Skin: anatomy and function

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

Get Medical Information / Alexandra Gordon

Anatomy

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

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

Each of the three main layers of the skin contains specialized cells, tissues, and appendages, and each has functions unique to the body.

Epidermis

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

This tough layer is mainly composed of keratin and epithelial cells packed in very compact sheets. It is in a state of constant renewal as new skin cells are constantly being created and old cells are shed in a process called peeling.

Important types of epidermal cells include:

  • Keratinocytes : the vast majority of the epidermis is made up of keratinocytes. Keratinocytes are cells that make keratin, the structural protein that makes up skin, hair, and nails. Keratin is what forms the skin's waterproof protective barrier.
  • Melanocytes : after keratinocytes, melanocytes are the second most abundant. These cells produce melanin, a protein that colors skin, hair, and eyes. Melanin also acts as a UV barrier to protect the skin.
  • Langerhans cells : They constitute only a small number of cells in the epidermis, but they have an important function. Langergan cells are specialized cells that, together with the immune system, protect the skin from foreign pathogens.
  • Merkel cells: These sensory receptor cells are most abundant in areas of high tactile capacity, such as the fingertips, lips, and around the hair shaft. These cells release a chemical that transmits information directly to the brain, allowing the skin to feel even the lightest touch.

The epidermis itself consists of four layers, and some areas have a special fifth epidermal layer.

Keratinocytes undergo radical changes as they move from the deepest layer of the epidermis, where they are 'born', to the upper layer, where they eventually exfoliate. The entire cell renewal process from birth to rejection takes about 28 days on average .

Layer is the term for a laminar layer.

Four layers of the epidermis:

  • Basal layer: it is the deepest layer of the epidermis, formed by a single layer of basal cells. It is from these columnar cells that keratinocytes are created. This layer also contains melanocytes and Merkel cells. The basal layer is also called the basal layer or germ layer.
  • Spiny layer: it is the thickest layer of the epidermis. As cells undergo mitosis (cell division) in the underlying layer, the newly formed keratinocytes are pushed up into the spinous layer. This layer also contains Langergan cells.
  • Granular stratum : As new keratinocytes are introduced into this layer, they continue to change in size and shape, becoming more solid and flat, creating a layer that has a granular appearance. The cell nucleus and organelles begin to die in this layer, leaving behind solid keratin.
  • Lucid Layer – This is a specialized fifth layer of the epidermis found only on the palms of the hands and the soles of the feet. This adds an extra layer of protection to these areas. The layer is made up of flattened dead cells.
  • Stratum corneum – Also called the stratum corneum, this is the top layer of the epidermis. It consists of densely packed keratinized cells. Upon reaching this layer, the keratinocytes die, flatten, harden, and are now called corneocytes. These cells create a protective waterproof barrier on the surface of the skin. As new corneocytes are created and pushed to the surface, the old corneocytes exfoliate.

Dermis

The dermis is the middle layer of the skin. The dermis is the layer that gives the skin its structure and elasticity.

The dermis consists of two layers: papillary and reticular.

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 many small blood vessels, phagocytes (defense cells that absorb pathogens), nerve fibers, and touch receptors called corpuscles.

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

In the reticular layer of the dermis are the following:

  • Sebaceous glands : The sebaceous glands are responsible for the production of an oily substance called sebum that lubricates the skin. The sebaceous glands are found everywhere except on the palms of the hands and the soles of the feet. The highest concentration of sebaceous glands is found on the face, scalp, and upper back.
  • Hair follicles : Hair follicles work closely with the sebaceous glands to help attract oil to the surface of the skin. The combination of a hair follicle and an oil gland is called a hair follicle. Hair follicles are found on most of the skin. They are absent on the palms of the hands, soles of the feet, lips, penis, and labia minora. It should be noted that the hair follicle passes through the epidermis and opens to the surface of the skin.
  • Vascular glands : They are also known as sweat glands and are of two types, eccrine and apocrine. The eccrine glands are spiral glands that produce sweat and play a key role in regulating body temperature. These glands also secrete small amounts of waste products such as urea, lactic acid, uric acid, and ammonia. Apocrines are numerous in the armpit and groin area and remain inactive until puberty. The apocrine glands produce sweat, which is easily digested by bacteria and causes bad body odor.
  • Erector pili muscle: The erector pili muscle is a tiny muscle that attaches to the base of the hair. When it shrinks, it gives you goose bumps and makes your hair stand on end.
  • Ceruminous glands : These specialized glands, found only in the dermis within the ear canal, create earwax.
  • Lymphatic vessels
  • Blood vessel
  • Sensory receptors

Subcutaneous layer

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

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

The subcutaneous layer also consists of loose connective tissue, larger blood vessels, and nerves. This layer helps connect the upper skin with the muscles below.

This layer differs in thickness depending on where it is on the body (it is thickest on the buttocks, palms, and feet), as well as the age, sex, and health of the person.

Anatomical variations

The thickness of the skin depends on the age. The skin gradually thickens until around age 40, when it changes direction and slowly becomes thinner. These changes occur mainly in the dermis.

There are some indications that men are biologically thicker in general than women. However, some studies have not found a significant difference between men's and women's skin thickness .

Skin pigmentation also varies from person to person. Pigmentation of the skin is primarily the result of melanin. Although most people have roughly the same number of melanocytes, the amount of melanin produced by these melanocytes varies greatly. The more melanin in the skin, the darker its color. Carotene and hemoglobin also play a role in skin pigmentation, but to a lesser extent.

Function

The skin has several important functions.

Protection

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

The skin creates a kind of armor, a physical barrier that prevents pathogens from entering the body. Also, sebum is slightly acidic, creating an environment that is not ideal for harmful microbes.

