A critical part of sorting your skin out is building a good relationship with your skin. The first step in doing so, is understanding its structure, major components and function. The skin is your largest organ (part of the integumentary system), which primarily acts as a barrier between your internal and external environments, something we take for granted. In this article, we will learn about every single layer of the skin, from the epidermis (the outermost layer of the skin) to the deep fascia (deepest part of the skin).
What is the skin structure in humans?
The skin is a complex organ which can be divided into three main layers:
- Epidermis (outermost layer).
- Dermis (main bulk of the skin).
- Hypodermis (connecting the skin to tissue).
The skin also contains ‘skin appendages’ which help maintain the skin in good condition. These appendages play a major role in homeostasis of the skin.
The skin is abundant with specialised cells such as macrophages, dendritic cells and melanocytes. These specialised skin cells all play a part in the skin function.
Epidermis Skin Structure
- Thickness: 180 μm.
- Location: outermost layer of the skin, above the dermis.
- Main Function: physical barrier to the external environment.
The epidermis is the outermost layer of the skin structure. This layer of the skin is non-viable (not alive). As the blood supply from the dermis does not reach here. The epidermis is in a constant state of renewal, with the formation of a new cell layer of keratinocytes at the stratum basale, and the loss of their nucleus and other organelles to form desiccated, proteinaceous corneocytes on their journey toward desquamation. Cells produced in the stratum basale gradually push existing cells up, moving them further away from the viable (live) layer, which leads to degradation and they eventually shed-off the surface of the skin. The cells in this layer of the skin are held together by desmosomes which interconnect the keratin of adjacent cells.
Stratum Corneum (Horny Layer)
- Thickness: 20-30 cell layers.
The stratum corneum (SC) is what you see when you look at your skin. It is the primary barrier of the skin responsible for regulating trans-epidermal water loss (TEWL), keeping pathogens out, and molecular exchange.
This layer is made up of cells called “corneocytes”, they are approximately 50 μm in length but only 1.5 μm thick, making them very flat cells. Corneocytes lack a nucleus and are made up of 70-80% keratin, making them similar to scales we associate with lizards! Corneocytes have a cornified cell envelope with a protein/lipid polymer structure which allows them to anchor intercellular lipids to the protein structures in the cornified envelope, which provides structure and barrier function to the stratum corneum.
These cells are suspended in an intercellular matrix made up of ceramides, free fatty acids, cholesterol, cholesterol esters and cholesterol sulfates, with a notable absence of phospholipids. Ceramides are bonded sphingoid bases and fatty acids, they are categorised based on their polarity. Fatty acids are typically long-chained saturated acids.Cholesterol and cholesterol esters reduce fluidity of intercellular lipid lamellae (increase stratum corneum rigidity). Natural Moisturising Factors (NMFs) act as humectants and plasticisers.
Stratum Lucidum
- Thickness: 2-3 cell layers.
This layer of the skin is only present on thick parts of the skin (for example: palms of your hands and soles of your feet). The stratum lucidum is so far from the viable (live) layer of the skin, that it loses cell nuclei and other organelles. Keratinisation increases (formation of keratin molecules) and the cells are compacted between the layers of skin, this creates flattened cell structure, similar to those found in the stratum corneum.
The palms of our hands and soles of our feet need these extra couple layers of skin to protect them from insults typically associated with limb extremities (holding, pushing, walking bare-foot).
Stratum Granulosum
- Thickness: 3-5 cell layers.
In the stratum granulosum, intracellular keratohyalin granules and membrane-coating granules contain lamellar subunits arranged in parallel stacks. Researchers believe these granules are the precursors of the intercellular lipid lamellae of the stratum corneum. This layer contains hydrolytic enzymes including stratum corneum chymotrypic enzymes (SCCE) – a serine protease associated with desquamation.
As the cells approach the upper layers of the stratum granulosum, the lamella granules are extruded into the intercellular space.
Stratum Spinosum
- Thickness: 8-10 cell layers.
In the stratum spinosum, skin structures change from columnar to polygonal. The cells in this layer still have all their organelles (including their nuclei), as they are close enough to the viable (live) layer of skin. These cells have an enlarged cytoplasm containing many organelles and filaments. The cells are not fully keratinised (unlike those in the stratum corneum), but they do contain keratin tonofilaments.
Stratum Basale (Germinativum)
- Thickness: 1 cell layer.
The stratum basal (germinativum) is only one cell thick, These cells are called keratinocytes. These are the only cells which undergo cell division in the epidermis. All of the above mentioned cells originated through cell division in this layer, and eventually degrade as they move further away from this viable (life) layer of the skin. The stratum basale is the only layer in the epidermis which contains skin appendages, namely; merle cells, Langerhans cells and melanocytes.
