In this lesson, you will learn about the layers, tissues, cells, and select functions of the skin as part of the integumentary system. Specifically, this lesson will cover:
Although you may not typically think of the skin as an organ, it is in fact made of multiple tissues that work together as a single structure to perform unique and critical functions. Recall from the introduction of the organ systems that the skin and its accessory structures make up the integumentary system, the organ system which provides the body with overall protection.
The integumentary system gets its name from the Latin word ‘integument’ which means ‘a covering.’ The integument, or skin, is made of multiple cells and tissues organized into two primary layers, the superficial epidermis and deep dermis. These are held to the underlying structures (i.e., bones, muscles) by a separate deeper layer called the hypodermis.
The deeper layer of skin contains multiple accessory structures such as sweat glands, oil glands, and neurons which you’ll see aid in temperature regulation, protection, and communication with the brain. The skin is also responsible for the production of hair and nails.
In this lesson, you will focus on the layers of the skin, the tissues and cells that make them up, and their functions. The accessory structures will be covered in detail in a future lesson.
Layers of Skin - The skin is composed of two main layers: the epidermis, made of closely packed epithelial cells, and the dermis, made of areolar and dense irregular connective tissue that houses blood vessels, hair follicles, sweat glands, and other structures. Beneath the dermis lies the hypodermis, which is composed mainly of adipose tissue.
term to know
Integument
The skin.
2. The Epidermis
The epidermis is the superficial layer of the integument (skin).
Integument -The skin (integument) is composed of the superficial epidermis and deep dermis. These layers are held to underlying structures by the deeper hypodermis.
The epidermis is composed of keratinized, stratified squamous epithelium and is organized into four or five layers of epithelial cells, depending on its location in the body. Skin that has four layers of cells is referred to as “thin skin.” Most of the skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet and has a unique fifth layer. From superficial to deep, the layers of the epidermis are listed below. The term ‘stratum’ is Latin for layer:
Stratum Corneum
Stratum Lucidum (thick skin only)
Stratum Granulosum
Stratum Spinosum
Stratum Basale
hint
There is a simple mnemonic that can help you remember the names of the epidermal layers in order from superficial to deep.
“Come, Let’s Get Sun Burned.”
The first letter of each word represents a layer of the epidermis in order.
Thick and Thin Skin -Thin skin (left) contains four layers in the epidermis and makes up the majority of the skin on the body. Thick skin (right) contains five layers in the epidermis and is only found in the palms of the hands and soles of the feet.
The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin. Recall that keratin is a water-resistant protein found in keratinized stratified squamous epithelial tissue. This protein gives hair, nails, and skin their hardness and water-resistant properties.
2a. Stratum Basale
Layers of the Epidermis - The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
The stratum basale is the deepest epidermal layer and is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale and eventually shed at the superficial surface. Another cell found dispersed among the basal cells in the stratum basale is a melanocyte, a cell that produces the pigment melanin. Melanin is a yellow, brown, or black pigment that gives hair and skin their color, and is also able to block ultraviolet (UV) radiation from the sun or other sources that might damage underlying tissues.
The stratum basale attaches the epidermis to the underlying dermis. Recall that epithelial tissues connect to underlying tissues through a basement membrane.
key concept
During development in the womb, the epidermis and dermis form a straight, horizontal boundary. As the growing fetus moves, it rubs against the inside of the uterus, pushing on the skin. This pressure causes the epidermal-dermal boundary to create folds. As you can see in the image below, portions of the epidermis fold down into the dermis. Simultaneously, portions of the dermis called dermal papillae (singular, dermal papilla) fold up into the epidermis. This folded boundary forms a stronger connection that makes it harder for the epidermis to be separated from the dermis.
Epidermal-Dermal Connection - The epidermis and dermis are connected through a basement membrane that is strengthened by folds in their boundary. Each projection of the dermis into the epidermis is a dermal papilla.
