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- Identify the components of the integumentary system
- Describe the layers of the skin and the functions of each layer
- Identify and describe the hypodermis and deep fascia
- Describe the role of keratinocytes and their life cycle
- Describe the role of melanocytes in skin pigmentation
Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system, which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue (Figure 1). The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain.The skin consists of two main layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers?
The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. 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. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum (Figure 2).
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. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers (Figure 3).
View the University of Michigan WebScope at to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?
The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made (Figure 4).
The stratum basale 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. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.
In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.
As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale (Figure 5). Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer.
View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?
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.
The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells (see Figure 4). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum corneum, and the accessory structures of hair and nails.
The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened (see Figure 4). These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water.
The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment (see Figure 4). 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 and the dehydration of underlying tissues, and provides a 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 layer and aim to keep the skin looking “fresh” and healthy.
The dermis might be considered the “core” of the integumentary system (derma– = “skin”), as distinct from the epidermis (epi– = “upon” or “over”) and hypodermis (hypo– = “below”). It contains blood and 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 (Figure 6).
The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (see Figure 6). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles.
Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. 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 turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.
The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves to connect the skin to the underlying fascia (fibrous tissue) of the 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 fat storage and provides insulation and cushioning for the integument.
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. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (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.
The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome (Figure 7).
Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production.
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 are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent.
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 (Figure 8).
The first thing a clinician sees is the skin, and 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 to claim that they are benign, at least to the individuals so afflicted.
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 (Figure 9). Neither albinism nor vitiligo directly affects the lifespan of an individual.
Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems. Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, 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. Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color. A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin becomes dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for “blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s pigmentation.
This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism?
The Individual Layers of Skin and Their Functions
Heather L. Brannon, MD, is a family practice physician in Mauldin, South Carolina. She has been in practice for over 20 years.
Casey Gallagher, MD, is board-certified in dermatology and works as a practicing dermatologist and clinical professor.
The skin is the largest organ, and it's one of the most complicated. It's ever-changing, and it contains many specialized cells and structures. The skin's primary function is to serve as a protective barrier that interacts with a sometimes-hostile environment.
It also helps regulate body temperature, gathers sensory information from the surrounding environment, and plays an active role in the immune system to protect the body from disease.
Learning how the skin functions begins with an understanding of the structure of the three layers of skin: the epidermis, the dermis, and subcutaneous tissue.
Anatomy of Human Skin
A human skin majorly comprises two layers (epidermis and dermis), but a layer that connects the skin to the bone is known as hypodermis (not a part of the skin). Here, we will briefly discuss each functional components of the epidermis and dermis layer of the human skin along with the diagrammatic representations.
It is the topmost layer of the human skin that possesses five sublayers.
- Stratum basale: It is the first layer above the basement membrane possessing column-shaped basal cells. The old basal cells shift upwards to the squamous cell layer. The basal cells get flattened and died once they become old. It possesses one receptor cell called Merkel cell that responds to the pressure exerted on the skin.
- Stratum spinosum: It possesses flattened cells, Langerhans cells and specialized melanocytes. The Langerhans cells are the dendritic cells that provide primary defence against the microbial invasion. Melanocytes synthesize melanin pigment and control the skin tone (higher the melanin production, darker is your skin colour). Melanosome vesicles act as a packaging material that store melanin and its number and size determine the skin tone.
- Stratum granulosum: It contains more keratinocytes that synthesize a colourless protein (keratin). The old keratinocytes will move upwards (towards the skin’s surface). Keratin strengthens the skin.
- Stratum lucidum: It only exists in the thick skin (soles, palms etc.).
- Stratum corneum: This layer is the thickest and an outermost squamous cell layer, which comprises dead and flattened corneocytes (lacks nucleus). It sheds off the dead keratinocytes (desquamation) after every 28-30 days and replaces them with the new keratinocytes.
Epidermis layer possesses four specialized cells:
- Merkel cells: It functions as a receptor cell that responds to the pressure.
- Melanocytes: It makes the melanin pigment that decides the human skin’s tone.
- Langerhans cells: These are the dendritic cells, forming the first line of defence against the outside environment.
- Keratinocytes: These form colourless protein keratin that provides strength.
