SKIN STRUCTURE

BASIC SKIN STRUCTURES

The skin is an ever-changing organ containing many specialized cells and structures. If any of the structures in the skin are not working correctly then a rash or inflammation is the result.

The cell membrane surrounds the cell. With a porous surface characteristic the cell membrane allows food and nutrients to enter and waste products to leave.

Chromosomes are located in the nucleus in the centre of the cell. They contain the genes which we have inherited from our parents. Genes are accountable for cell function and cell production.

Cytoplasm is the liquid within the cell membrane consisting of many random organelles, with each having a specific role within the cell.

Cells tend to carry out specific functions. When groups of cells sharing the same function, shape or size are evident they are then defined as tissues. When tissues are grouped together they create a structural unit also known as an organ.

FUNCTIONS

Protection – The skin functions as a protective barrier from a sometimes hostile environment.

The outer surface of the skin is bactericidal, which is important for the skin to avoid damaging micro-organisms. The cells on the outer surface form a chemical and physical wall restricting the absorption of numerous substances.

Melanin is a pigment in the skin that absorbs menacing rays of ultra-violet light.

The skin’s natural oil and sebum stops the skin from losing vital water, functioning as a waterproof coating, which defends the skin from dehydration.

The skin also shields underlying structures from physical damage.

Body Temperature Regulation –The skin maintains the proper body temperature through heat loss and sweat so that the body can function well.

Excretion – The surface of the skin transports small amounts of excess products such as water and salt away from the body.

Sensitivity – Skin allows us the ability to feel, touch, pressure, pain, heat and cold. Skin gathers sensory information from the environment and has an important role in the immune system protecting the body from potential harm

Nutrition – The skin produces a significant amount of our Vitamin D and it is also used as storage for fat, which supplies an energy reserve.

Moisture Control – Within the deeper layers of the skin is where the movement of moisture is controlled.

There are three layers within the skin: the Epidermis, the Dermis and Subcutaneous layer. The basement membrane is situated between the Dermis and Epidermis acting as a ‘glue’ securing the layers together. If the Dermis separates away from the epidermis, then body fluids fill the area forming a blister.

The subcutaneous layer or fat layer is positioned directly beneath the Dermis and Epidermis composed of cells containing fatty deposits. The thickness of the subcutaneous layer varies throughout the body and from person to person. For example it is very thin around the eyes.

The subcutaneous layer or fatty layer:

Has a protective purpos
Operates as an energy source due to excess fat being stored
Reinforces muscles and bones to escape injury
Is important in the regulation of body temperature.

THE EPIDERMIS

Based directly above the Dermis, it is the outer layer of skin and is made up of 5 layers:

Stratum Basale
Stratum Spinosum
Stratum Granulosum
Stratum Licidum
Stratum Corneum

The surface layer forms the outer skin which is what we are able to see and touch. The Epidermis’ main role is to protect everything beneath it from the hostile external substances that surround us.

The main type of cell found in the Epidermis is Keratinocyte which produces the protein Keratin. Keratin causes the skin to be tough and reduces the progression of substances to and from the body.

During a time frame of approximately four weeks, cells make their journey from the bottom layer of the Epidermis to the skin’s surface (the top layer), changing shape and structure.

Cellular change occurs due to cell division forming the cell. As the cell matures its structure alters and moves upwards and outwards and once it dies it becomes an empty shell which is eventually shed.

THE GERMINATIVE ZONE

In this zone the cells of the Epidermis are living cells.

STRATUM GERMINATIVUM

Also known as the Basal layer, it is the bottom layer of the Epidermis separated from the underlying Dermis by a basement membrane. It is formed from a layer of cells shaped like columns with large nuclei. These cells divide constantly replacing cells in the higher layers. Mitosis is the term that describes this process of cell division and it takes around 14 days for cells to move through this layer. As the cells move into the higher layers, they flatten and eventually die.

STRATUM SPINOSUM

Otherwise known as the Prickly layer is composed of elongated cells, which have a surface of spiky spines which operate as a mechanism of connecting to nearby cells.

SPECIALIZED EPIDERMAL CELLS

These are two important cells also based in the Germinative Zone of the Epidermis:

LANGERHAN CELLS

Absorb and dispose of foreign bodies that invade the skin. The Langerhan cell is the frontline defence of the immune system in the skin.

MELANOCYTE CELLS

Produce the skin pigment melanin which contributes to our skin colour. Melanocytes are triggered to produce Melanin by ultra-violet rays. Their role is to protect the other epidermal cells from the damaging effects of ultra-violet.

