The Anatomy of the Skin

The skin is the largest organ of the human body. Its main functions are protection, thermal regulation, and sensation. The skin is made up of three main layers: the epidermis, the dermis, and the subcutaneous layer.

Anatomy

The skin is part of the integumentary system, which also includes the nails, hair, and exocrine glands. It is an incredibly large organ, accounting for 15% of an adult's total body weight.

Total skin thickness varies depending on where it is found on the body. The thickest skin is found on the back, the palms of the hands, and the bottoms of the feet, where it can be up to 3 millimeters (mm) thick. The thinnest skin is found on the eyelid, where the epidermis measures just 0.05 mm with very little dermis and subcutaneous fat.

The three major layers of the skin each contain specialized cells, tissues, and appendages, and each performs unique functions for the body.

Epidermis

The epidermis is the outermost layer of the skin, the layer of skin which is visible. The epidermis is also the thinnest of the three skin layers. It is an avascular skin layer, so it does not contain blood vessels.

This tough layer is chiefly made up of keratin and epithelial cells, stacked in tightly-packed sheets. It is in a state of constant renewal, as new skin cells are continuously created while older cells are sloughed away in a process called desquamation.

Important cell types of the epidermis include:

• Keratinocytes: The vast majority of the epidermis is made up of keratinocytes. Keratinocytes are cells that manufacture keratin, the structural protein that makes up skin, hair, and nails. Keratin is what forms the protective, water-resistant barrier of the skin.
• Melanocytes: After keratinocytes, melanocytes are the second most numerous. These cells produce melanin, the protein that gives color to the skin, hair, and eyes. Melanin also acts as a barrier, protecting the skin from UV light.
• Langerhans cells: These account for just a small number of cells within the epidermis, but have an important function. Langerhan's cells are specialized cells that work with the immune system to protect the skin from foreign pathogens.
• Merkel cells: These touch receptor cells are most numerous in high tactile areas such as the fingertips, lips, and around the hair shaft. These cells secrete a chemical that relays information directly to the brain, allowing the skin to sense even the lightest touch.

The epidermis itself is made up of four layers, with some areas having a specialized fifth epidermal layer.

Keratinocytes go through radical changes as they travel from the deepest layer of the epidermis where they are "born," to the top layer where they eventually slough away. The entire cell turnover process from cell birth to sloughing away takes, on average, about 28 days to complete.

The four layers of the epidermis are:

• Stratum basale: This is the deepest layer of the epidermis, and is made up of a single layer of basal cells. It is from these column-shaped cells that keratinocytes are created. Melanocytes and Merkel cells are also found in this layer. The stratum basale is also called the basal layer or stratum germinativum.
• Stratum spinosum: This is the thickest layer of the epidermis. As cells undergo mitosis (cell division) in the layer below, the newly-formed keratinocytes are pushed up into the stratum spinosum. Also found in this layer are Langerhan's cells.
• Stratum granulosum: As new keratinocytes are pushed up into this layer they continue to change in size and shape, becoming harder and flatter, creating a layer that has a granular appearance. The cell nucleus and organelles begin to die in this layer, leaving behind hard keratin.
• Stratum lucidum: This is the specialized fifth layer of the epidermis, and it is found only on the palms of the hands and soles of the feet. It adds an extra layer of protection to these areas. The layer is made up of dead, flattened cells.
• Stratum corneum: Also called the horny layer, this is the uppermost layer of the epidermis. It is made up of tightly-packed, keratinized cells. Once they've reached this layer, keratinocytes have died, flattened, hardened, and are now called corneocytes. These cells create the waterproof, protective barrier of the skin surface. As new corneocytes are created and pushed to the surface, old corneocytes are shed.

Dermis

The dermis is the middle layer of the skin. The dermis is the layer that gives skin it's structure and elasticity.

The dermis has two layers: the papillary layer and the reticular layer.

The papillary layer is the layer closest to the epidermis. The dermis and epidermis are connected by finger-like projections called dermal papillae. The dermal papillae send nutrients to the epidermis through a process called diffusion. Within the papillary layer are an abundance of small blood vessels, phagocytes (protective cells that ingest pathogens), nerve fibers, and tactile receptors called corpuscles.

The reticular layer is the thicker of the two dermal layers. It is primarily made up of fibers of collagen and elastin. This gives the dermis strength and allows it to stretch.