The main function of the stratum basale is cell division. The stratum basale produces new cells ever day. The life cycle from synthesis in the stratum basale to desquamation (shedding) from the stratum corneum is approximately 28 days. This means your skin totally re-news itself every 28 days!
Epidermis Skin Cell Types
Corneocytes
Corneocytes are derivatives of keratinocytes. These cells are non-viable with no organelles. They are keratinised, flat and rigid. Corneocytes are found most notably in the stratum corneum, but undergo the process of cornification all the way from the stratum spinosum.
Keratinocytes
Keratinocytes are found in the stratum basale (germinativum) layer alone. They are the only cells in the epidermis which actively undergo cell division. These cells contain all their organelles and are one of the only cells which undergo molecular exchange with the deeper layers of the skin.
Langerhans Cells
Langerhans cells are dendritic cells (major antigen presenting cells in the skin). These cells are generated in the bone marrow. When activated by antigens, they migrate from the epidermis to the dermis and onto the regional lymph nodes where they initiate an immune response. They play a key role in allergic dermatitis.
Melanocytes
Melanocytes produce melanin, high molecular weight polymers that provide pigmentation to the skin, hair and eyes. Their function is to protect the skin against UV radiation. They absorb potentially harmful UV radiation and prevent these rays from damaging the sensitive structure below the epidermis. Melanocytes produce two forms of melanin: eumelanin (black-brown) and pheomelanin (yellow-red).
Merkel Cells
Merkel cells are associated with nerve endings. They are pressure sensitive and able to sense changes in our environment. These cells are concentrated in touch sensitive areas such as the fingertips and lips.
Basal Lamina
The stratum basale (germinativum) is attached to the basal lamina (basement layer) which separates the epidermis from the dermis. The cells are attached to the basement layer via hemidesmosomes.
Papillary Dermis Skin Structure
- Thickness: 300–400 μm.
- Location: beneath the epidermis, above the reticular dermis.
- Main Function: provide sufficient nutrients to the epidermal layer.
The papillary layer is the top layer of your dermis. It’s significantly thinner than the reticular dermis. It consists of collagen fibres, fibroblast cells, fat cells, blood vessels (capillary loops), nerve fibres, touch receptors (Meissner corpuscles) and cells that fight bacteria (phagocytes). The papillary dermis extends to the basement layer of the epidermis layer. They form a strong bond that connects like interlocking fingers.
The papillary layer forms irregular projections known as the dermal papillae. They interlace with the epidermal ridges and contain small vessels organised in subpapillary vascular plexus. The papillae are less abundant and smaller in thin skin that has minimal mechanical stress, when compared to in areas of thicker skin (i.e., palms and soles) where they tend to form curved parallel lines (finger tips).
Papillary Dermis Skin Cell Types
Papillary Fibroblasts
Dermal fibroblasts control the structure and composition of the dermal extracellular matrix, including collagen, elastin, proteoglycans, and structural glycoproteins. The stroma formed by fibroblasts serves as the framework that physically supports the epithelium and also regulates the structure/functions of the epithelial cells and epidermal morphogenesis.
Mast Cells
Mast cells are able to mount a rapid immunological response by releasing pre-stored inflammatory mediators, they are a part of the front line of defence against pathogens invading the body. These cells have been reported to be very important as regulators of epidermal barrier function and skin homeostasis and play important roles in wound healing and skin ageing.
Macrophages
Macrophages are present during all phases of wound repair, and they outnumber all other inflammatory cells. During the initial phase, a temporary wound matrix is formed to achieve homeostasis followed by tissue formation and tissue maturation & remodelling.
Thin Collagen Fibres (Type I, II & III)
Collagen is abundant in the papillary dermis, but serves a slight different function to the collagen in the reticular dermis. The most abundant type of collagen in the papillary dermis is Type III collagen which connects the basement layer of the epidermis to the reticular (main body) dermis.
Sensory Nerve Fibres
Sensory nerve fibres are abundant in the papillary dermis. They connect to sensory receptors such as mechanoreceptors (pressure or distortion), nociceptors (pain), and thermoreceptors (temperature). These messages are then transmitted to the central nervous system and physiological actions are likely to result if significant changes are detected in order to reach homeostasis.
Dendritic cells
Dendritic cells (also known as T cells) are professional antigen presenting cells that inform the fight against invasive pathogens while enforcing tolerance to self and harmless environmental antigens. They capture pathogens and receive signals from pathogens that influence the outcome of immune responses.
Reticular Dermis Skin Structure
- Thickness: 600 μm.
- Location: in between the papillary dermis and hypodermic.
- Main Function: provide strength, rigidity and elasticity to the skin.
The reticular layer is the bottom layer of your dermis. It’s thick, and it contains blood vessels, glands, hair follicles, lymphatics, nerves and fat cells. A net-like structure of elastin fibres and collagen fibres surrounds the reticular dermis. These fibres support your overall skin structure, as well as allow it to move and stretch. This is the ‘bulk’ of the skin which houses skin appendages which perform all the skin functions.