Recall that thick skin (palms of the hands and soles of the feet) contains five epidermal layers. Due to its increased thickness, the formation of dermal papillae in thick skin forms ridges on the superficial surface of the epidermis called epidermal ridges. On the fingers, these are collectively known as fingerprints and serve to increase friction, making it easier to hold a glass jar without it slipping. Since these are formed based on movement within the womb, every individual person’s epidermal ridges are unique, even identical twins will be different. However, certain general patterns (swirl, loop, etc) are inherited. A person’s epidermal ridges do not change with growth or age but can be altered by injuries that alter the epidermal-dermal boundary. Epidermal ridges also exist on the palm of the hand, sole of the foot, and toes.
Palm Print - The dermal papillae formed during development form epidermal ridges in thick skin (palms of the hand and soles of the feet). These form unique identifying patterns, commonly referred to as fingerprints.
2b. Stratum Spinosum
The stratum spinosum is the second layer of the epidermis, superficial to the stratum basale, and is named based on the spiny shape of the cells. However, this shape is an artifact of the removal from the tissue and staining. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of 8 to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale. Interspersed among the keratinocytes of this layer is a type of cell with long extensions called the Langerhans cell, which performs phagocytosis, engulfing bacteria, foreign particles, and damaged cells that occur in this layer.
The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum.
2c. Stratum Granulosum
The stratum granulosum is the third layer of the epidermis and is named for its grainy appearance due to increased production and accumulation of keratin. These 3 to 5 layers of cells become flatter and their cell membranes thicken. The nuclei and other cell organelles disintegrate as the cells begin to die, leaving behind the keratin and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.
2d. Stratum Lucidum
The stratum lucidum is a thin layer of the epidermis only present in thick skin. This layer is named based on being seemingly translucent (lucidum, clear). The keratinocytes that compose the stratum lucidum are dead and flattened.
2e. Stratum Corneum
The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment. The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes, dehydration of underlying tissues, and provides mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, upper layers and aim to keep the skin looking “fresh” and healthy.
did you know
The epidermis is renewed approximately every 4 weeks. This means that the stratum basale undergoes mitosis to generate new cells while the stratum corneum sheds cells.
Furthermore, you might wonder where all of that dead skin goes. Your dead skin cells make up the majority of the dust that settles around your room. This provides ample food for microscopic organisms called dust mites that eat the dead skin cells that your body no longer needs.
terms to know
Epidermis
The superficial general tissue layer of the skin.
Keratinocyte
A cell of the epidermis that produces and/or stores keratin.
Stratum Basale
The deepest layer of the epidermis.
Basal Cell
A stem cell located in the stratum basale that forms keratinocytes.
Melanocyte
A pigment producing cell found in the stratum basale responsible for skin and hair color.
Melanin
A protein pigment produced and secreted by a melanocyte.
Dermal Papillae
Superficial extensions of the dermal layer of the skin.
Epidermal Ridges
Superficial projections of the epidermis that form on thick skin, forming fingerprints.
Stratum Spinosum
The epidermal layer superficial to the stratum basale.
Langerhans Cell
A phagocytic cell located in the stratum spinosum.
Stratum Granulosum
The grainy epidermal layer superficial to the stratum spinosum.
Stratum Lucidum
The epidermal layer found between the stratum spinosum and granulosum, found only in thick skin.
Stratum Corneum
The superficial layer of the epidermis.
3. Dermis
The dermis might be considered the “core” of the integumentary system (derma, skin), as distinct from the epidermis (epi, upon / over) and hypodermis (hypo, below). It contains blood vessels, lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts.
Layers of the Dermis - This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer.
3b. Papillary Layer
The papillary layer is the superficial layer of the dermis and is made of loose, areolar connective tissue. Recall that areolar tissue forms a mesh of collagen, elastic, and reticular fibers among fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels. This layer fills in space and provides support for the superficial epidermis. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin, and sensory nerve fibers.
3c. Reticular Layer
Deep to the papillary layer is the much thicker reticular layer, composed of dense irregular connective tissue. Recall that dense, irregular tissue contains tightly packed, randomly oriented collagen fibers, providing strength in all directions. The reticular layer provides strength.
try it
Try this short experiment to experience the strength of the skin.
Place a finger on the skin on the back of your hand.
Push the skin—towards your fingers, towards your wrist, and to either side.
