The epidermis

The epidermis is the most superficial layer of the skin and consists of five layers of cells:

• the basal cell layer (stratum germinativum) — innermost layer

• the prickle cell layer (stratum spinosum)

• the granular layer (stratum granulosum)

• the clear layer (stratum lucidum)

• the horny layer (stratum corneum) — outermost layer.

In most areas of the body the epidermis is approximately 35-50 micrometres thick. It is thicker on the palms and soles of the feet (up to several millimetres) and thinner around the eye.

The epidermis holds a large amount of water, with the basal cell layer holding the highest percentage (approximately 80 per cent). Water is held in the spaces between the cells; each subsequent layer has less water, with the horny layer containing approximately 15 per cent.

Hair

Pain receptor

Touch receptor (Meissner's corpuscle)

Capillary network

Sebaceous gland

Erector pili muscle

Heat receptor (Ruffini endings) Hair follicle

Cold receptor (Krause corpuscle)

Motor nerve

Hair bulb

Vein

Artery

Nerve endings

Subcutaneous fat

Subdermal muscle layer

Sweat (eccrine) gland

Pacinian corpuscle (pressure receptor)

Epidermis

Dermis

Subcutaneous layer

Subdermal muscle layer

Deep fascia

Sweat (eccrine) gland

Pacinian corpuscle (pressure receptor)

Fig 2.1 Structure of the skin

Dead cells t

The upward movement of cells replaces those lost from the skin's surface

Dead cells

Horny layer

Basal cell layer

Blood capillaries of dermal papillae

Horny layer

Clear layer Granular ^ layer

Prickle cell layer

Basal cell layer

Fig 2.2 Layers of the epidermis

Blood capillaries of dermal papillae

IN PRACTICE

The intercellular lipids in between the epidermal cells are responsible for hydration, epidermal firmness and smoothness. They protect against transepidermal water loss, which can result in dehydration. They also provide protection against any offending or injuring substances invading the skin.

One very important group of these lipids is ceramides, which are also contained within the ingredients of some skin-care products.

It is important to recognise that the cell renewal process is responsible for the production of these essential lipids, and if the cell cycle slows down, the production of lipids slows down and results in dryness and dehydration.

Functions of the epidermis

The epidermis has three primary functions:

• protecting the body from the external environment, particularly the sun

• preventing excessive water loss from the body

• protecting the body from infection.

The basal cell layer (stratum germinativum)

This is the deepest of the five layers and therefore lies at the base of the epidermis. It consists of a single layer of column cells on a basement membrane, which separates the epidermis from the dermis. This layer of the epidermis is concerned with cell regeneration. The basal cells within this layer are constantly reproducing, producing new cells that constantly divide. As new cells are formed by division, they push adjacent cells towards the skin's surface.

Approximately 95 per cent of the cells within the epidermis are keratinocytes, which produce a protein substance called keratin. Keratin is what makes the epidermal cells more resilient and protective as they are pushed towards the skin's surface.

At intervals between the column cells, which divide to reproduce, are the large star-shaped cells called melanocytes, which form the pigment melanin, the skin's main colouring agent. This layer also contains tactile (Merkel) discs that are sensitive to touch.

Prickle cell layer (stratum spinosum)

This is the thickest layer of the epidermis. It is known as the prickle cell layer because each of the rounded cells contained within it has short projections which make contact with the neighbouring cells and give them a prickly appearance. The tiny hair-like structures on the prickle cells will eventually become desmosomes, which are small disc-shape attachments that provide strength and integrity by holding the upper level of epidermal cells together.

The cells of this layer are living and are therefore capable of dividing by the process mitosis.

The cells of this layer make special fats called sphingolipids, and when these cells reach the top layer (horny layer), these lipids play an important role in the retention of moisture in the skin.

It is in the prickle cell layer that the keratinocyte has a major role to play in skin barrier defence. Keratinisation refers to the process that skin cells undergo when they change from living cells with a nucleus to dead cells without a nucleus. The keratinisation process is well under way in the upper cells of this layer as the cells migrate upwards.

