Skin: layers and their functions

The skin is considered the body's largest organ: it covers an area of approximately 1.5-2 square meters and performs dozens of functions, from protection and thermoregulation to participation in metabolism and the immune response. Anatomically, the skin consists of three main layers: the epidermis, the dermis, and the subcutaneous fat, also known as the hypodermis. Each layer has its own structure and cell composition, but they all work together as a unified system. [1]

The epidermis is the superficial, stratified squamous epithelium. It provides the primary physical barrier, preventing microbes and allergens from penetrating and limiting water loss. The epidermis forms the skin's texture and color and is involved in the healing of superficial injuries. It contains keratinocytes, melanocytes, Langerhans cells, and other specialized cells. [2]

Beneath the epidermis lies the dermis, a layer of dense connective tissue. The dermis gives the skin its strength and elasticity thanks to collagen and elastic fibers. This layer contains blood vessels, nerve endings, hair roots, sebaceous glands, and sweat glands. The dermis is divided into a more superficial papillary layer and a deep reticular layer, which merge smoothly into each other. [3]

The hypodermis, or subcutaneous fat, consists of fatty and loose connective tissue. It serves as a shock absorber, protecting bones and internal organs from impact, storing energy, and helping retain heat. The thickness of this layer varies greatly depending on the body region, age, gender, and overall fat mass. In many people, it is changes in the hypodermis with age that cause sagging tissue and loss of facial volume. [4]

It's important to understand that skin isn't simply "three vertical layers." Between them, there's a complex system of transitions, vascular and neural connections, a basement membrane, and buffer zones. The layers influence each other: the condition of the epidermis depends on the blood supply to the dermis and the support from the hypodermis, and changes in the deep structures gradually affect its appearance. [5]

Table 1. Main layers of the skin and their functions

Layer of skin	Main functions
Epidermis	Barrier from the external environment, prevention of water loss, formation of skin color and texture
Dermis	Strength and elasticity, blood vessels, nerves, skin appendages
Subcutaneous adipose tissue	Shock absorption, thermal insulation, energy storage

Epidermis: surface barrier and its microstructure

The epidermis consists of several layers of keratinocytes that gradually move from the basal layer to the surface, changing shape and internal contents along the way. The classic scheme distinguishes the basal, spinous, granular, and stratum corneum layers. In some areas, such as the palms and soles, a stratum lucidum is also distinguished. Each layer is responsible for specific stages of cell maturation. [6]

The basal layer contains stem and progenitor cells. These divide and ensure continuous renewal of the epidermis. Also located here are some melanocytes, which produce melanin and transfer it to keratinocytes, forming the individual skin color and protecting the cells from ultraviolet radiation. The basal layer connects the epidermis to the dermis via the basement membrane. [7]

In the spinous and granular layers, keratinocytes begin to synthesize keratin, the protein filaggrin, and lipids, which subsequently form the framework and intercellular "cement" matrix of the stratum corneum. Filaggrin and its fragments are particularly important for the strength and hydration of the stratum corneum, as well as for maintaining a slightly acidic environment on the skin's surface. Disruptions in the filaggrin structure are associated with a weakened barrier and a predisposition to atopic dermatitis. [8]

The stratum corneum consists of corneocytes—dead, nucleated cells filled with keratin and surrounded by a lipid matrix. This layer provides the primary physical protection and significantly limits the penetration of external substances. Ceramides, cholesterol, and free fatty acids form lamellar structures between the corneocytes, which play a key role in barrier function and the prevention of transepidermal water loss. [9]

A hydrolipid film and acid mantle, commonly discussed in cosmetology, form on the surface of the stratum corneum. Their condition depends on underlying processes: if lipid synthesis, enzyme function, and corneocyte maturation are disrupted, no cosmetic products can fully compensate for the barrier defect. At the same time, properly selected emollients can partially restore the structure of intercellular lipids and improve the protective properties of the stratum corneum. [10]

Table 2. Layers of the epidermis and their roles

Epidermal layer	Main processes
Basal	Cell division, initiation of differentiation, melanogenesis
Spiky	Formation of intercellular contacts, synthesis of keratin
Grainy	Accumulation of filaggrin and lipid granules
Horny	Formation of a strong barrier, limiting water loss

Dermis: strength, elasticity and the "filling" of the skin

The dermis is a layer of dense connective tissue containing collagen and elastic fibers, amorphous intercellular substance, blood vessels, nerves, immune cells, and skin appendages. It is divided into the papillary dermis and the reticular dermis, which have no distinct boundary. The papillary dermis is looser and richer in capillaries, while the reticular dermis is denser and contains the bulk of the collagen. [11]

The papillary dermis forms dermal papillae, which project into the epidermis and increase the contact area between the layers. These papillae contain a dense network of capillaries and nerve endings responsible for the sensation and nutrition of the superficial layers of the skin. With age and photodamage, these papillae flatten, which impairs the connection between the epidermis and dermis and makes the skin more vulnerable. [12]