It is an intermediate layer between the basement membrane and the subcutis. It has two sub-layers, namely papillary and reticular dermis. The dermis layer is the thickest portion of the skin that constitutes about 90% of the human’s skin. It comprises most of the specialized cells and structures. A papillary dermis is present underneath the basement membrane, while a reticular region is associated with the lower region of the hypodermis layer.
- The papillary layer appears thin and comprises of loose connective tissues. It possesses some irregular projections called papillae or epidermis ridges. It includes an extensive network of the blood vessels that supplies essential nutrients to the epidermis layer and supports keratinocytes’ formation.
- The reticular layer appears thick and contains compactly packed or irregular connective tissues. It possesses collagenous, elastic and reticular fibres along with the pilosebaceous unit. Collagen (mostly type-1 and 3) and elastin proteins are the components constituting the formation of the reticular layer, in which a former provides strength and a latter gives elasticity to the human skin.
Let us discuss the specialized cells and structures that exist in the skin’s dermis layer:
It comprises hair follicle, arrector pili muscle, sebaceous gland and sweat gland that colloquially termed as epidermal invagination.
- Hair follicle: It originates from the follicular base called hair bulb. Each hair follicle contains different cells and connective tissues. The living cells in the dermis layer’s hair bulb will divide actively to build a hair shaft. The network of blood vessels nourishes the cells dividing in the hair bulb.
- Arrector pili muscle: It is composed of smooth muscle fibres and attached to a follicular bulge. Contraction of this muscle results in piloerection or goosebumps.
- Sebaceous gland: It is an exocrine gland that secretes oil into the skin. The sebaceous gland is associated with the hair follicle. It functions as a lubricating gland of the skin that maintains the body’s moisture. Oil glands are absent in the palms and soles.
- Sweat gland: It is also called sudoriferous gland. Apocrine and eccrine are the two common sweat glands that differ in location, function and secretory product. The apocrine sweat gland is found in the hairy skin (armpits, groin area etc.) and eccrine sweat gland is located all over the body. The apocrine and eccrine gland’s primary function is to contribute to body odour and regulate the body’s temperature, respectively. Numerous adipose tissues contribute to the lining of sweat glands.
Mechanoreceptors & Specialized Cells
Pacinian and Meissner corpuscles are the cutaneous receptors, which respond to the vibratory sensation. Skin includes specialized cells like fibroblasts, macrophages, adipocytes, and mast cells. Fibroblasts are the specialized cells responsible for the production of collagen, elastic and reticular fibres along with the extracellular matrix. Macrophages (histiocytes) assist the immune system. Adipocytes provide insulation and hair follicle regeneration. Mast cells generate an inflammatory response.
It is not a part of the skin but found associated with the lower region of the dermis layer. It is also called subcutaneous and hypodermis layer. Subcutis allows the components of the skin to communicate with the underlying fibrous tissues of bones and muscles. A network of loose connective tissues, fat cells and elastin proteins contribute the subcutis layer.
Functions of Human Skin
Skin performs various functions like:
Skin shields the body and provides primary defence against physical, chemical and biological damage. Also, it prevents excessive water loss or maintains body homeostasis.
Skin can regenerate new cells by replacing the old, dead or damaged cells. Skin regeneration is one of the skin’s important features, as the skin cells renew after 28-30 days. The dead cells from the lower epidermis reach the skin’s surface where they grow hard and sloughed off from the skin.
Skin functions as a site of vitamin-D synthesis, in which the substance (7-dehydrocholestrol) isomerizes into cholecalciferol or previtamin-D3 in the presence of UVB-light. This inactive form (cholecalciferol) further hydroxylates in the liver and kidney to produce an active form of vitamin-D (calcitriol). In the bloodstream, calcitriol travels as a hormone and functions to regulate calcium and phosphate synthesis, promoting bones’ remodelling.
Human skin possesses mechanoreceptors, thermoreceptors and nociceptors that can sense the pain exerted from the deep pressure, temperature and noxious stimuli.
Thermoregulation is one of the skin’s major functions, which can either occur through blood vessels and sweat glands. The blood vessels dissipate heat out from the skin as the body temperature increases through vasodilation. Oppositely, the blood vessels retain heat as the body temperature decreases through vasoconstriction. Similarly, the sweat glands in the skin’s dermis layer transpirate heat out by releasing electrolytes out of skin through sweating or perspiration.
