Skin is the largest organ in our body, making up roughly 15% of our entire body weight. Its primary function is to act as a barrier and protect our bodies from the surrounding environment. In addition, our skin regulates our body temperature, receives sensory stimulation and facilitates the excretion of waste products.
These notes will outline the Anatomy of the Skin and its function.
Anatomy of the Skin Part 1: Skin Structure
The skin make-up is divided into three primary layers:
1. The Epidermis is the outermost layer, and it has no blood supply and creates a waterproof barrier to protect our body. It also plays a significant role in the colour of our skin tone. It is further subdivided into five layers: Stratum Basale, Stratum Spinosum, Stratum Granulosum, Stratum Lucidum and Stratum Corneum.
90% of the cells in the Epidermis are Keratinocytes. They are first produced at the bottom part of the Epidermis, and as they age, they start to differentiate (change their shape) and migrate towards the surface layer. As they differentiate, the keratinocytes lose their nuclei and cytoplasmic organelles, and they are left with an insoluble cornified pocket that contains the plasma membrane and lipid cells.
When the keratinocytes reach this cornified state, they are called Corneocytes, their last living phase. Corneocytes are regularly replaced through a process called Desquamation, and they are continuously renewed from lower epidermal layers.
2. The Dermis is the middle layer, containing nerve endings, connective tissue, hair follicles, blood vessels, lymphatic vessels and apocrine glands (aka. sweat glands). It is also subdivided into two layers, the top layer known as the Papillary Dermis and the bottom layer known as the Reticular Dermis.
3. The Hypodermis is the deepest layer and is also known as the subcutaneous tissue, made up of fat and connective tissue.
Anatomy of the Skin Part 2: The Epidermal Layers
As we mentioned above, the Epidermis (the outermost layer of the skin) comprises five layers.
1. Stratum Basale/ Germinativum – Basal or Germinal Layer
The Basal Layer is the deepest layer of the Epidermis and contains several keratinocytes, some proliferating (replicating) and others non-proliferating. Despite their state, all keratinocytes are attached to the basement membrane by hemidesmosomes which are multiprotein complexes that provide stability for the cells.
This layer also contains Melanocytes, connected to the keratinocytes and other strata through dendrites. And lastly, the basal layer also has Merkel cells which are highly innervated cells that help in the light touch sensation. Given their function, these cells are most commonly found in areas of the body that are sensitive to touch, such as the fingertips and lips.
2. Stratum Spinosum – Spinous layer
After the keratinocytes are produced in the basal layer, they are pushed into the Spinous layer's second layer of the skin. Here, the keratinocytes change their attachment to desmosomes and start to develop their internal structures known as Golgi (the squiggly membrane inside a cell). The Golgi contain lamellar bodies (almost like little fat pockets) enriched with polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes.
In addition, the middle of this layer contains Langerhans, immunologically active cells.
3. Stratum Granulosum – Granular Layer
After fully maturing their internal structures in the Spinous layer, the keratinocytes are pushed into the Granular layer, where they lose their nuclei and develop a granular-looking cytoplasm. As the breakdown of these internal structures occurs, the lipids inside the lamellar bodies are also released, and they make their way into the extracellular space through a process known as exocytosis. Once in the extracellular space, they form an impenetrable lipid barrier that blocks off the water and protects the skin.
4. Stratum Lucidum – Translucent Layer
The translucent layer is only found in the soles of our feet and the palm of our hands because those areas require more 'cushioning' in the skin. This layer comprises three to five layers of dead, flattened keratinocytes that do not have distinct boundaries and are filled with Eleidin, a clear intracellular protein similar to Keratin.
5. Stratum Corneum – The Cornified Layer
The Cornified layer is the final layer of the skin, and it is the one that comes in contact with the surrounding environment. It is composed of 10 to 30 layers of polyhedral and anucleated corneocytes, with the palms and soles having the most layers. Corneocytes are surrounded by a protein envelope filled with water retaining keratin protein, attached through Corneodesmosomes and wrapped in the extracellular space by stacked layers of lipids.
Anatomy of the Skin Part 3: The Epidermal Function
Altogether the five layers mentioned above make up the Epidermis, and when they work together, their primary function is to serve as a barrier in five different ways:
1. Physical Barrier: The Epidermis provides mechanical resistance from its surrounding environment through the attachments that keratinocytes have together via cell-cell junctions and the presence of cytoskeletal proteins.
2. Chemical Barrier: It protects our body from harmful chemical compounds through the presence of highly organised lipids, acids, hydrolytic enzymes and antimicrobial peptides.
3. Immunologically Active Barrier: The epidermal layer has humoral and cellular constituents of the immune system to protect our body from microbial pathogens. The Langerhans cells, part of the skin immunity, are often depleted by advancing age, UV radiation and topical or oral steroids.
4. Hostile environment for pathogens: The Epidermis has an acidic pH of 5.0, non-pathogenic microorganisms on the surface, and pushes water contents toward the surface. All of which makes it very difficult for pathogenic organisms to survive and grow.
5. Water Barrier: It also regulates the amount of water released from the body into the atmosphere through Trans epidermal Water Loss (TEWL).
Anatomy of the Skin Part 4: The Function of the Skin
Apart from the specific barrier functions of the Epidermis, the three skin layers have additional purposes. Our skin allows us to sense our surroundings such as pain, temperature and pressure. It triggers Vitamin D production, a very important factor in our endocrine system as it facilitates the absorption of Calcium and normal bone metabolism. Skin is also involved in our exocrine system as it by releasing excess water, urea and ammonia and secreting sebum, sweat and pheromones.
Lastly, our skin regulates our body temperature by conserving or releasing heat a concept known as thermoregulation and it balances our body's water volume.
Anatomy of the Skin Part 5: The Blood Supply and Lymphatic Supply
If you scroll up we mentioned that the Dermis is split into two layers, the reticular layer and the papillary layer. The area in between the layers is supplied with Plexuses which are a bunch of blood vessels, lymphatic vessels and nerves. The blood that arrives at these plexuses is coming from the large blood vessels and capillaries of the systemic circulation.
The lymphatic system has a similar approach, the lymph vessels run along the skin's blood vessels and eventually return the lymph fluid back to the main system.
Anatomy of the Skin Part 6: The Skin Microbiome
The skin flora consists of many bacteria and fungi living on the skin's surface that contributes to the skin's physiology. In most cases, these florae are non-pathogenic, meaning they do not cause harm or any diseases. They are often split into two groups:
Commensal, meaning they are not harmful to their host, and
Mutualistic, meaning they offer a benefit to the host.
Both the commensal and the mutualistic flora prevent transient pathogenic organisms from colonising the skin surface. They do this by competing for nutrients, secreting chemicals against them and stimulating the skin's immune system to fight them off.
The type of skin flora will vary from one individual to another and from one part of the body to another. Typically, most individuals have several non-pathogenic skin bacterial flora, including:
Staphylococcus species
Corynebacterium
Brevibacterium
Acinetobacter
When an infection occurs, it would likely have been caused by pathogenic bacteria such as:
Staphylococcus Aureus which can cause folliculitis and impetigo
Streptococcus Pyogenes can cause Erysipelas, cellulitis and impetigo