Within the reticular layer of the dermis are found:

• Sebaceous glands: The sebaceous glands are responsible for secreting an oily substance called sebum, which lubricates the skin. Sebaceous glands are found everywhere, except for the palms and soles of the feet. The highest concentration of sebaceous glands is on the face, scalp, and upper back.
• Hair follicles: The hair follicles work closely with the sebaceous glands to help draw oil to the skin's surface. The combination of hair follicle and sebaceous glands together is called the pilosebaceous unit. Hair follicles are found over the majority of the skin. They are absent on the palms of the hands, soles of the feet, lips, penis, and labia minor. It should be noted that the hair follicle extends up through the epidermis, opening at the surface of the skin.
• Sudoriferous glands: These are also known as sweat glands, of which there are two types—eccrine and apocrine. Eccrine glands are coiled glands which produce sweat and are key in helping regulate body temperature. These glands also excrete small amounts of waste product such as urea, lactic and uric acid, ammonia. Apocrine are numerous in the armpit and groin area and aren't active until puberty. The apocrine glands produce a type of sweat that is easily digested by bacteria and is responsible for causing body odor.
• Arrector pili muscle: The arrector pili muscle is a tiny muscle that is attached to the base of a hair. When it contracts, it creates a goosebump and makes the hair stand on end.
• Ceruminous glands: These specialized glands, only found in the dermis within the ear canal, create earwax.
• Lymph vessels
• Blood vessels
• Sensory receptors

Subcutaneous Layer

The upper two layers of the skin sit atop the subcutaneous tissue. This layer is sometimes called the hypodermis or panniculus.

This layer is primarily made up of fatty tissue called adipose tissue. This is where the body reserves its fat stores.

The subcutaneous layer is also made up of loose connective tissue, larger blood vessels, and nerves. This layer helps connects the upper skin to the musculature below.

This layer varies in thickness depending on where it's found on the body (it's thickest on the buttocks, palms, and feet) as well as age, sex, and health of an individual.

Anatomical Variations

Skin thickness varies by age. The skin becomes progressively thicker until about age 40, when it reverses its course and slowly thins. These changes occur mostly in the dermis.

There is some indication that men, biologically, have overall thicker skin than women. Some studies, though, have found no significant difference between the thickness of male versus female skin.

Skin pigmentation also varies from individual to individual. Skin pigmentation is chiefly the result of melanin. While most people have about the same number of melanocytes, the amount of melanin produced by those melanocytes varies greatly. The more melanin the skin has, the darker the skin color. Carotene and hemoglobin also play a part in skin pigmentation, but to a lesser degree.

Function

The skin carries out several critical functions.

Protection

The main purpose of the skin is to serve as an organ of protection⁠—from injury, infection, UV radiation, and moisture loss.

The skin creates a type of armor, a physical barrier to prevent pathogens from entering the body. In addition, sebum is slightly acidic, creating an environment that isn't ideal for harmful microbes.

But, if the skin is damaged (by a cut, scrape, burn, etc.) it creates a chink in the armor allowing those pathogens access to the body. This can allow an infection to take hold.

The subcutaneous layer specifically acts as a cushion to protect the more delicate bones and musculature beneath.

The skin also protects the body from UV rays. As mentioned before, melanin acts as a type of shield, blocking UV light so it cannot penetrate farther than the upper skin tissues. Sun exposure triggers the melanocytes to create more melanin, as the skin tries to protect itself from further damage (in other words, the skin tries to create a stronger shield). The creation of melanin is what causes the skin to tan, and is an indication of sun damage.

The skin is also key to preventing excess water loss. The epidermis creates a barrier that helps slow down the evaporation of water, as well as prevents excess water from being absorbed into the skin while bathing or swimming.

Sensation

The plethora of nerve endings found in the skin allows the human body to detect sensations of pressure, temperature, and pain. Sensory receptors are found throughout the skin, are especially numerous throughout the dermis.

Thermoregulation

The skin helps to maintain the body temperature within a very specific range.

When the body is getting too cool (hypothermia) the arrector pili muscles cause the hair to stand up, giving you goosebumps. The thin layer of air trapped between the hair and the body acts as an insulator helping to warm the body.

The blood vessels within the dermis also constrict, a process called vasoconstriction. Constricting the vessels at the skin's surface allows the skin to cool while maintaining the warmed blood for the body's core and critical organs.

When the body is becoming too warm, the sudoriferous glands release sweat. As the sweat evaporates, it cools the skin.

The blood vessels also play a role here in cooling the body by dilating (vasodilation). The vessels relax, allowing more blood to flow from the body's core, bringing heat with it. The heat then dissipates through the skin.

Synthesis of Vitamin D

The skin is responsible for producing the majority of vitamin D that is needed by the body. The skin contains molecules called 7-dehydrocholesterol. When these molecules are hit by UVB rays of sunlight, they are converted into vitamin D3. Vitamin D3 is then converted into the active form of vitamin D through the kidneys.

The amount of sun needed to get an adequate amount of vitamin D varies widely and is dependant on a host of different factors including skin tone, season, location (near the equator versus northern latitudes), time of day, and the amount of skin that is exposed. It's suggested you follow your healthcare provider's recommendation for the right amount of sun exposure for you.

Vitamin D supplements are also an option.

Associated Conditions

There are hundreds of conditions that affect the skin, and they have a wide range of causes.