The reticular dermis consists primarily of dense bundles of collagen and elastin, which form the skin's framework, providing resistance to stretching and mechanical stress. This layer contains hair roots, sebaceous and sweat glands, larger blood vessels, and nerve trunks. The space between the fibers is filled with a gel-like matrix rich in hyaluronic acid and other glycosaminoglycans, which retain water. [13]

Fibroblasts, the main cells of the dermis, synthesize collagen, elastin, and components of the extracellular matrix and are also involved in wound healing and tissue remodeling. Macrophages and mast cells form the immune component of the dermis, responding to damage and infection. Chronic inflammation, photoaging, and other processes disrupt the balance between matrix breakdown and synthesis, leading to thinning and sagging skin. [14]

Most of the sensations a person experiences in the skin are associated with receptors located in the dermis. This is where mechanoreceptors, thermoreceptors, and nociceptors are found, transmitting signals of pain, pressure, touch, heat, and cold. The dermis also contains a dense network of blood vessels that supply nutrition not only to the dermis but also to the lower layers of the epidermis.

Table 3. Structural elements of the dermis

Element	Role
Collagen fibers	Strength and tensile resistance
Elastic fibers	Elasticity and the ability to return to shape
Intercellular matrix	Water retention, metabolism
Fibroblasts	Matrix synthesis, participation in healing
Vessels and nerves	Nutrition, sensitivity, thermoregulation

Subcutaneous fat tissue: support and the “hidden architect” of contours

Subcutaneous adipose tissue, or hypodermis, consists primarily of fat cells arranged in lobules with layers of connective tissue containing blood vessels and nerves. This structure serves as mechanical protection, cushioning impacts and pressure, and acts as a "cushion" supporting the overlying layers of skin. Furthermore, the hypodermis helps connect the skin to the underlying muscles and fascia, maintaining a certain degree of mobility. [16]

Hypodermal adipose tissue is an important energy reserve. Adipocytes accumulate triglycerides and, when needed, release them as free fatty acids. Adipose tissue is also involved in the production of several hormones and signaling molecules, including leptin and some estrogens, which influence metabolism and inflammation. This explains why changes in the fat layer can affect not only the facial contour but also the overall metabolic state. [17]

One of the key functions of subcutaneous fat is thermal insulation. Due to the relatively low thermal conductivity of fat, this layer helps retain heat and protects internal organs from hypothermia. However, excess adipose tissue impairs heat transfer, increasing stress on blood vessels and joints, which can lead to systemic health problems. [18]

With age, the volume of adipose tissue in different areas of the body changes unevenly. On the face, some fat deposits shrink and shift, particularly in the cheeks and under the eyes, leading to the formation of hollows, folds, and changes in facial contour. In other areas, conversely, fat may accumulate more rapidly. These age-related changes in subcutaneous fat are closely linked to the perception of "facial aging." [19]

In severe skin injuries, such as deep burns or severe pressure ulcers, the hypodermis can also be affected. Loss of this layer deprives the tissue of important mechanical and metabolic support, impairs healing, and increases the risk of scarring. Therefore, reconstructive surgery places great emphasis on restoring not only the skin's surface but also its deeper layers. [20]

Table 4. Functions of subcutaneous fat tissue

Function	Meaning
Depreciation	Protecting bones and internal organs from impacts
Thermal insulation	Heat retention, protection from hypothermia
Energy reserve	Triglyceride reserve
Endocrine activity	Production of hormone-like substances
Skin support	Formation of contours and volume

How skin layers work together and change with age

The epidermis, dermis, and hypodermis do not function in isolation. When the epidermis is damaged, signaling pathways in the dermis are activated, fibroblasts begin to synthesize additional matrix components, and blood vessels dilate and attract immune cells. Subcutaneous fat cushions mechanical stress and participates in the delivery of energy substrates to repair tissues. As a result, a coordinated healing process is initiated. [21]

The condition of the stratum corneum barrier directly affects how the deep structures respond to external influences. When the lipid matrix is disrupted and ceramide levels decrease, irritants and allergens penetrate the epidermis more easily, inflammation increases, and immune cells in the dermis are activated. This can manifest as dryness, itching, redness, and exacerbations of chronic dermatoses. [22]

With age, changes occur in all layers. The epidermis becomes less smooth, keratinocyte renewal slows, and the stratum corneum can become drier and denser. In the dermis, the number of fibroblasts decreases, collagen and elastin density decreases, and the papillary layer thins. Simultaneously, the boundary between the epidermis and dermis smooths, which impairs the skin's mechanical stability. [23]

In many people, the hypodermis thins partially with age in the face and back of the hands, while it can increase in other areas. This leads to a combination of morphological changes: the skin becomes less elastic due to changes in the dermis, and the reduction of fat pads reveals folds and enhances the appearance of flabbiness. This complex aging cannot be explained solely by "poor skin tone"—it is associated with the restructuring of all layers. [24]

