tissue. An injection by hypodermic needle goes, of course, into this layer. Eighty per cent of the body’s fat is stored in the hypodermis, cushioning us, sometimes a little more than we would like. In women, most fat is stored in this subcutaneous layer; in men it’s more likely to be in the abdomen, around their organs.⁶ There it is known as visceral fat or, in the common parlance, a beer gut. Beneath the hypodermis is the muscular system. There is remarkable activity both between and within the skin’s three layers – the epidermis, the dermis and the hypodermis – and between each of these layers and the rest of the body. This dynamism is why the skin is an organ, part of a system, not a carapace separate from the rest of us like a turtle’s shell or a Gore-Tex jacket.

The epidermis is the layer of skin we can see. Its thinness, on average just 1 millimetre thick, belies its complicated structure and accentuates its awesomeness. The epidermis itself has four layers; five if you count the extra layer found in the thickest areas of human skin, the palms of the hand and the soles of the feet. We rarely have cause to think about the thickness of our skin except metaphorically, when we feel insulted, but actually its thickness varies across the body. The thinnest skin is that of the scrotum and the eyelids.

When my son Toby was studying biology for his high-school final exams I asked him if he knew anything about human skin. ‘Bricks-and-mortar structure’ was his perfunctory answer. Monty Lyman – doctor and research fellow at the University of Oxford, and author of The Remarkable Life of the Skin – is another writer who appears frequently in this book. He expands on that idea, observing that the secret of the epidermis lies in its ‘multi-layered living brickwork: keratinocyte cells. The epidermis is made up of between fifty and one hundred layers of keratinocytes, named after their structural protein, keratin.’ Lyman evocatively describes the assemblage of keratinocytes as ‘biological chainmail’, saying that if you were to ‘zoom in on the back of your hand to about 200x magnification you would see tough, interlocking keratin scales resembling an armadillo’s armour’.⁷ An image to recall next time you apply hand sanitiser.

Keratinocytes are not only crucial, they’re always on the move. If we start at the deepest layer of the epidermis, attached to the dermis below, with all its nerves and blood vessels, we find the stratum basale, which means – helpfully – deepest layer. Keratinocytes are produced here, before they divide. These new cells push upwards into the layers above, ensuring that a new epidermis forms every few weeks. It is in this way that our skin constantly remakes itself. As University of Sydney dermatology professor Pablo Fernández-Peñas told me, ‘Most people don’t even know they change their skin. I present this to my students by saying, “Imagine you find a new partner. How long would you have to wait on a desert island until you could tell them, ‘Nobody has touched my skin ever?’ A month!” All your surface is new surface. It’s amazing.’

I’m inclined to agree. Also in the basal layer or stratum basale, sitting at the bottom but still only a millimetre from the top, are two kinds of cells named after German scientists. Merkel cells made me think of the formidable former German chancellor. Unlike Angela Merkel, however, who in official photographs often stood out in a sea of men, these cells can be hard to identify. Merkel cells are named after nineteenth-century anatomist Friedrich Merkel, who called them tastzellen, or touch cells, when he first described them in 1875. Merkel cells make possible the sense of light touch – for example, running your hands over silk, or feeling language beneath your fingertips if you read Braille.⁸ They are found in parts of the body of ‘high tactile acuity’ according to the ‘Rook Book’, a standard dermatology textbook used by medical students.⁹ ‘Erogenous’ is not the sexiest of words, until you pair it with ‘zone’.

The other cell named after a German physician and anatomist, Langerhans, are dendritic cells, which means they serve an immune function. Writing about the immune system lends itself to military metaphors, so it’s not surprising that Langerhans are often referred to as ‘immune sentinels’. This is because their job is, first, to detect an unknown invader and, second, to trigger the immune system when pathogens launch an attack. Once they’ve identified a suspicious bacteria or allergen, they engulf it, Pac-Man-like, and break it into smaller pieces called epitopes. Monty Lyman writes that Langerhans cells use the information from these epitopes like a barcode, moving from the epidermis to the lymph nodes to share the epitopes’ specifications and raise the alarm. One scientific paper I read described this process rather delightfully as ‘cross talk’. Lyman writes that following a visit from Langerhans cells, ‘T Cells are able to signal to other cells and organize a coordinated immune response against any invader.’¹⁰