Benign Skin Lesions

These are noncancerous growths that are common and not harmful. (Although if you notice a new growth, or changes in an existing one, you should have it looked at by a practitioner.)

• Birthmarks such as port-wine stains or hemangiomas
• Moles
• Skin tags
• Seborrheic keratosis

Inflammatory Rashes/Conditions

There is a vast array of inflammatory conditions that can affect the skin. Some are temporary, while others are chronic. Some may require treatment while others will heal on their own. They often resemble one another, so it's always a good idea to get a diagnosis from a healthcare provider.

• Acne
• Allergic rashes including contact dermatitis and hives
• Atopic dermatitis
• Keratosis pilaris
• Psoriasis
• Rosacea
• Seborrheic dermatitis

Injuries

The skin is vulnerable to all sorts of injuries. In most cases, the skin is able to heal through it's remarkable, and complex, process. Serious injuries should always be treated by a healthcare professional. Common skin injuries include:

• Abrasions
• Bruises
• Blisters
• Burns (including sunburn)
• Cuts
• Ulcers

Cutaneous Infections

Infections can take hold whenever there is a break in the skin barrier that allows microbes to pass through. Infection can be caused by bacteria, viruses, or fungi. They include:

• Boils and abscesses
• Cellulitis
• Fungal infections (like ringworm and athlete's foot)
• Panniculitis (inflammation of the subcutaneous tissue often caused by infection but can also be triggered by an injury)
• Warts

Viral Infections

Many conditions that don't originate in the skin can cause cutaneous symptoms. They include:

• Chickenpox
• Herpes virus—both genital herpes and cold sores
• Measles
• Pityriasis rosea

Pigmentation Disorders

These are disorders that affect the way the skin produces melanin. Pigmentation conditions can either cause an increase in color (hyperpigmentation) or a loss of color (hypopigmentation). Some pigmentation conditions are treatable while others are not.

• Freckles and "age spots"
• Melasma
• Pityriasis alba
• Vitiligo

Cancer

Skin cancer is most often linked to excessive sun exposure. Most forms of skin cancer are highly treatable, but early detection is key.

There are three types of skin cancer:

• Basal cell carcinoma
• Squamous cell carcinoma
• Melanoma

Genetic Conditions

Certain genetic conditions can cause the skin to not function as it should. Most are fairly rare. They include:

• Albinism (can also be classified as a pigmentation disorder)
• Pidermolysis bullosa - A group of conditions that causes extremely fragile skin which easily blisters or erodes
• Hereditary ichthyosis - A condition that causes an overgrowth of exceptionally dry, scaly skin
• Xeroderma pigmentosum

Tests

There are several tests that are performed on the skin to help diagnose various conditions that can affect this organ.

Biopsy

A skin biopsy is a procedure in which cells or skin tissue is removed for examination under a microscope. A biopsy is used to check for skin cancer, infection, and to help identify certain rashes.

There are three main techniques used to do a skin biopsy: punch, shave, and excision.

• Punch biopsy: A circular cookie-cutter-like tool is used to remove a small piece of skin
• Shave biopsy: A blade or scalpel is used to shave off a piece of the skin surface
• Excisional biopsy: The entire lesion is removed

You will be given a local anesthetic prior to the biopsy. Stitches may be used to close the biopsy site in some cases.

Patch Test

Patch tests are common done to help identify possible causes of contact dermatitis. Adhesive patches with small pads impregnated with common allergens are placed on the back and left for a period of 48 hours. After the patches are removed, the skin is checked for signs of irritation, redness, or swelling. This allows one to know the substances that trigger contact dermatitis.

Woods Lamp Examination

A Woods lamp is a type of black light that allows a medical professional to detect things that aren't easily seen with the naked eye.

During an exam, you will sit in a dark room. The healthcare provider holds a Woods lamp close to your skin to look for color changes. The presence of certain fungi or bacteria will appear in particular colors. The borders of hyperpigmentation or hypopigmentation are more easily seen under a Woods lamp as well.

Skin Prick Test

The skin prick test is a test that is performed on the skin, but isn't used to diagnose a skin condition. Instead, skin prick tests are used to determine which substances a person may be allergic to. This includes things that cause allergic rhinitis and food allergies.

Skin prick tests are typically done on the back or on the arm. A device with small points, which have been dipped in allergen extracts, is used to prick or scratch the skin's surface. After 15 to 20 minutes, the skin is examined. Any inflamed bumps or wheals indicated a positive reaction.

A Word From Verywell

For an organ that is so visible and familiar, the skin is surprisingly complex. As the largest organ of the human body, the skin is responsible for many important functions. There are hundreds of conditions that can affect the skin; many of them look incredibly similar and are difficult to distinguish one from another. If needed, see a healthcare provider for help in diagnosing and treating your skin condition.