This layer also includes Langerhan cells, which set up an immune response to foreign bodies. These special defence cells are spread out among the keratinocytes and their role is to mop up invading foreign bodies that have found their way into the body and transport them to specialised white blood cells where they are neutralised.

Granular layer (stratum granulosum)

This layer consists of distinctly shaped cells that resemble granules, which are filled with keratin and produce intercellular lipids (the substances that

IN PRACTICE

The horny layer of the epidermis is an important layer in relation to understanding skin problems, as it is the part of the skin that is directly affected by the external environment. It also plays a key role in helping to contain moisture in the rest of the skin and in regulating the natural moisture flow out from the deeper layers to be lost eventually in evaporation from the skin surface. This natural moisture flow is known as transepidermal water loss (TEWL).

Without adequate retained moisture, skin can become dry and unhealthy.

Under normal conditions, up to 15 per cent of the horny layer consists of water, which is vital to enable the stratum corneum to work. The natural functions of the skin do not work as efficiently when the horny layer contains less than 10 per cent of water and it becomes dry.

fill the spaces between the upper epidermal cells) from structures called lamellar bodies.

These lipids help form a strong cement-like structure to prevent the absorption of harmful substances by the skin and help to maintain hydration of the lower layers. The cells of this layer ultimately create an appearance of a wall of bricks (cells) and mortar (lipids).

As the cells move into the granular layer, a number of changes occur to the keratinocyte. The keratinocyte becomes less flexible, more granular in appearance, and the keratin within the cell has become completely hardened, thereby completing the keratinisation process.

As the cells move further up into the granular layer, further changes to the keratinocyte occur. The desmosomes begin to dissolve, preparing the corneocyte (dead skin cell) for desquamation, the process in which the outer layer of dead cells is continually being shed.

Clear layer (stratum lucidum)

This layer consists of transparent cells which permit light to pass through. The cells in the clear layer are filled with a substance called eleidin, which is produced from keratohyalin and is involved in the keratinisation process.

This layer is considered to be an important transitional stage in the development of the top layer of the epidermis (the horny layer). The keratinocyte has almost reached its final destination by the time it reaches this layer; during its upward movement it has gone through a process of change, which eventually will result in the formation of dead skin cells, known as corneocytes. Keratinisation is therefore complete by the time cells have reached the clear layer.

The clear layer is very shallow in facial skin, but thick on the soles of the feet and the palms of the hands, and is generally absent in hairy skin.

Horny layer (stratum corneum)

This is the most superficial outer layer, consisting of dead, flattened keratinocytes, now known as corneocytes. The horny layer is thought of as a permeability barrier, as the cells of this layer form a waterproof covering for the skin and help to prevent the penetration of bacteria.

The horny layer is the end result of the change that occurs when new live cells are produced in the basal layer, then pushed upwards by newer cells until they reach the surface where they dry out and are sloughed off. The outer layer of dead cells is continually being shed through desquamation.

Life span of the epidermal cells

In normal skin, it takes approximately 28-30 days for a cell produced by the basal layer to move through the epidermis to the surface. The rate of regeneration is partly determined by the rate at which the outer layer is being desquamated. With age this process is greatly reduced and by the age of 50, it is said to take about 37 days to complete the same process.

When the cells of the horny layer are lost quickly (for instance due to skin injury or sunburn), the process speeds up as the cells are replaced more

KEY FACTS

quickly from below. Removing the outer layers of the skin with a chemical peel will also speed up replacement.

The predominant cell of the epidermis is the keratinocyte. In their upward metabolic process, they undergo a series of chemical changes, transforming from soft cells into flat scales that are constantly rubbed off.

An essential factor in beautiful skin is the healthy metabolism of keratinocytes. If the keratinocyte formation is not functioning properly in the epidermal layers, it cannot generate an aesthetically pleasing horny layer. The healthy balance of all the essential elements (water, lipids, etc.) is needed in order to ensure the health of the keratinocytes and the skin is not impaired.

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