The Skin

Skin Anatomy

The skin makes up around 12% of an adult’s body weight. The skin has several important functions which include:

Skin is made up of 3 major layers known as the Epidermis, Dermis and the Subcutaneous.

The Epidermis

This is the outermost layer of the skin. There are various layers of cells within the epidermis, the outermost of which is called the stratum corneum (or horny layer). The layers can be seen clearly in the diagram of the skin. The surface layer is composed of twenty-five to thirty sub-layers of flattened scale-like cells, that are continually being exfoliated off by friction and replaced by the cells beneath.

The surface layer is considered the real protective layer of the skin. Cells are called keratinised cells because the living matter within the cell (protoplasm) has changed to form a protein (keratin) which helps to give the skin its protective properties.

New skin cells are formed in the deepest layer of the epidermis. This layer is known as the stratum basale. New cells being to gradually move from this layer towards the stratum corneum to be shed. As they move towards the surface, the cells undergo a process of change from a round, living cell to a flat, hardened cell.

The layers of the

epidermis from top to bottom are known as:

• Stratum Corneum/Horny Layer
• Stratum Lucidum/Clear Layer (only found in the palms on the
• hands and soles of the feet)
• Stratum Granulosum/Granular Layer
• Stratum Spinosum/Prickle Cell Layer
• Stratum Basale/Basal or Germinative Layer

The epidermis is the outermost layer of the skin and comprises of four cell types, keratinocytes, melanocytes, Langerhans cells and Merkel cells. The epidermis is also divided into layers comprising of living and non-living cells comprising of the stratum corneum, stratum granulosum, stratum spinosum and stratum basale.

The stratum corneum is made up of corneocytes and lipids and referred to as the epidermal barrier. It functions as an evaporative barrier that maintains the skin’s hydration and suppleness and protects the body from microbes, trauma, irritants and UV radiation by acting as a physical barrier. Corneocytes contain the skins natural moisturising factor (NMF), which maintains the hydration of the stratum corneum. Corneocytes are bound together to each other by corneodesmosomes. A lipid bilayer surrounds the corneocytes, which comprise two layers of phospholipids that have hydrophilic heads and two hydrophobic tails. The epidermis requires a constant cell turnover to maintain its integrity and to function effectively. Young, healthy skin renews every 28 days, which is the time it takes for the keratinocyte to migrate from the living basal layer of the epidermis to the stratum corneum’s surface and desquamate during the renewal process.

Melanin production

Melanin pigment, which determines the skin’s colour and causes hyperpigmentation, is primarily concentrated within the epidermis and, in some conditions, is found within the dermis (in cases of melasma). There are two types of melanin pigment, pheomelanin and eumelanin. Pheomelanin is yellow to red in colour and is found in lighter skin tones. Eumelanin is brown to black in colour and is the predominant type of melanin in darker skin types. Melanin synthesis (melanogenesis) occurs when melanocytes in the basal layer of the epidermis. The key regulatory step is the initial enzymatic conversion of tyrosine to melanin by tyrosine. Melanin is packaged into melanosomes, intracellular organelles within the melanocyte; these are then distributed to surrounding epidermal keratinocytes. Melanin has a protective physiologic role in the skin to protect the nuclei of the keratinocytes by absorbing harmful UV radiation: and eumelanin has the greatest UV absorption capabilities. When the skin is exposed to UV radiation, melanin synthesis is upregulated, which is observed by the darkening of the skin as we tan. The number of melanocytes for both light and dark skin tones are similar; however, the quantity and distribution of melanin within the epidermis differ. Lighter skin tones have less melanin per square centimetre and smaller melanosomes that are closely aggregated in membrane-bound clusters. Darker skin tones have more melanin and larger melanosomes that are distributed singularly.

Dermis Layer

The dermis is a tough and elastic layer containing white fibrous tissue interlaced with yellow elastic fibres.

The dermis is an expanse layer and contains:

• Blood vessels
• Lymphatic capillaries and vessels
• Sweat glands and their ducts
• Sebaceous glands
• Sensory nerve endings
• The erector pili – which involuntary activates tiny muscles attached to the hair follicle in cold weather to trap heat.
• Hair follicles, hair bulbs and hair roots

The dermis lies beneath the epidermis and divided into the more superficially dermis and deeper reticular dermis. The most predominant cell in the dermis is the fibroblast, which is abundant in the papillary dermis and sparse in the reticular layer. Fibroblasts synthesize most components of the dermal extracellular matrix (ECM), which includes structural proteins such as collagen and elastin, glycosaminoglycans such as hyaluronic acid, and adhesive proteins such as fibronectin and laminins.

Glycosaminoglycans

Glycosaminoglycans (GAGs), also known as mucopolysaccharides, are polysaccharides that deal with the support and maintenance of skin structural proteins such as collagen and elastin. Frequently occurring glycosaminoglycans include hyaluronan and chondroitin sulphate, which function as water-binding molecules that can hold nearly 1000 times their own weight. This ability may serve to provide moisture for other skin components (i.e., collagen and elastin). For this reason, the use of glycosaminoglycans in skincare are renowned for being excellent ingredients for increasing overall hydration. Lastly, glycosaminoglycans may also inadvertently supply anti-ageing benefits.