Langerhans have long memories. This is why if you brush against poison ivy you won’t get the itchy allergic rash the first time, but the second time you are exposed your now-sensitised skin will react as it would to infection, by itching, swelling and even blistering.¹¹ Langerhans are mainly found in the layer above the basale, the spinosum, or spiny layer. It gets this name because the cells in the spinosum are linked by web-like filaments called desmosomes, which are the mortar that holds the keratinocyte bricks together. These filaments are themselves made up of a protein called filaggrin, a word that will be familiar to people who have eczema and to their doctors.

Still within the epidermis, as we rise above the stratum spinosum and move closer to the surface, we arrive at the granular layer, the stratum granulosum. Here our hero cells, keratinocytes, turn into something expressible in verb form: they keratinise. Being now so far away from the capillary layer of the dermis means they can no longer draw on nutrients. They flatten and harden as they die, acting as a shield that protects the body.

Finally, we reach the top, the stratum corneum, or horny (hard) layer of the epidermis. We find layers within layers: this one is twenty to thirty layers thick. All the keratinocyte cells, which, now effectively dead, are officially called corneocytes or squames, are anucleated, meaning their nucleus has disintegrated. (Squames means scales; we all have scaly skin like that armadillo.) Yes, all the cells that cover the outside of your body are dead. But they are dead in a good way, with thick plasma membranes surrounding lots of keratin. The dead cells protect the living ones inside, which will soon replace them. It’s the circle of life playing out on your skin.¹²

The idea of dead skin isn’t so novel, but the constant journeying of the cells to the top layer of the epidermis was a revelation to me. The terminology of non-melanoma skin cancers (basal cell carcinomas, Merkel cell carcinomas, squamous cell carcinomas and so on) now made more sense. So did the beauty industry’s obsession with exfoliating. I recalled a lesson in primary school, which may have been a formal part of the late-1970s natural science syllabus or simply a helpful life lesson. Our teacher, who clearly knew her audience, set an everyday kind of experiment for our homework. We were to have a bath, making sure to get the dirt off, and then examine whatever we rubbed off our bodies after our initial scrub. What we could expect to see, she promised, was dead skin cells.

Forty-something years later, by the time I finished my crash course through the five layers of the epidermis I was a little befuddled by all the skin-cell names. So, I decided to do a quiz on DermNet, one designed for medical students, to check that I was taking it all in. Reader, I’m pleased to say I only got one wrong (about fingernails). But the quiz reminded me that there is another cell type found in the epidermis to which we need to be introduced, because, directly or indirectly, it gets attention in every chapter of this book.

Meet the melanocyte. Melanocytes, which are made deep down in the stratum basale, produce melanin. This is the pigment that gives our skin and hair its colour, and gets a shout-out in AOC’s beauty tutorial. As soon as you start thinking about the many wrongheaded notions of skin colour and race, the outsized cultural role of the melanocyte looms large.

Before we zoom back out into the world beyond our skin, it’s time to dive down to the dermis, passing through the basement layer of the epidermis that anchors it to the dermis below. The dermis is the engine room of the skin, packed with dense connective tissue, blood and lymph vessels, nerve fibres, hair follicles and glands primed to pump sweat and sebum. Rook’s Textbook of Dermatology states that ‘the dermis . . . has a remarkable capacity for retaining water’.¹³ (I can’t pretend to have read all four volumes of the text, but this was one of the more hyperbolic sentences I came across in this even, unemotive tome.)

