MBLEx Review: Integumentary System Anatomy and Physiology

As massage therapists, one of the systems of the body that we influence the most is the integumentary system. Even though our techniques are often focused on treating deeper structures like muscle tissue and fascia, we can only do this by working through the skin.

What is the integumentary system? The integumentary system is one of the major systems of the body. It consists of the skin and accessory structures including hair, nails, and glands. The skin is composed of 3 layers: epidermis, dermis and hypodermis (or subcutaneous). The primary function of the integumentary system is to serve as a barrier to the outside world and to protect the body.

Massage therapists reviewing for the MBLEx in 2023 will find questions about the integumentary system in the Anatomy & Physiology and the Benefits & Effects content areas. This post on the integumentary system is fairly comprehensive and includes more detail than you will need for the MBLEx exam. However, current massage therapy students may find this thoroughness useful for anatomy class exams in school.

Structure of the integumentary system

The integumentary system’s main organ is the skin. It is composed of primarily of connective tissue underneath a layer of epithelial tissue. Skin also has accessory structures or appendages including the hair, nails and glands.

Remember that there are 4 basic types of tissue in the body: connective, epithelial, nervous, and muscle tissue. Connective tissue is the most abundant type of tissue in the body. Epithelial tissue is found not only in the skin, but in many other structures including blood vessels, the GI tract and the kidneys.

The skin is the largest organ of the body, in both surface area and mass. It accounts for 7% – 16% of the body mass, and covers about 2 square meters. Skin ranges in thickness from 0.5 mm at the eyelids, to 6 mm at the palmar and plantar surfaces.

There are 3 layers of skin:

• Epidermis is the most superficial
• Dermis is the middle layer
• Hypodermis (or subcutaneous) is the deepest layer

Some sources say that there are only 2 layers of skin, and they don’t count the hypodermis as a layer. However most sources including the American Academy of Dermatology consider the hypodermis a layer of the skin, so that’s how I’ll present it here.

Healthy skin and overall health go hand in hand. Damaged skin creates an opening for pathogens to invade the body and cause illness. The appearance of the skin can reflect information about a person’s health, age, nutrition, hydration or disease.

For example, skin can be cyanotic (blue) indicating lack of oxygen in the blood. It can be jaundice (yellowish) when someone has certain liver diseases. Erythema skin is reddened which can be due to heat exposure, hypertension or infection. Skin can be pale due to low blood pressure, anemia or fear.

Epidermis

The epidermis is the most superficial layer of the skin. It varies in thickness, from 0.05 mm at the eyelids to 1.5 mm at the palms of the hands and soles of the feet.

There are no blood vessels that supply the epidermis. Nutrients from the dermis diffuse upwards into the epidermis, particularly the deepest layers of the epidermis (stratum basal and spinosum).

The epidermis layer of the skin is primarily composed of keratinocytes. These are specialized cells that produce keratin.

There are two types of skin: thin and thick. The difference is in the epidermis.

Thin skin is what covers most of the body. The epidermis of thin skin has 4 layers (no stratum lucidum), has hair, has more sebaceous glands and fewer sweat glands, and has fewer sensory receptors.

Thick skin is on the palms of the hands and soles of the feet. It includes an additional epidermis layer: stratum lucidum. Thick skin is hairless, has more sweat glands, more sensory receptors, and no sebaceous glands.

Layers of the epidermis

There are 4 to 5 layers or stratum of the epidermis. “Stratum” means horizontal or sheetlike layered appearance. Here are the layers of the epidermis, from superficial to deep:

1. Stratum corneum. The flattened cells are dead by the time they reach this superficial layer. These cells are continually shed and replaced with cells from the deeper layer.
2. Stratum lucidum. This is the extra layer of the epidermis that is only present at the palms of the hands and plantar surface of the feet. It is what makes thick skin.
3. Stratum granulosum. Epidermal cells at this layer begin keratinization to produce a protein called keratin. Cells begin to decay and die in this layer.
4. Stratum spinosum. Contains about 10 layers of keratinocytes. This layer provides the epidermis with strength and flexibility.
5. Stratum basal. This is the base layer of the epidermis, and is also referred to as the stratum germinatum. It is formed by a single row of cuboidal or columnar cells that receives nourishment from the dermis. These cells reproduce rapidly through mitosis and generate the cells of the epithelium.

*Test tip: a pneumonic to help remember the epidermal layers is: “Come, Let’s Get Sun Burned.”

All epithelial cells are created in the stratum basal. Then as new cells are created, they push the previously made cells towards the surface. In other words, every epithelial cell starts off at the stratum basal layer, then moves upward through each layer until it reaches the stratum corneum.