Examples of common glycosaminoglycans are chondroitin 6-sulfate, keratan sulphate, heparin, dermatan sulphate, and hyaluronate.

Glycosaminoglycans (GAGs) have widespread functions within the body. They play a crucial role in the cell signalling process, including regulation of cell growth, proliferation, promotion of cell adhesion, anticoagulation, and wound repair.

The GAG’s retain water and form a gel substance through which ions, hormones and nutrients can freely move.

The main component of this gel is hyaluronic acid, which is a large polysaccharide made of glucuronic acid and glucosamine that attract water and is increased in tissues under repair or growth.

Fibroblast

A fibroblast is a type of cell that is responsible for making the extracellular matrix and collagen. Together, this extracellular matrix and collagen form the structural framework of tissues in humans and plays an important role in tissue repair. Fibroblasts are the main connective tissue cells present in the body.

Elastin 

The same as collagen, elastin is present in many structures in the body, not just in the skin. Elastin makes up only around 3% of the skin, whereas collagen makes up 70% of the dry mass of skin. Degradation of elastic fibres is associated with UV exposure, and elastosis is one of the key features of photo-aged skin.

The fact that new elastin fibres are not produced is a challenge in the aesthetic industry.

Collagen 

Collagen is an abundant protein; it is the main component of connective tissue and is found not only in fibrous tissue like the skin but also in tendons, ligaments, cartilage, bones, corneas and blood vessels.

There are 18 collagen subtypes, 11 of which are in the dermis of the skin.

Types of collagen 

The basal lamina serves as structural support for tissues and as a permeable barrier to regulate movement of both cell and molecules.

The dermal-epidermal junction contains type IV collagen, laminin and highly specialised type VII collagen.

During wound healing, type III collagen appears in the wound about four days after the injury. Wound collagen or type III is immature collagen tissue and does not provide a great deal of tensile strength. It is initially deposited in the wound in a seemingly random fashion.

It will take approximately three months for type III collagen to mature into type I collagen.

As skin ages, reactive oxygen species, associated with many aspects of ageing, lead to increased production of the enzyme collagenase, which breaks down collagen. Then fibroblasts, the critical players in firm, healthy skin, lose their normal stretched state. They collapse, and more breakdown enzymes are produced. People in their 80s have four times more broken collagen than people in their 20s.

Immune functions of the skin 

Langerhan cells are ‘guard’ cells, found mainly in the Stratum Filamentosum (Spinosum) but start in the dermis.

They move across the skin and are stimulated to action by the entry of foreign materials, acting as macrophages to engulf bacteria.

If someone has a bad immune system, any micro wound treatment will not be as effective.

Subcutaneous Layer

This is the deepest layer of the skin and located beneath the dermis. It connects the dermis to the underlying organs. The subcutaneous layer is mainly composed of loose fibrous connective tissue and fat (adipose) cells interlaced with blood vessels. This layer is generally around 8% thicker in females than in males. The functions of this layer include insulation, storage of lipids, cushioning of the body and temperature regulation.

Skin Ageing

The visible signs of ageing are a combination of physiologic and environmental factors known as intrinsic and extrinsic factors. Over-exposure to ultraviolet (UV) radiation is one of the main factors responsible for skin damage, commonly referred to as sun damage, photoaging, actinic damage and UV-induced ageing. Other extrinsic factors that contribute to the ageing process include smoking, diet, sleep habits and the consumption of alcohol. Photoaging will present in the clinic with one or more of the following conditions:

Textural changes

• Wrinkles
• Dry or rough skin
• Solar elastosis
• Dilated pores
• Sagging and lax skin

Pigmentation 

• Hyperpigmentation such as lentigines, darkened freckles, mottled pigmentation
• Poikiloderma or civatte
• Hypopigmentation
• Sallow discolouration

Vascular changes 

• Telangiectasias
• Erythema

Degenerative changes 

• Benign such as seborrheic keratoses, sebaceous hyperplasia, cherry angiomas
• Preneoplastic and neoplastic, actinic keratoses, basal and squamous cell cancers and melanomas

Photoaged skin has slower, much more disorganised keratinocyte maturation and increased cellular adhesion relative to younger skins. These factors reduce the desquamation process and result in a rough and thickened stratum corneum that has an impaired barrier function. The stratum corneum also has a poor light reflectance which presents as sallow, dull skin. Water escapes more easily from the skin, causing dehydration. This disrupted barrier also allows an increase in penetration of irritants which can be associated with skin sensitivity and erythema. Sun-damaged skin has signs of pigmentary changes due to overactivity melanocytes and disorganised melanin deposition in the epidermis. Areas with excess melanin are evident as hyperpigmentation, and areas with melanin deficits are shown as hypopigmentation.

In the dermis, chronic UV exposure is very damaging to the ECM. Structural proteins such as collagen are degraded due to the upregulation of enzymes (e.g. matrix metalloproteinases) and weakened due to cross-linkage. This accelerated collagen degradation combined with reduced collagen synthesis that occurs over time contribute to the formation of fine lines and wrinkles. In some cases of advanced sun damage, solar elastosis occurs, which consists of tangles masses of damaged elastin proteins in the dermis, seen as deep wrinkling, sallow complexion and thickening of the skin. Abnormal dilation of dermal blood vessels is also common, leading to visible erythema and telangiectasias.