Dermis Skin Appendages
Pilosebaceous Unit
Hair follicles are present at a fractional area of about 1/1000 of the skin surface, except on the lips, palms of the hands, and soles of the feet. The sebaceous gland associated with each hair follicle secretes sebum, which is composed of free fatty acids, triglycerides, and waxes. Sebum protects and lubricates the skin, and maintains the skin surface at a pH of about 5. The arrector pili muscle attaches the follicle to the dermis and allows the hair to respond to cold and fear.
Eccrine Sweat gland
Humans have approximately 2–4 million eccrine sweat glands in total and are found on both glabrous (palms, soles) and non-glabrous (hairy) skin. Gland density is not uniform across the body surface area. The highest gland densities are on the palms and soles and respond to emotional as well as thermal stimuli.
Apocrine Sweat Glands
Apocrine sweat glands are located primarily in the axilla, breasts, face, scalp, and the perineum.These glands differ from eccrine glands in that they are larger and open into hair follicles instead of onto the skin surface. In addition, although present from birth, the secretory function of apocrine glands does not begin until puberty. Furthermore, apocrine glands produce viscous, lipid-rich sweat, which also contains proteins, sugars, and ammonia. Moreover, many species generally regard apocrine glands as scent glands involved in the production of pheromones (body odor), although humans have a rudimentary social/sexual function for these glands.
Apoeccrine Sweat Gland
Apoeccrine glands develop from eccrine sweat glands between the ages of approximately 8 to 14 years and increase to as high as 45% of the total axillary glands by age 16–18. They are intermediate in size, but as the name suggests, apoeccrine glands share properties with both eccrine and apocrine glands. Like apoeccrine glands, apoeccrine glands are limited in distribution, as they are contained to only the axillary region. The function of this secretion is unknown, but unlikely to play a significant role in thermoregulation since evaporation is inefficient in the axilla region.
Capillaries
A large vascular network located within the dermis regulates body temperature, supplies nutrition and oxygen to tissues, eliminates toxins and waste products from tissues, and promotes immune response and wound healing. The skin has arteriovenous anastomoses in addition to tiny capillaries. They enable the fast blood flow necessary for temperature adjustment by allowing direct shunting of up to 60% of cutaneous blood flow between arteries and veins.
Hypodermis (Superficial Fascia) Skin Structure
- Thickness: 1,000 μm.
- Location: beneath the dermis, bottom-most layer of the ‘skin.
- Main Function: storage and supply of energy.
Superficial fascia is found directly under the skin and superficial adipose layers. It can show stratification both grossly and microscopically. Traditionally, people describe it as consisting of membranous layers with loosely packed, interwoven collagen and elastic fibers. This layer of the skin contains vital skin appendages and connects the skin to the rest of the body.
Hypodermis Skin Appendages
Artery
Arteries carry oxygenated blood to organs, including the skin. Their blood carries nutrients vital for cell division. These are delivered via capillaries which reach from the hypodermis up to the papillary dermis.
Vein
Veins carry deoxygenated blood and waste produced by cells from organs, which are later re-oxygenated or disposed of.
lymphatic vessels
These vessels help regulate fluid levels in the body, receive waste products from tissues, and transport a fluid called lymph that defends the body against infection.
Deep Fascia Skin Structure
- Thickness: 2,000 – 10,00 μm.
- Location: beneath the skin, connects to organs and muscles.
- Main Function: support and protect muscles and other soft tissue skin structures.
Deep fascia surrounds bones, muscles, nerves, and blood vessels. It is commonly has a more fibrous consistency and rich in hyaluronan as compared to the other subtypes. Deep fascia tends to be highly vascularized and contain well developed lymphatic channels. In some instances, deep fascia can even contain free encapsulated nerve endings, such as Ruffini and Pacinian corpuscles. There are two types of deep fascia.
Aponeurotic fascia
It forms into sheets of pearly-white fibrous tissue to attach muscles needing a wide area of attachment. Aponeurosis can thin into a tendon and become a point of origin or insertion for other muscles. Some examples of aponeurotic fascia include the fascia of limbs, thoracolumbar fascia, and rectus sheath. The thicker of the two subtypes normally easily separates from the underlying muscle layer. It comprises two to three parallel collagen fibre bundles.
Epimysial fascia
Also known as the epimysium, this is the connective tissue sheath surrounding skeletal muscle and can, in some cases, connect directly to the periosteum of bones. Some major muscle groups enveloped in epimysium include muscles of the trunk, pectoralis major, trapezius, deltoid, and gluteus maximus. The thinner of the two subtypes, on average, connects more tightly to the muscle via septa that penetrate the muscle layer.
