All the cells in the epidermis originate ultimately from a single layer of basal cells, called the basal layer, which sits on the basement membrane. The 'daughter cells' produced by this basal layer gradually move upwards, lose their central nucleus, and start to produce skin proteins called keratins (hair is made up of a similar, but harder, material) and fats called lipids. They are now known as keratinocytes. As they move upwards through the skin thickness their form slowly changes. The altered cells form distinct layers, which naturally blend into each other.
Skin Care Myth:
Cucumbers on Eyes
Cucumber contains a special ingredient that helps to reduce swelling around the eye or bags under the eyes.Pleasantly cooling, but no sorcery involved!
Fact: More than 90% of the cucumber consists of water. It is the cooling effect of the water in the cucumbers on eyes, together with increased humidity, that reduces the swelling. Some products contain extracts of cucumber, which in high concentration may help to improve the hydration of the skin. Gentle massage also helps.
Skin Care Myth:
Skin Cells and Skin Repair
It is important for skin regeneration to have at least eight hours of sleep a night. While we are sleeping new skin cells are formed, waste is removed and the skin prepares itself for the day ahead.
Fact: Skin repair continues day and night, and is not dependent on the length of sleep.
Information on Skin Lipids
What are lipids?
As the daughter cells move upwards their shape flattens, and they become joined by spiny processes to make another recognisable layer, known as the spiny layer. These cells make special fats called sphingolipids. When the cells reach the stratum corneum these lipids will play an important part in the retention of moisture in the skin.
A model of the 'bricks and mortar' arrangement of the cells of the stratum corneum. The flattened cells (corneocytes) are held together by attachments called desmosomes. The lipids (natural fats) between them help to conserve moisture since water cannot pass through them easily.
As the cells migrate further upwards they develop characteristic granules; they now form part of the granular layer. In the upper cells of this layer these granules discharge and fill up the spaces between the cells with lipids, ultimately creating an appearance of a wall of bricks (cells) and mortar (lipids).
The different forms of the cells of the epidermis: as the cells move upwards they gradually change shape.
As the cells rise into the top layer of the epidermis - the stratum corneum, sometimes called the horny layer or the cornified layer - they take the form of flattened discs, tightly packed together. These flattened cells, now called corneocytes, are effectively dead. The number of layers of cells in the stratum corneum depends on the site on the body; on the sole of the foot the stratum corneum is at its thickest, and is there made up of hundreds of layers of densely packed cells.
The stratum corneum acts as an outer 'covering' to the skin, able to resist scrapes and scratches on the outside and helping to keep water on the inside. In this respect it is rather like the bark of a tree.
Sections of the epidermis, compared: (top) aged 18, (bottom) aged 80. The reasons why the epidermis changes throughout life are discussed in Chapter 4, 'Skin and aging'.
What Causes Dry Skin Problems?
Throughout a person's life, from birth to death, the cells of this layer are continually being worn away and replaced from below with new cells. The wearing process is called desquamation, and the flattened scales of dead skin are called squames (see page 9). Each squame is only about a micrometre thick but some 35 micrometres across. Desquamation tends to slow down as we grow older. In any particular part of the body, however, the results of the processes of cell loss and replacement are that the skin tends to remain the same overall thickness.
In normal skin, it takes about 30 days for a cell produced by the basal layer to move through the epidermis to the surface. The rate of movement is partly controlled by the rate at which the outer layer is being lost. When stratum corneum cells are being lost quickly -perhaps after sunburn - they are replaced more quickly from below. In skin that has been injured (grazed, for instance) the process speeds up dramatically. Artificially removing the outer layers by the cosmetic process of peeling (exfoliation) also tends to speed up replacement.
The stratum corneum is a very important layer from the point of view of understanding skin, skin problems, skin care and the beneficial effects of cosmetics such as skin moisturizers. It is the part of the skin that forms the junction of the body with the outside world, and it is directly affected by the outside environment, by harsh soaps, by skin care products and by the sun.
The stratum corneum acts as an outer 'hide' that can resist injury and help to conserve water in the skin.
It plays a key role in helping to contain moisture within the rest of the skin, and in regulating the natural moisture flow out from the deeper layers to be lost eventually by evaporation from the skin surface. This flow is known as transepidermal water loss (TEWL), and it is important to understand the factors that influence it. Without adequate retained moisture, skin can become dry and unhealthy.
Under normal conditions up to 15% of the stratum corneum consists of water. This water is vital to enable the stratum corneum itself to work. The natural functions of the skin do not work as well when the stratum corneum contains less than 10% of water, and it becomes dry.
In the epidermis the spaces between the cells are packed with fats, or lipids, made by the body. One very important group of these lipids is the ceramides, which are also ingredients of some skin care products.
Removal of lipids leads the stratum corneum to break down.
Hydrate Dry Skin with Skin Moisturizers
The lipids of the epidermis play a vital role in healthy normal skin, as they help the stratum corneum to regulate natural water loss. If they are removed by harsh soaps or detergents, or by damage such as a burn, the skin loses some or all of its ability to retain water, becomes dry and will start to break down.
The epidermis also contains natural enzymes, which are important for getting rid of old skin cells. Enzymes need moisture in which to work, so dryness (desiccation) of the stratum corneum worsens dry and unhealthy looking skin.
The pictures (below) of skin from the palm of the hand illustrate how the condition of the stratum corneum can vary. One is dry and the stratum corneum is not well hydrated: the other has been moisturized and the layer is now healthy. The same can be true of the skin of the face.
Parched, dry skin on the hands...
...can be improved by treating with moisturizer.
Here's a face in desperate need of our best facial moisturizer:
Seen still more closely, facial skin can be equally dry (as seen here)..