THE SKIN: THE BODY'S HEAVIEST ORGAN
CHARACTERISTICS OF THE SKIN
The skin is the heaviest organ in the human body, accounting for 16% of total body weight. The surface area of adult skin is 1.5 to 2 m2. The surface area of facial skin represents only 3.5% of the total skin surface. Skin thickness varies according to location, from 0.5 to 4 mm. The thinnest skin is found on the eyelids (0.5 mm), and the thickest on the heels (4 mm). Facial skin is 2 to 3 mm thick. Skin pH is acidic: between 4.2 and 6.1. The skin contains 5 million pilosebaceous follicles (hairs) and 3 million eccrine sweat glands (responsible for sweat production).
THE SKIN'S MAJOR ROLES, THANKS TO THE EPIDERMIS
The skin is highly complex, made up of 3 layers: epidermis, dermis and hypodermis. A dermal-epidermal junction exists between the dermis and epidermis, enabling them to attach. It is a mechanical support for adhesion, but also a selective barrier for controlling molecular and cellular exchanges between the two compartments. In particular, it enables keratinocytes to adhere and migrate during wound healing. Cutaneous functions are mainly due to the epidermis. The skin plays the following roles:
- The skin acts as a protective barrier, shielding the body from microorganisms.
- It prevents water loss from the body, thus protecting against skin dryness.
- The skin protects the body from the sun's UV rays, thanks to the production of melanin in melanocytes and its transfer to keratinocytes. This pigment is responsible for skin color.
- The skin plays various roles: sensation (thanks to Merkel cells), regulation of body temperature, excretion and absorption, synthesis of vitamin D, an energy reserve and thermal insulation (due to the accumulation of fats).
THE EPIDERMIS: THE SUPERFICIAL LAYER OF THE SKIN
THE 5 SUB-LAYERS OF THE EPIDERMIS
The epidermis is the outermost layer of the skin. Its average thickness is 100 µm. It is 50 µm for the eyelids and 1mm for the palm of the hand. The epidermis is non-vascularized: there are no blood vessels. It is, however, innervated: Merkel cells in the basal layer are linked to a nerve. The epidermis is made up of 5 different layers, each composed of cells stacked one on top of the other.
- The basal layer. This is the deepest layer, linked to the dermal-epidermal junction. It contains the keratinocyte stem cells. They proliferate through the various layers, reaching the stratum corneum and enabling epidermal renewal.
- The spinous layer. Here, keratinocytes have begun to differentiate. Each keratinocyte in this layer has desmosomes on its surface (like spines) to bind to the others.
- The granular layer. The keratinocytes in this layer synthesize in keratinosomes what the stratum corneum needs. Keratinosomes contain lipids (cholesterols, ceramides and fatty acids), proteases and proteins, which are released into the stratum corneum.
- The transition layer. This may or may not be present, depending on the thickness of the skin.
- The stratum corneum. This is the outermost, most rigid layer of the skin, protecting the underlying layers. This is where keratinocytes mature. They become corneocytes, i.e. dead, flattened, nucleus-less cells glued together. The stratum corneum is an important protective layer, resembling a brick wall of corneocytes with ceramides, cholesterols and fatty acids between them. These cells cannot synthesize ceramides themselves, as they have no nucleus, and rely on the granular layer.
EPIDERMAL CELLS: KERATIN AND MELANIN SYNTHESIS
There are many different cell types in our skin, the most common of which are keratinocytes and melanocytes. Let's take a look at their roles:
- Keratinocytes are the skin's outer covering, accounting for 80% of its cells. They enable epidermal renewal and cohesion. They act as a barrier between the internal and external environment. They also protect against drying out, by keeping water inside the body. They provide protection against the sun's UV rays, since they accumulate melanin, the pigments that give color to the skin when tanning. Keratinocytes also synthesize keratin, present in the epidermis as alpha keratin. This is a protein made up of amino acids (alanine, leucine, cysteine) linked together by peptide bonds.
- Melanocytes are the second most important cell type, accounting for 5-10% of epidermal cells. Melanocytes synthesize melanin and store it in vesicles called melanosomes. Melanin is the color pigment that protects against the sun's UVB rays and darkens during tanning.
- Merkel cells in the basal layer are linked to nerve endings, enabling the sensation of touch. Other nerve cells (touch, pressure, heat) are mainly found in the dermis.
- Langerhans cells are immune cells found mainly in the spinous layer. They protect against external attacks. They are sentinel cells: they are able to recognize antigens (external attacks) and present them to certain cell types (leukocytes and B, T and helper lymphocytes), which then produce antibodies (means of defense).
THE DERMIS: A LAYER RICH IN COLLAGEN AND ELASTIN
The thicker dermis lies beneath the epidermis. They are linked by a dermal-epidermal junction. The dermis is a dense, fibro-elastic connective tissue. It is 0.5 to 1mm thick on the face, 0.6mm thick on the eyelids, and 3 to 4mm thick on the hands. The dermis is composed of papillary and reticular dermis. It is vascularized by blood vessels. In the papillary dermis, we find venules, which carry blood rich in CO2, and arterioles, whose blood is rich in oxygen and nutrients.
DERMIS CELLS: COLLAGEN AND ELASTIN SYNTHESIS
The main cell type in the dermis is the fibroblast. Fibroblasts are firm, elongated cells. They synthesize collagen, elastin and proteoglycans. This layer also contains leukocytes and macrophages: cells of the immune system. The dermis has a structural function, due to its composition in collagen and elastin.
Elastin is a major protein in our body's elastic tissues, such as the skin. It is a fibrous, extensible protein whose function is to maintain skin elasticity.
Proteoglycans are proteins that play a hydrating role in the dermis, as they retain water. The dermis also contains hyaluronic acid, both in free form and in proteoglycans. Hyaluronic acid helps fill intercellular spaces, but also plays a role in hydration and tissue cohesion.
Collagen is a fibrillar, rigid, non-stretchable protein. It is made up of 3 main amino acids (glycine, proline and lysine) and 3 modified amino acids. Collagen gives structure, rigidity and strength to the dermis. The papillary dermis has type III collagen (rather loose) and the reticular dermis type I collagen (fibrous and fragile).
THE HYPODERMIS: AN ENERGY RESERVE AND THERMAL INSULATOR
The hypodermis is the deepest layer of the skin, varying in thickness from 1 to 6mm, particularly between men and women. It is a fatty tissue, vascularized by subcutaneous vessels. It accumulates fat, creating an energy reserve. It also acts as a thermal insulator.
The hypodermis is made up mainly of adipocytes, inside which vesicles contain triglycerides, an important source of energy. Adipocytes secrete adipokines: proteins that function like hormones. Adipokines include adiponectin (which reduces cardiovascular risk and inflammation) and leptin. Leptin is a satiety hormone. It promotes lipolysis (the release of triglycerides), increases energy expenditure, and reduces food intake.
Cutaneous vascularization is due to the presence of arteries and veins. The body's blood vessels are made up of endothelial cells and smooth muscle cells, which can contract and relax. The papillary dermis contains blood vessels in the form of venules and arterioles, while the hypodermis contains subcutaneous vessels.
The functions of cutaneous vascularization are :
- Regulation of body temperature, i.e. vasodilation to remove heat from the body, or vasoconstriction to reduce blood flow in capillaries.
- Nutrition, thanks to the nutrients and waste products circulating through the arteries.
- Hemostasis: slowing blood flow in capillaries.
- Immunity and maintenance of arterial pressure.