The skin’s secretions like sebum (from sebaceous glands) and sweat (from sweat glands) act as an antimicrobial agent. Sebum is the waxy lipid material that lubricates the skin that forms a slightly acidic environment to restrict the direct contact of microorganisms like bacteria, fungi etc. Similarly, the sweat glands release electrolytes (NaCl) that also inhibit the microbial interaction.
|Composition||It is composed of stratified epithelium, keratinocytes, melanocytes and Langerhans cells||It is composed of the pilosebaceous unit, specialized cells and mechanoreceptors||It is made of subcutaneous fat|
|Functions||Immune defence, decides skin tone, protects against uv-damage and pathogens||Pliability, thermoregulation, tensile strength, wound-healing and elasticity||Insulation, calorie reserve and shock absorber|
Fun Facts about the Human Skin
- It’s the largest organ of the human integumentary system, but small intestine is the largest organ found in our body’s interior.
- The skin comprises 19 million skin cells.
- There are about 20 blood vessels, and 1000 nerve endings exist within the skin.
- The skin possesses approximately 650 sweat glands.
- Skin contributes approximately 12-15% of the body’s weight.
- A skin forms the first line of defence against the physical, chemical and biological damage.
- The skin thickness varies accordingly with the different body parts and depends on the person’s age, gender and eating habits. Our feet have a maximum thickness (1.4mm), while our eyelids possess the thinnest skin (0.2mm).
- The skin tone is decided by the melanin pigment released by the melanocytes in the epidermis layer. More is the melanin pigment, darker the colour of your skin.
- Legs contain a few oil glands, due to which they are considered as the driest body part.
Therefore, we can conclude that the human skin possesses an epidermis and dermis layer where each performs distinct roles. Skin performs multitasking, as it acts as a barrier or shock absorber by sensing the physical or chemical (UV-light, physical damage and trauma etc.), maintains equilibrium between the body fluids, regulates cooling effect in summers, and replaces the old or dead cells. So, it becomes necessary for us to keep our skin healthy by avoiding excessive UV-exposure, unhealthy diet, and stress.
Layers of the Skin
The skin is composed of 3 layers namely:
3 Layers of the Skin (Source: Wikimedia)
This is the outer most superficial layer which is made up of 5 inner layers. They are Stratum basale, Stratum spinosum, Stratum granulosum, Stratum lucidum, and Stratum corneum.
In general, skin’s epidermal layer is subjected to constant wear & tear from external factors such as sunlight, chemicals such as soaps, and pollution.
Dermis cover the significant portion of the skin’s layer. The dermis layer has connective tissues, blood vessels, oil and sweat glands, nerves, hair follicles, and other structures.
The dermis is made up of two inner layers namely – a thin upper layer called the papillary dermis, and a thick lower layer called the reticular dermis.
3. Subcutaneous Layer
Subcutaneous layer is also known as hypodermis. The hypodermis is the innermost layer of the skin. This layer hosts fat and connective tissues that house larger blood vessels and various nerves.
The primary function of the hypodermis is to act as an insulator for regulating the body temperature.
Why are tattoos permanent though skin cells die and get replaced?
The answer lies hidden in the second layer of the skin (Dermis). The permanent tattoo ink is injected till the dermis layer so that it stays permanent. If it is put on the outer layer, then it will be worn out as time progresses. That is why permanent tattooing is always a harrowing and painful experience.
The amount and type of epidermal melanin is the main factor that determines skin complexion and UV sensitivity. Melanin is a large bio-aggregate composed of subunits of different pigment species formed by oxidation and cyclization of the amino acid tyrosine [10,25,26] ( Figure 2 ). Intriguingly, the intermediates of melanogenesis may have important regulatory roles in the skin . Melanin exists in two main chemical forms: (1) eumelanin, a dark pigment expressed abundantly in the skin of heavily pigmented individuals, and (2) pheomelanin, a light-colored sulfated pigment resulting from incorporation of cysteines into melanin precursors . Eumelanin is much more efficient at blocking UV photons than pheomelanin, thus the more eumelanin in the skin, the less UV-permeable is the epidermis . Fair-skinned people who are almost always UV-sensitive and have high risk of skin cancer have little epidermal eumelanin and therefore “realize” much more UV than darker-skinned individuals. Therefore, the fairer the skin, the more damaging UV exposure will be. In fact, pheomelanin levels are similar between dark-skinned and light-skinned individuals, and it is the amount of epidermal eumelanin that determines skin complexion, UV sensitivity and cancer risk. Data suggest that pheomelanin may promote oxidative DNA injury and melanomagenesis by generating free radicals in melanocytes even in the absence of UV .