But if the skin is damaged (by a cut, scratch, burn, etc.), a gap forms in the armor, allowing these pathogens to enter the body. This can allow the infection to develop.

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

The skin also protects the body from ultraviolet rays. As mentioned above, melanin acts as a kind of shield, blocking ultraviolet light, so it cannot penetrate beyond the upper tissues of the skin. Exposure to the sun causes the melanocytes to produce more melanin as the skin tries to protect itself from further damage (in other words, the skin tries to build a stronger shield). Melanin production causes the skin to tan and is a sign of sun damage.

The skin also plays a key role in preventing excessive water loss. The epidermis creates a barrier that helps slow the evaporation of water and also prevents excess water from entering the skin while swimming or swimming.

Sensation

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

Thermoregulation

The skin helps maintain body temperature within certain limits.

When the body gets too cold (hypothermia), the muscles that contract the pili make the hair stand on end and give you goose bumps. A thin layer of air trapped between the hair and the body acts as an insulator, helping to warm the body.

Blood vessels in the dermis also contract, a process called vasoconstriction. Vasoconstriction on the surface of the skin allows the skin to cool while retaining warm blood for the body's core and critical organs.

When the body gets too hot, the sweat glands produce sweat. When sweat evaporates, it cools the skin.

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

Synthesis of vitamin D

The skin is responsible for the production of most of the vitamin D that the body needs. The skin contains molecules called 7-dehydrocholesterol. When these molecules are exposed to UVB rays from the sun, they turn into vitamin D3. Vitamin D3 is then converted through the kidneys into the active form of vitamin D.

The amount of sunlight needed to get enough vitamin D varies widely and depends on many different factors, including skin color, time of year, location (near the equator versus northern latitudes), time of day and the amount of exposed skin. We suggest that you follow the advice of your healthcare professional in choosing the right amount of sunlight for you.

Vitamin D supplements can also be used.

Related conditions

There are hundreds of conditions that affect the skin and have a wide variety of causes.

Benign skin lesions

These are common, non-malignant neoplasms that do not cause harm. (Although, if you notice new growth or changes to an existing one, you should see a doctor.)

Inflammatory rashes / conditions

There are many inflammatory conditions that can affect the skin. Some are temporary, others are chronic. Some may need treatment, while others heal on their own. They are often similar to each other, so it is always a good idea to get a diagnosis from your healthcare provider.

Trauma

The skin is vulnerable to all kinds of injuries. In most cases, the skin can heal through this wonderful and complex process. Always consult a doctor in case of serious injury. Common skin lesions include:

Skin infections

Infections can spread as long as the skin barrier through which germs pass is broken. The infection can be caused by bacteria, viruses, or fungi. They include:

Viral infections

Many non-skin conditions can cause skin symptoms. They include:

Pigmentation disorders

These are disorders that affect the way the skin makes melanin. The pigmentation condition can cause an increase in color (hyperpigmentation) or a loss of color (hypopigmentation). Some pigmentation conditions can be treated while others cannot.

Cancer

Skin cancer is most commonly associated with excessive sun exposure. Most forms of skin cancer respond well to treatment, but early detection is key.

There are three types of skin cancer:

If you have a sore that doesn't heal or keeps coming back, a new mole or skin lesion, or a change in the size, shape, or color of an existing mole, you should see your doctor.

Genetic conditions

Certain genetic conditions can cause the skin to malfunction. Most of them are quite rare. They include:

  • Albinism (can also be classified as a pigmentation disorder)
  • Pidermolysis bullosa is a group of conditions that cause extremely fragile skin that blisters or breaks easily.
  • Hereditary ichthyosis is a condition that causes an overgrowth of exceptionally dry and scaly skin.
  • Xeroderma pigmentosa

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 tissue are removed from the skin for examination under a microscope. A biopsy is used to look for skin cancer, infection, and certain rashes.

There are three main methods used for skin biopsies: punch, shave, and excision.

  • Stroke biopsy: A round instrument similar to a cookie cutter is used to remove a small piece of skin.
  • Post-shave biopsy: A blade or scalpel shaves off part of the skin's surface.
  • Excisional biopsy: The entire lesion is removed.

You will be given a local anesthetic before the biopsy. In some cases, stitches may be used to close the biopsy site.

Patch test

Patch tests are usually done to identify possible causes of contact dermatitis. Adhesive patches with small pads soaked in common allergens are placed on the back and left for 48 hours. Once the spots are removed, the skin is checked for signs of irritation, redness, or swelling. This allows you to identify the substances that cause contact dermatitis.

Woods Lamp Inspection

A Woods lamp is a type of black light that allows a healthcare professional to detect things that are difficult to see with the naked eye.

During the exam, you will sit in a dark room. A healthcare professional holds a Woods lamp to your skin to monitor color changes. The presence of certain fungi or bacteria will appear in certain colors. The limits of hyperpigmentation or hypopigmentation are also easier to see under a Woods lamp.

Skin prick test

A skin prick is a test that is done on the skin, but is not used to diagnose a skin condition. Instead, skin pricks are used to determine what substances a person may be allergic to. This includes substances that cause allergic rhinitis and food allergies.

Skin injections are usually given in the back or arm. A small tip device that has been moistened with allergen extract is used to prick or scrape the surface of the skin. After 15-20 minutes, the skin is examined. Any swollen lump or blister indicates a positive reaction.

Get the word of drug information

For such a visible and familiar organ, the skin is surprisingly complex. As the largest organ in 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 hard to tell apart. If necessary, consult your doctor for help in diagnosing and treating your skin condition.

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