Pinch your skin and pull—upwards and at multiple angles
Did your skin break? Hopefully not! The majority of strength that skin has comes from the collagen fibers in the reticular layer of the dermis and can withstand a surprising amount of tension.
This layer is well vascularized and has a rich nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastic fibers provide some elasticity to the skin, enabling movement and recoil. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin tension by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.
make the connection
If you're taking the Anatomy & Physiology I Lab course simultaneously with this lecture course, it's a good time to try the activity Skin Layers and Organ Anatomy: Follow a skin cell’s journey! in Unit 3 of the lab course. Good luck with your investigation!
terms to know
Dermis
The second general tissue layer of the skin, composed of connective tissues.
Papillary Layer
The superficial layer of the dermis, composed of areolar connective tissue.
Reticular Layer
The deep layer of the dermis, composed of dense irregular connective tissue.
4. Hypodermis
The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer deep in the dermis and serves to connect the integument to the underlying tissue of bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of lipid storage, providing energy storage, insulation, and cushioning.
IN CONTEXT Everyday Connection - Lipid Storage
The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma.
Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others), as well as genetic factors. Fat distribution changes as our bodies mature and age. Males tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do females (breasts, hips, thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, although this measure is, in fact, derived from a mathematical formula that compares body weight (mass) to height. Therefore, its accuracy as a health indicator can be called into question in individuals who are extremely physically fit.
In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a major health threat. Recent studies indicate that a distressing percentage of our population is overweight and/or clinically obese. Not only is this a problem for the individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle—specifically in diet and exercise—are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes.
terms to know
Hypodermis
A layer of connective tissue connecting the integument to the underlying bone or muscle.
5. Skin Color
The color of skin and other bodily structures is influenced by a number of pigments, including melanin, hemoglobin, and carotene, among others. Differences in the color of skin can be based on genetics, disorders or conditions, nutrition, injuries, or the environment.
5a. Melanin
Recall that melanin is a pigment-protein produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The number of melanocytes present in the skin is the same no matter a person’s skin or hair color. The difference is in how much melanin a melanocyte produces. Dark-skinned individuals produce more melanin than those with pale skin.
Skin Pigmentation - The relative coloration of the skin depends on the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes.
Melanin is produced by melanocytes and transferred into keratinocytes by a vesicle called a melanosome. Recall that melanin can act as a shield to protect a cell from the damaging effects of UV light which can cause DNA mutations. Exposure to the UV rays of the sun or a tanning salon causes keratinocytes to secrete chemicals that stimulate melanocytes to increase melanin production. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. However, it requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns but are more protected than pale-skinned individuals. Sun tans are also impermanent for two reasons. First, melanosomes are eventually destroyed by fusion with lysosomes which will break down the melanin. Second, the skin is continually renewed as the stratum corneum sloughs off.
Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin and, in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer.
Moles - Moles range from benign accumulations of melanocytes to melanomas. These structures populate the landscape of our skin.
While melanin can protect cells from harmful DNA damage, too much melanin can have detrimental effects. Through UV light exposure, the skin plays a role in the production of Vitamin D, an important nutrient involved in calcium absorption. Therefore, too much melanin can lower Vitamin D and calcium levels in the body. You will see in a future lesson how this can affect the structural integrity of bones and the skeletal system.
IN CONTEXT Disorders of the Integumentary System
The first thing a clinician sees is the skin, so the examination of the skin should be part of any thorough physical examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard for the afflicted individuals to claim that they are benign.
Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes.
In vitiligo, the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches (see image below). Neither albinism nor vitiligo directly affects the lifespan of an individual.
Individuals with vitiligo experience depigmentation that results in lighter-colored patches of skin. The condition is especially noticeable on darker skin.
5b. Hemoglobin
Hemoglobin (Hg) is the protein pigment that provides blood the ability to carry and transport oxygen throughout the body. When blood is oxygenated, hemoglobin is bright red and the blood vessels carrying it are seen as red. When oxygen is used up, the blood is deoxygenated, hemoglobin is dark red, and the blood vessels are seen as blue (a trick your skin plays with light).