Systemic factors such as ultraviolet radiation, smoking, sleep deprivation, poor diet, and chronic diseases accelerate age-related changes. Ultraviolet radiation damages DNA and collagen, disrupting the function of epidermal and dermal cells. Smoking impairs microcirculation and increases collagen breakdown. All of this leads to premature deterioration of skin quality, which always affects all three layers. [25]

Table 5. Age-related changes in skin layers

Layer	Major age-related changes
Epidermis	Slow renewal, dryness, uneven stratum corneum
Dermis	Decreased collagen and elastin, thinning, less firmness
Hypodermis	Changes in fat volume and distribution, loss of support
Layer connection	Flattening of the dermoepidermal junction, less resistance to stress

A practical look: skincare tailored to your skin type

Understanding the structure of your skin helps you make informed skincare choices. Gentle cleansing and regular use of products that replenish stratum corneum lipids and natural moisturizing factor components are key to maintaining the epidermal barrier. Clinical studies show that creams with a combination of ceramides, cholesterol, and fatty acids, similar to natural ingredients, can improve barrier function and reduce skin sensitivity. [26]

The dermis is important for two purposes: protection from damage and stimulation of healthy remodeling. UV radiation is considered one of the main enemies of collagen, so regular photoprotection reduces the breakdown of fibers and slows the appearance of wrinkles. Certain active ingredients, such as retinoids and certain peptides, when used correctly, can stimulate collagen synthesis and improve dermal density, but require careful administration, especially for sensitive skin. [27]

Subcutaneous fat is less amenable to localized cosmetic treatments, but is highly dependent on overall lifestyle. Fluctuations in body weight, prolonged nutritional deficiencies, excessive sun exposure, and smoking affect the condition of fat deposits and the structure of connective tissue. For some age-related changes in the face, invasive techniques such as fillers or lipofilling are used to partially compensate for the loss of subcutaneous volume, but this is the domain of aesthetic medicine. [28]

Intensive treatments require an understanding of which layer they target. Chemical peels and ablative lasers primarily target the epidermis and superficial dermis, stimulating renewal and remodeling. Non-ablative lasers and radiofrequency techniques target deeper, the dermis, to enhance collagen synthesis. Deeper effects on the hypodermis are achieved through plastic surgery or lipomodeling. [29]

With any intervention, it's important to remember that excessive aggression toward one layer can negatively impact others. Too frequent, traumatic procedures weaken the barrier, provoke chronic inflammation, and can accelerate aging. Therefore, proper care always takes into account the skin's anatomy, combining delicate support of the epidermis, protection of the dermis from damage, and gentle care of the subcutaneous fat. [30]

Table 6. Care goals for different skin layers

Layer	Main tasks of care	Examples of approaches
Epidermis	Maintaining barrier and moisture	Gentle cleansing, emollients, acid mantle maintenance
Dermis	Collagen and elastin protection, healthy remodeling	Photoprotection, retinoids, controlled procedures
Hypodermis	Maintaining healthy volume and structure	Stable body weight, general healthy habits, invasive methods if necessary

A short Q&A session

Question: Is it possible to "nourish" the dermis with creams if they are applied only to the surface?
Most skin care ingredients work primarily within the epidermis and at the dermis-stratum corneum interface. Deep penetration of large molecules is limited by the barrier function of the stratum corneum. However, by improving the barrier, reducing inflammation, and protecting against UV rays, such products also indirectly affect the condition of the dermis. For a more pronounced effect on the dermis, professional treatments are used. [31]

Question: If the stratum corneum consists of "dead" cells, why is it so important?
Corneocytes do not contain a nucleus, but the stratum corneum is a complex structural system that regulates the penetration of substances, water loss, and protection from microbes. Even minor damage to this "brick-and-mortar" barrier increases skin sensitivity and the risk of dermatoses. The goal of skin care is to maintain its integrity and not "scrape" it away with aggressive methods. [32]

Question: Do "hypoderm" or "fat layer" creams make sense?
Currently, there is no convincing evidence that topical cosmetics can selectively and deeply affect subcutaneous fat. Changes in the hypodermis largely depend on the overall health of the body, hormonal levels, and body weight. Topical creams can improve the condition of the superficial layers and the visual appearance of the skin, but they are not a tool for targeted treatment of fatty tissue. [33]

Question: Is it possible to completely "restore" the skin's structure to its youthful appearance?
Some changes are reversible: it's possible to improve the barrier, reduce dryness, strengthen the collagen framework, and correct volume through aesthetic procedures. However, it's impossible to completely restore the original architecture of all layers. A realistic goal is to slow down further changes as much as possible, improve skin quality, and make age-related changes softer and more harmonious. [34]