Melanin Biosynthesis. Melanin, a large bioaggregate composed of pigmented chemical species, is found in two major forms: the brown/black highly UV-protective 𠇎umelanin” pigment and the red/blonde UV-permeable “pheomelanin”. Both eumelanin and pheomelanin are derived from the amino acid tyrosine. Tyrosinase is the enzyme that catalyzes the rate-limiting synthetic reaction for both melanin species and when defective causes albinism. Incorporation of cysteine into pheomelanin results in the retention of sulfur into the pigment, which yields a light color to the final melanin product and may contribute to oxidative injury in the skin. The melanocyte stimulating hormone (MSH)–melanocortin 1 receptor (MC1R) signaling axis is a major determinant of the type and amount of melanin produced by melanocytes in the skin.
Dermis – The Layer that SUPPORTS
The dermis is the middle layer, beneath the epidermis. Its primary job is to provide structural support and moisture to the skin, and to connect the epidermis to blood vessels, lymph vessels, and nerves.
The dermis provides the scaffolding for the epidermis. Think of the epidermis as a mattress, and the dermis as the box springs. If the box springs (dermis) weaken, the mattress (epidermis) sags.
This scaffolding is called the Extracellular Matrix (ECM), an important support system for dermal cells. The matrix consists primarily of structural proteins (collagen and elastin) surrounded by a large amount of a ‘gel’ consisting of water, glycosaminoglycans, and proteoglycans . This gel-like fluid provides plumpness to the skin and is known as the Ground Substance .
The dermis is divided into 2 sub-layers :
- Papillary Dermis (upper part of dermis)
- Reticular Dermis (lower part of dermis, the bulk of the dermis)
The Papillary Dermis contains blood vessels (capillaries) that provide oxygen and nutrients to skin cells in the epidermis and lymph vessels that carry away waste from the epidermis. It also contains nerves and elastic fibers (which is why sagging primarily occurs here).
The Reticular Dermis contains dense collagen and elastin fibers, hair follicles, sebaceous glands, sweat glands, and larger blood vessels. There is a lot more collagen than elastin, so this is the area where wrinkles form.
The dermis also contains immune cells as well as fibroblasts , which are specialized cells that make collagen, elastin, glycosaminoglycans, and proteoglycans.
Between the epidermis and dermis lies a thin area called the Dermal-Epidermal Junction . It is also known as the Basement Membrane (Rete Pegs) . The rete pegs look like ridges. As the skin ages over time, these ridges flatten out and the junction gets thinner. This flattening of the rete pegs is a major site of aging in the skin.
What is the mammalian skin made of?
The mammalian skin is composed of two primary layers – epidermis and dermis, with a basement membrane in between.
What is epidermis?
The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body’s surface, responsible for maintaining fluids in the body and prevents pathogens from entering the body.
The epidermis contains no blood vessels and the cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the skin. The melanocytes present in the epidermis are responsible for producing melanin which gives the skin its colour.
What is basement membrane?
The epidermis and the second layer, dermis are separated by the basement membrane, which is a thin sheet of fibres. The basement membrane controls the traffic of cells and molecules between the two layers of the skin, through it’s complex structuring.
What is dermis?
The dermis is the layer of the skin beneath the epidermis and consists of connective tissue, cushioning the body against stress and strain. The dermis provides tensile strength and elasticity to the skin through a complex layering of extracellular matrix of collagen fibrils, microfibrils and elastic fibres embedded in layers of vital cellular fluids and molecules that help maintain the strength of skin.
The dermis is divided into two layers, the Papillary region and Reticular region.
1. Papillary region of dermis
This layer is composed of areolar connective tissue, and is named after the finger like projections called the papillae that extend toward the epidermis. The papillae increase the surface area of the dermis, providing extra strength.
2. Reticular region of dermis
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic and reticular fibres that weave through it. These protein fibres give the dermis its properties of strength, extensibility and elasticity. Also located within the reticular region are the roots of the hair, sweat glands, sebaceous glands, receptors, nails and blood vessels.