Blood - Blood changes color depending on how much oxygen is bound to hemoglobin. Oxygenated blood is bright red (left) while deoxygenated blood is dark red (right).
did you know
Blood and Skin Color - The blood present underneath the skin plays a role in its color. (a) The presence of extra oxygenated blood can turn the skin red such as when you blush. (b) The lack of blood can turn the skin white such as during poor peripheral circulation. (c) The presence of deoxygenated blood can turn the skin blue.
When the body sends additional oxygenated blood to an area of skin, it adds red to its natural color. This occurs during blushing, embarrassment, or inflammation. When blood is moved out of a region of the body, it takes away the red, causing a pale-skinned person to turn white. This occurs with very low blood pressure, poor peripheral circulation such as Reynauds, or when a person is “scared white.” Lastly, if the blood in a region of the body turns from oxygenated to deoxygenated, the skin will turn bluish, a condition called cyanosis. This occurs when blood is not allowed to leave an area such as a tourniquet or bruise.
When hemoglobin is broken down, it changes colors. This can occur naturally in the body or underneath the skin in a bruise. Below is a quick list of the colors hemoglobin can cause in a bruise (and the body).
Red = oxygenated hemoglobin.
Blue = deoxygenated hemoglobin.
Purple = Mixed oxygenated and deoxygenated blood.
Yellow = hemoglobin broken down (bilirubin, urobilin), makes urine yellow.
Brown = hemoglobin broken down (stercobilin), makes feces brown.
Green = hemoglobin broken down (bile), makes bile green.
Hemoglobins Many Colors - Hemoglobin can provide a variety of colors depending on its oxygen status or breakdown status. All of these colors are naturally found in the body in blood, bile, urine, or feces but can also be found in a bruise.
5c. Other Pigments
Other changes in the appearance of skin coloration can be indicative of dietary practices or diseases associated with other body systems.
Carotene, a green, yellow, orange, or red pigment in many vegetables can be stored in adipose tissue. Excessive consumption of these pigments can alter skin tone.
Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin (a breakdown product of hemoglobin), leading to the skin appearing yellow or jaundiced (jaune is the French word for “yellow”).
Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin.
Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color.
did you know
Many additional substances, if consumed or applied topically, can have an effect on the color of your skin, including silver. Start by looking up the condition of argyria. You can also look up a photo of Paul Karason, who went by “Blue Man” or “Real Papa Smurf.”
Paul Karason was once described as a white male. Paul had what he described as a bad case of dermatitis, inflammation of the dermis in his skin that affected his face. After many different treatments that didn’t work, Paul decided to try his own remedy, silver. Silver has some antimicrobial properties that some claim to be beneficial to the body. He began applying silver to his skin as a topical agent. Over time, he found that his dermatitis improved. He eventually began making a solution of silver and water that he drank as well. While the treatment seemed to help his skin as well as other ailments (arthritis, acid reflux, etc), his skin began to turn blue.
We think of silver as a silver color. However, when a silver salt (remember, a salt is an ionically bonded compound) has light applied to it, it turns into a bluish hue. This is a known process used in photography, but also seems to be how the body processes silver when taken into the body. Silver is converted into a salt when absorbed but when stored in the adipose tissue of the hypodermis, is converted back to elemental silver by light, giving off its blue tint.
For people who turn blue due to silver, it is a condition called argyria. This condition is permanent and cannot be reversed. In 1999, the Food and Drug Administration (FDA) made a ruling stating that “all over-the-counter (OTC) drug products containing colloidal silver ingredients or silver salts for internal or external use are not generally recognized as safe and effective.”
terms to know
Melanosome
A cellular vesicle that transports melanin from melanocytes into keratinocytes.
Hemoglobin
A red protein pigment in the blood that binds oxygen.
summary
In this lesson, you learned about the integumentary system, focusing on the layers of the skin. You learned to identify the layers of the epidermis—the stratum basale, stratum spinosum, stratum granulosum, 'stratum lucidum, and stratum corneum—and their individual functions . You also learned about the papillary and reticular layers of the dermis. You learned about how the skin is attached to underlying tissues by the hypodermis. Lastly, you learned about how melanin, hemoglobin, and other pigments can affect the skin color.
Try this short experiment to experience the strength of the skin.