The cells and layers of the epidermis become flatter as they move closer to the surface. Epithelial cells live for 3-4 weeks, from the time they are created to the time they are soughed off.

Specialized cells of the epidermis

Keratinocytes

Keratinocytes are the most common type of skin cells. They produce keratin, a tough protein that protects the skin from environmental damage, and forms the hair and nails. Keratin forms a physical barrier that makes the skin waterproof, and prevents most bacteria from invading the body. 90% of cells in the epidermis are keratinized.

Melanocytes

Melanocytes produce the pigment melanin which contributes to skin color and provides protection from damaging UV light. Melanocytes make up about 8% of the epidermal cells, and are located in the stratum spinosum near the stratum basal. The number of melanocytes is about the same in all races. However people with darker skin have melanocytes that produce more pigment.

Langerhans cells

Langerhans cells are produced in red bone marrow. They migrate to the stratum spinosum layer of the epidermis and assist the body’s immune response. These cells are easily damaged by UV radiation.

Merkel cells

Merkel cells are found in the deepest layers of the epidermis, at the dermal-epidermal junction. They function in the sensation of light touch.

Dermis

The dermis is located beneath the epidermis. This layer varies in thickness from 0.3 mm to 3.0 mm. This is referred to as “true skin”.

The dermis gives the skin its strength and elasticity. It is primarily composed of the fibrous connective tissues collagen and elastin. Collagen makes up about 70% of the dermis. Most of the accessory skin structures are located in the deeper parts of the dermis.

Accessory skin structures of the dermis

The accessory structures (appendages) of the skin begin in the dermis, but some of them like the hair and nails extend to the surface of the skin.

Nerve endings

The dermis contains sensory (afferent) nerve endings. This is how the integumentary system interfaces with the nervous system. So these nerve endings are an integral part of both systems.

Many of these nerve endings have specialized structures at the distal end that are designed to detect specific types of stimulation. Sensory receptors in the integumentary system can be classified as:

• Mechanoreceptors detect mechanical forces like pressure, vibration or stretch.
• Thermoreceptors detect changes in temperature.
• Nociceptors respond to painful (noxious) and potentially damaging stimuli.

Merkel’s discs

Merkel’s discs are mechanoreceptors located in the stratum basal and hair follicles. They can detect and discriminate light touch and pressure. Merkel nerve endings are abundant in the fingertips.

Meissner’s corpuscles

These are mechanoreceptors that can detect light touch and discriminate between light touch and heavy pressure. Meissner’s corpuscles are abundant in areas that are particularly sensitive such as hands, feet and lips. These mechanoreceptors also detect low-frequency vibration.

Ruffini endings

Ruffini endings, also called bulbous corpuscles, are mechanoreceptors that detect skin stretch and sustained pressure. They contribute to proprioception by detecting changes in joint angle. Ruffini endings can also detect heat.

Pacinian corpuscles

Pacinian corpuscles are also called lamellar corpuscles. They lie deeper in the dermis and hypodermis, and can detect vibration, deep pressure, and stretch. Pacinian corpuscles are also proprioceptors.

Free nerve endings

Free nerve are cutaneous nociceptors, which means they detect pain. They also detect pressure, temperature and chemical stimuli. Free nerve endings don’t have a specialized organelle at the end of them, and are abundant throughout the body.

Glands

*MBLEx tip: the glands of the skin are exocrine glands. This means that they secrete their substances to the surface via ducts.

Sebaceous glands

Sebaceous glands are located in the dermis throughout most of the body, except for the palms of the hands and soles of the feet. These glands produce and secrete an oily substance called sebum. They are attached to each hair follicle so it can diffuse the sebum towards the surface.

Sebum is composed of a mixture of fats, proteins, salts and pheromones, and is slightly acidic with a pH of 4.5 – 5.5. Sebum enhances the skin’s barrier function by inhibiting bacterial growth on the skin surface and assisting keratin to waterproof the skin. Sebum also lubricates the epidermis and hair, keeps the skin soft and pliable, and prevents dehydration from excessive water loss through the skin.

Sudoriferous glands

Sudoriferous glands are sweat glands that are located over most of the body.

• Eccrine sweat glands are all over the body. These secrete cooling sweat directly onto the skin to help regulate body temperature and for removal of waste products like urea and excess salts.
• Apocrine sweat glands are located at the axilla and genitals and secrete sweat in response to emotion such as stress, fear, or excitement. These glands begin to function at puberty. Sweat from these glands is slightly oily and contains pheromone compounds.