The quantity and distribution of Melanocytes varies depending on a person’s race. In a white Caucasian person the Melanin tends to be destroyed by the time it reaches the Granular layer. With stimulation from artificial or natural ultra-violet light, Melanin is also visible in the upper Epidermis.

In black skin Melanin exists in larger quantities throughout all the epidermal layers. At this level of epidermal protection black skin is able to handle bright ultra-violet light. This added protection allows less ultra-violet light to penetrate the Dermis, lowering the prospect of premature ageing from exposure to ultra-violet light. The more even quality and spread of Melanin across the body also means that people with darker skin are less likely to be at risk of developing skin cancer.

Skin colour also increases when the skin is warm because blood capillaries dilate at the surface bringing blood closer to the surface so that heat can be allowed to leave the body.

STRATUM GRANULOSUM

Also described as the Granular layer, it is the third layer composed of between one and three layers of cells which have become flattened. The nucleus of the cell has begun to dismantle, creating what appears to be diamond shaped granules within the cell cytoplasm. These granules are known as Keratohyaline granules that later form Keratin. At this stage cells form a new combined layer.

THE KERATINISATION ZONE

This is also known as the Cornified Zone. This is where cells begin to die and are finally shed from the skin. At this stage the cells gradually flatten and the cell cytoplasm is substituted by the hard protein keratin.

STRATUM LUCIDUM

Can also be called the Clear layer or Lucid layer, is most pronounced in the palms of hands and the soles of feet. The cells in these places have lost their nuclei and instead, contain a clear substance called Eledin.

STRATUM CORNEUM

The final layer is known as the Cornified layer it is formed up of non-nucleated, disintegrated, overlapping and flattened cells consisting mainly of Keratin. They are dead skin cells that are shed every three weeks by a process called desquamation. They form a protective barrier for the skin by reflecting ultra-violet light away from the skins surface. Black skin has a thicker Stratum Corneum than white Caucasian skin and it withstands stronger ultra-violet light better.

THE DERMIS

Found underneath the Epidermis, it is much thicker and is composed of dense connective tissue. It is composed of two layers: The Papillary layer and the Reticular layer. The Dermis is composed of three types of tissue: Collagen, Elastin and Reticular fibres.

RETICULAR LAYER

This layer is a combination of protein fibres in the Dermis. These fibres permit the skin to expand, contract and perform supple movements.

The network consists of two protein fibres: Yellow Elastin and white Collagen fibres. Elastin fibres give the skin elasticity and Collagen fibres provide it with strength. Fibroblasts are specialised cells that produce the fibres and are maintained in a gel called the Ground Substance.

When the network is strong, the skin will be youthful and firm. As the fibres harden and fragment, the network begins to breakdown thus losing its elasticity. This is when the skin begins to show visible signs of ageing.

A key cause of damage to this network is unprotected exposure of the skin to ultra-violet light and to weather. A sudden increase in body weight can also lead to the skin losing elasticity (During puberty or pregnancy for example). As a result skin has stretch marks and the lost elasticity in the skin cannot be restored.

NERVE ENDINGS

Numerous types of sensory nerves endings are situated in the Dermis. They register touch, pressure, pain and temperature. They send messages to the central nervous system and the brain, alerting us of what is taking place on the skin’s surface.

THE PAPILLARY LAYER

This layer contains a thin arrangement of Collagen and Elastin fibres. Papillae are tiny bulges, which extend into the Epidermis and comprise of both nerve endings and blood capillaries. Papillae are especially noticeable in the palms of hands and soles of feet. This layer contains nerve fibres and blood vessels which supply a source of nutrition to the upper epidermis.

GROWTH AND REPAIR

The body’s blood system of arteries and veins constantly carries blood to the capillary networks in the skin and removes it again. Nutrients and oxygen reside in the blood and are necessary for the skin’s health, maintenance, growth, and remove waste products.

DEFENCE

Structures accountable for protecting the skin from damaging foreign bodies are located within the Dermis.

The Mast cells burst when stimulated during inflammation or allergic reactions and release Histamine. This triggers the blood vessels nearby to enlarge and in result more blood is brought to the irritated area.

In the blood and also in the Lymph and the connective tissue, are another group of cells called the Macrophages. They destroy micro-organisms and absorb dead cells and other excess products.

Lymph vessels in the skin transfer a fluid containing waste products for example used blood cells. The waste products are disposed of and usable protein is recycled to be used by the body once again.