We don’t need beverage manufacturers to remind us that hydration is important. Water is a key influence on skin’s mechanical properties – its function as a barrier, its elasticity and suppleness – and its physical properties at a molecular level. The term ‘natural moisturising factor’ (NMF), which scientists admit is a bit loose (and easily co-opted by cosmetics companies), is central to this interplay. NMF is a finely balanced combination of chemical substances that helps to maintain hydration in the top layer of the epidermis, the stratum corneum. One of many relatable examples being the way NMF makes your skin swell and wrinkle in a hot bath to prevent the influx of too much water.¹⁴

Through each layer of my study, I realised I was subconsciously looking out for a word I know to be crucial to skin, courtesy of countless magazine advertorials and because many women I see have had their faces injected with it. Collagen, a protein, is found in the dermis (and elsewhere in the body), where it forms fibrous bundles that strengthen the skin. One characteristic of ageing, particularly at the onset of menopause in women, is reduced production of collagen in the dermis. Another familiar word frequently cited in pseudoscientific advertising is elastin. Elastin also resides in the dermis. True to its name, it makes the skin elastic and pliable.

Sweating regulates the body’s temperature and, as we will see, played a role in the evolution of our species. How does it work? Eccrine sweat glands are distributed over the entire surface of the body. They are made up of secretory coils in the lower dermis, which sounds more like a boiler for heating than something devised for cooling. Sweat glands operate via a duct that runs through the dermis. Sweat, which is 99 per cent water, seeps out through the funnel-shaped pores of our skin and into the air around us. As Bill Bryson writes in his book about the human body, this sweat ‘cools the body as it evaporates, turning us into a kind of living air conditioner’.¹⁵ Heat-containing molecules from the body are gone, thereby cooling the skin and the blood vessels of the dermis. The cooled venous blood then circulates to the core of the body, so our temperature doesn’t keep rising.¹⁶

During moderate exercise, sweat glands can produce one litre of sweat an hour. You do need to put back what the sweat takes out, as advertisements for specially formulated sports drinks exhort, although drinking water does the job in most cases. The 1 per cent of sweat that isn’t water is generically referred to as ‘salts’, but more specifically, as athletes know, it is electrolytes, fatty acids, urea (as found in urine) and lactic acid. We don’t need to do much to work up a sweat; even while sitting at a desk, the ambient temperature can provoke sweating so light that we don’t even notice. Over the course of a day, a sedentary adult can lose 450 millilitres of ‘invisible sweat’. But there are different kinds of sweating.

Imagine you are working in an air-conditioned office when you receive a group email announcing that your department is being restructured and roles will be cut. As you try to remain calm, a colleague who has suddenly become your possible adversary tells you that the video of a recent presentation you gave where a swarm of bees somehow got into the conference room is going viral. This incident is being shared and re-shared around the world with the hashtag #GetThemAwayFromMe. With your heart pounding and sinking at once, you look at the clock and realise you have a report due in twenty minutes, and then you accidentally spill coffee all over your keyboard. All these feelings – fear, embarrassment, anger – with their accompanying adrenaline, trigger what is known as emotional or mental sweating. When we describe a stressful situation with the phrase ‘I broke out in a cold sweat’, it is a biological fact. As professor of clinical anatomy Adam Taylor explained in an article (which also discussed why some people don’t sweat at all), ‘Adrenaline causes the blood vessels to narrow and a few sweat glands to become active – producing a drop in skin temperature and a cold sweat.’¹⁷

As well as the eccrine glands, we also sweat from our apocrine glands, which are located in the armpits, nipples and groin. Sweat itself is odourless, but its proteins and lipids are attractive to the bacteria that live on these parts of our skin, which metabolise the sweat and turn it into our unique personal scent. The resultant odour may be repulsive in a primal, get-me-out-of-this-smelly-taxi way. Or, in your partner, attractive in a primal, get-me-into-the-bedroom way. Smell emanating from the apocrine glands is one of the indefinable factors of human sexual attraction. Throwing a bucket of cold water on the mysteries of eros, the author of the relevant chapter in the Rook Book writes dispassionately, ‘The exact role of apocrine glands in humans is unknown.’¹⁸

Skin has other glands that are crucial but can manifest in ruinous ways: the sebaceous glands that secrete sebum, or oil. There is a small sack, a sump if you will, attached to hair follicles that transmits a mixture of oil, fat and waxy substances to the skin surface. This is terrific for waterproofing and lubricating the skin, and is 100 per cent necessary. But our hormones, particularly those that are released at puberty, can prompt overproduction of sebum, which leads to the embarrassment – sometimes the nightmare – of acne. Whether you have oily, dry or ‘normal’ skin is thanks, in part, to the level of activity of your sebaceous glands, which become less busy as you age.