Hair

Hair is produced by the epithelial cells deep in the dermis layer of the skin, at the hair papilla. The hair root penetrates the dermis and epidermis, and is surrounded by the hair follicle. The shaft is the part of the hair that protrudes through the surface of the skin at the pores and is visible. Hair is made of keratinized cells and gets its color from melanin pigments in the papilla.

Hair has the function of protecting the scalp from sunlight and injury. It helps to protect the eyes and filters the air in the nostrils. Hair is also sensitive to light touch due to the receptors at the base of the follicles. Hair can also assist with thermoregulation. Arrector pili muscles erect the hairs and create goosebumps. These are involuntary skeletal muscles.

Nails

Nails are composed of compact keratinized cells that grow out of the base of the nail called the lunula. As new cells are created, they push the older cells out. Nails function to protect the fingertips and toes as well as help with manipulating objects.

Hypodermis or subcutaneous layer

The hypodermis, also called the subcutaneous or subcutis, layer is the deepest and thickest layer of the skin. It consists mostly of adipose tissue and loose connective tissue known as superficial fascia.

The hypodermis contains collagen and elastin fibers, and larger blood vessels and nerves. It connects the skin to the underlying tissues and organs.

The subcutaneous layer provides insulation and cushioning to the deeper tissues, like muscle and bone. The subcutaneous fat in the hypodermis is an insulator that helps with thermoregulation. It also serves as additional fuel storage for energy production. This layer becomes thinner as people age, which is why the elderly can be more sensitive too cold.

Functions of the integumentary system

The integumentary system plays an important role in protecting the body from the outside world. Healthy skin is a strong and resilient protective barrier. Skin and its accessory structures help the body to maintain homeostasis. See common pathologies that affect the integumentary system.

We are constantly being exposed to environmental threats including:

• Pathogens like bacteria and viruses
• Toxins, chemicals and pollutants
• The elements: water, wind, cold, heat, UV
• Injury: cuts, scrapes, impact

The integumentary system has 6 primary functions:

1. Protection
2. Sensation
3. Thermoregulation
4. Absorption & Excretion
5. Immunity
6. Synthesis

Let’s look at each of the functions of the skin in more depth.

Protection

The primary function of skin is to provide protection for all of the underlying tissues. It is a waterproof and insulating shield that creates a physical, chemical, and biological barrier to the outside world. Skin provides protection against injury and infection, and serves as storage of fat that can be used for fuel.

Physical barrier
Intact skin is one of the body’s best defenses against injury and infection. Skin creates a physical barrier primarily due to the keratinized cells. These cells in addition to glycolipids create a waterproof surface that also protects the body from water loss (dehydration). Hair protects the scalp from UV radiation. The adipose of the hypodermis protects the body from temperature changes. Epithelial tissue of the skin is able to heal after being injured.

Chemical barrier
Skin serves as a chemical barrier to infection due the secretions like sebum that creates a slightly acidic surface which slows down growth of foreign bacteria. The chemical pigment melanin creates UV protection.

Biological barrier
The skin provides protection from infectious organisms. Langerhans cells are a type of dendritic cell that protects the body from infection. Skin also contains phagocytic cells and skin flora which protect the body from pathogens.

Sensation

The integumentary system has cutaneous sensory receptors for that detect pain, temperature, pressure, vibration and movement.

• Free nerve endings (nociceptors) detect painful stimuli
• Meissner’s corpuscle can detect and discriminate light touch
• Pacinian corpuscle detects deep pressure
• Ruffini endings detect skin stretch and pressure
• Merkel discs detect light tough
• Hair follicle receptors detect movement of the hair

Thermoregulation

The integumentary system helps the body to maintain an optimal temperature through thermoregulation. The dermis layer contains the majority of the blood vessels of the skin, and is able to assist with thermoregulation. Adipose tissue in the hypodermis also serves as an insulator.

The skin can help the body to dissipate heat and cool down by dilating the superficial blood vessels (arterioles) to lose heat through evaporation. Dilating of blood vessels is called vasodilation. This is why skin becomes flushed when hot. This process is assisted by sweat which speeds up the evaporation process.

When the body is cold, the blood vessels constrict (vasoconstriction) in order to reduce heat loss. The arrector pili (erector pili) muscle will cause the hair to stand up. It is involuntary skeletal muscle tissue attached to the hair follicle. This creates more of a dead air space around the body to slow heat loss through evaporation and creates goosebumps.

Other ways that the integumentary system can help the body with thermoregulation includes using thermoreceptors in the skin to detect cold or heat. This sends a message to the nervous system to respond accordingly. This response could include:

• Triggering the muscles to shiver and generate heat
• Stimulating the endocrine system to speed up metabolism
• Causing the circulatory system to shunt blood from the cold extremities and conserve heat
• Influencing the person to make a conscious environment modification, like put on a jacket or move

Long-term exposure can also produce adaptive changes within the skin to help with thermoregulation. For example, people living in higher latitudes will develop more blood vessels at the extremities to prevent frostbite. People living in hot climates will develop more efficient cooling mechanisms. This can occur over a timeframe of hours to weeks in a process called acclimatization. And it can occur over generations in a process called adaptation.

Absorption & Excretion

Even though the skin is waterproof due to keratin and skin oils, some substances can pass through the skin. The skin can absorb fat soluble molecules, organic solvents, resins from certain plants (like poison ivy), and topical medications.

The skin is able to assist the body with eliminating some waste materials like ammonia, urea and excess salts and water by sweating.

Immunity

The integumentary system works with the immune system to protect the body from infections. Skin has dendritic cells called Langerhans cells in the epidermis. These cells are a type of white blood cell that trigger immune reactions.

Secretions from the sebaceous glands (sebum) reduce surface bacterial growth by creating a slightly acidic environment.

The surface of healthy skin has a layer of microorganisms called skin flora or skin microbiota. These microorganisms are usually bacteria or fungal, and are either not harmful, or are beneficial. They colonize the skin so that foreign microorganisms have a hard time taking over.

Synthesis

Skin can synthesize vitamin D when exposed to sunlight. Vitamin D is a fat soluble vitamin that regulates the levels of calcium and phosphate in bones. It is also a hormone that regulates calcium absorption from dietary sources.

How the integumentary system interacts with other systems

Nervous system

The integumentary system and nervous system work together in receiving sensory information from the environment. Nerve receptors in the skin provide the nervous system information about pain, pressure, temperature, etc. The autonomic nervous system regulates peripheral blood flow and sweat glands.

Muscular system

Sensory receptors in the skin can warn the body of potentially threatening conditions that require a response. Like detecting cold and triggering muscle shivering to generate heat. Learn more about skeletal muscle contraction, anatomy and physiology here.

Arrector pili muscle attaches to the hair follicle and enables the hairs to stand up when it contracts. The muscle layer of the blood vessels in the dermis controls the blood flow by causing the vessels to constrict or dilate.

Skeletal system

Vitamin D is synthesized by the skin when exposed to sunlight. Vitamin D helps the body to absorb calcium from the diet. The skeletal system needs the mineral calcium for the bone matrix.

Lymphatic and immune system

Intact skin provides the body with its first line of defense against infection. The layered structure of the skin, and the skin oils, prevents most pathogens like bacteria and viruses from penetrating the skin and invading the body.

Langerhans cells as previously mentioned assist with immune response. Vitamin D synthesized by the skin also has an important role in the body’s immune function.

Digestive system

The skin synthesizes vitamin D which helps the digestive system with calcium uptake from the food we ingest. The digestive system uses the dietary fats and oils to provide the skin with the lipids it needs to keep the skin healthy, and make the protective sebum and other oil compounds of the skin and hair.

Circulatory system

The integumentary system interacts with the circulatory system due to the blood vessels at the dermis and hypodermis. The capillaries of the skin can absorb medications into the bloodstream. Blood vessels in the skin can constrict or dilate to regulate body temperature. Blood brings healing nutrients to repair the skin if it gets damaged.

Other body systems

The integumentary system interacts with other body systems to a lesser extent.

Urinary system. The kidneys can adjust the amount of salts and electrolytes they eliminate to compensate for those compounds lost through sweat.

Reproduction system. Sensory receptors of the skin contribute to sexual arousal.

Endocrine system. Vitamin D is also a hormone.

How does massage therapy effect the integumentary system?

Therapeutic massage and bodywork treatments are often used to treat deeper structures like muscle tissue and fascia by working through the skin. However massage therapy can provide several benefits the integumentary system itself.

These benefits are due to the physiological effects of massage techniques on the skin. The skin is sensitive to touch, pressure, vibration and temperature, which are all components of therapeutic massage technique.

Here are a few specific ways that massage therapy can effect and benefit the integumentary system:

• Stimulate sensory receptors in the skin that activate the parasympathetic nervous system and produce systemic relaxation, and increase peripheral circulation
• Reduce stress on the skin by reducing edema if it is present
• Reduce superficial adhesions if present
• Mobilize scar tissue and minimize the appearance of scars
• Increase the strength of scar tissue in all directions
• Aid in exfoliation and moisturizing of the skin
• Improve the tone, flexibility and elasticity of the skin
• Increase local circulation of blood and lymph. Blood brings in nutrients and oxygen and removes waste products from the dermis and hypodermis.