Where there are follicles there must be hair. Hair is technically an appendage of the skin. I once joked to a friend that the money I had spent on my hair over the years would be enough to buy a small car. (It should have been a reality check rather than a joke, because that phantom car has since grown into a luxury vehicle.) So, when it comes to hair, I have extensive knowledge and depleted funds to show for the various treatments I have tried. Among these was keratin treatment, which, now that I know what keratin is, I see in a new light. Hair may be long, short, frizzy, lustrous, curly, kinky or straight, but however it looks, all hair is made up of flexible strands of dead, keratinised cells.

The only truly hairless skin (technical term ‘glabrous skin’) is our lips, nipples, genitalia, palms of the hands and soles of the feet. Most of the hair on our bodies is pale and fine ‘vellus’ hair. Quite a few people would like more of that and less of the thicker, coarser ‘terminal’ hair, that includes pubic, chest and armpit hair. What is the purpose of hair on these sites of extensive, ongoing depilation? The science about the reasons for the distribution of hair is, unfortunately, divided, just as the reasons for baldness are disputed. Male pattern baldness may affect half of all men over fifty. It causes so much anxiety that overcoming baldness is a multi-million-dollar industry. Hair – men’s and women’s – has become another adjective to put in front of that all-encompassing word ‘wellness’.

Fingernails and toenails are other skin appendages. One scientific source described fingernails as ‘great tools for scratching’. Perhaps not how a nail salon might present them in its promotions for manicures and pedicures, but accurate nonetheless, particularly in an evolutionary sense. But that’s not all: nails also protect the tips of our digits and are useful for picking, pinching and poking at things. You won’t be surprised to learn that these hard, translucent structures are, too, made of keratin.

Finally, let us consider two skin features that are unique to each of us: one we’re born with, the other we can choose to acquire. The dermis and epidermis are locked together, as we’ve seen. Dermal papillae are protuberances that lie just under the hair follicle, sticking out into the epidermis above. They form dermal ridges which, on the hands and feet, we know as fingerprints. Their arches, whorls and loops, reflecting the undulations of the underlying dermis, are formed in the womb. Unlike most of our skin, they don’t change over the course of our lives. Fingerprinting is a trusted biometrical tool for identifying someone, not to mention a mainstay of detective shows. Even identical twins have different fingerprints. Fingerprints have openings for sweat glands too. So, combining fingerprints with our other theme of sweating, should an anxious and inexperienced cat burglar or murderer, sweaty and without gloves, leave moisture from their fingers on surfaces or objects – say, a doorknob or knife – their prints can be traced. But, in what could easily become a plot twist, fingerprinting forensics usually destroys any DNA left behind at a crime scene.¹⁹

If keratinocyte skin cells are replaced every month or so, you might wonder why tattoos are permanent. Lyman has the best technical description of tattooing I’ve read. He writes about what happens when a needle full of ink is pushed through the skin of the epidermis (which is, remember, only 1 millimetre thick on average) into the deeper dermis:

The needle fires into your skin at roughly a hundred times a second, intentionally causing many tiny wounds, alerting the body to damage . . . The ink is sucked up by capillary action in the dermis, the ink particles waiting for your immune cells to rush into the damaged area . . . macrophage cells (‘big eaters’ from the Ancient Greek) detect the pigment particles as foreign and try to eat them up. But their eyes are bigger than their stomachs, and many of these macrophages end up stuck, with the pigment trapped inside them.

So, while it’s true that the top layer of our skin renews itself, ‘the inked-up cells in the dermis stay there for the remainder of our lives, locked in forever like intricate fossils in a cave wall’.²⁰

And so our introduction to the basic structure of skin comes to an end. Biology tells us a lot, but is only part of the story of skin. How did we come to know what skin is and how it works? And who are the experts that step in when something goes wrong?