Our Skin – Structure and Function

Structure of the Skin

Just as we care for the health of our other organs, we should care for the health of our largest organ, the skin. Depending on the size and circumference of a person, the surface area of the skin is between 1.5 and 2 m². Our skin is layered and consists of

Epidermis, the upper skin
Corium, the dermis or sclera
Subcutis, the subcutaneous fatty tissue

Together, the epidermis and corium are also called cutis. The skin appendages are embedded in these layers: the hair, the sebaceous, sweat and scent glands.

1. Epidermis

The epidermis forms the uppermost layer of skin and thus also the surface of our body. It is itself non-vascular, but is interlocked with the underlying dermis which supplies it with nutrients. In the epidermis, there are isolated additional cell groups embedded with various functions such as the keratinocytes, which form keratin and provide the skin with protection and stability, the melanocytes, which form the brown skin pigment melanin, or the Langerhans cells, which serve as the body’s immune defence. The epidermis is between 0.4 mm (eyelids) and 1 mm (soles of the feet) thick and is again divided into 5 layers:

Germ layer: This layer consists mainly of basal cells, which are the germ cells of the epidermis. They divide every 200 – 400 hours and thus supply new cells that migrate to the surface and replace the cells that were rejected there. The melanocytes also sit in this layer and form the skin pigment.
Prickle cell layer: The prickle layer contains daughter cells of the basal cells, which are only connected by “prickles”. This structure (with cleft spaces) absorbs pressure and tension and thus gives the epidermis stability. The Langerhans cells, which serve the immune defence also sit in this layer.
Granular layer: The granular cells contain small granules which probably produce eleidine. This oily substance saturates the granular layer and the following cornea, keeping them supple.
Glossy layer: This layer is elastic and the cornea can slide on it, protecting it from sudden impacts. The glossy layer is only formed in areas of high mechanical stress such as the hands and feet.
Corneal layer: The corneal layer consists of corneocytes and cell remains (scales). It forms the uppermost barrier of the skin, which protects against chemical and mechanical influences and also prevents the loss or penetration of water and substances. The corneocytes are therefore surrounded by a keratin coat for protection. It should be noted that the active agents in our cosmetic products thus may also have problems with penetration. They usually do not even penetrate the horny layer, let alone deeper layers.

2. Corium

The corium is located under the epidermis and isa well supplied with blood, connective tissue, which ensures the supply of the epidermis with nutrients. It is interspersed with sensory and vegetative nerves. The dermis consists of 2 – 4 % elastin fibres, which form an elastic network in the skin. From the age of 30 onwards, the production of these fibres is reduced, which later leads to saggy aged skin.

The dermis also contains mast cells which collect substances such as histamine and serotonin. They are the effector cells of immediate allergic reactions and are involved in inflammatory reactions.

3. Subcutis

The subcutaneous fat tissue is stretchy connective tissue that is interspersed with fat cells. The name of this layer already indicates that it stores fat. Its thickness therefore also depends on diet and serves as protection against shock and cooling. It is not present in the ear, nose and eye areas.

Glands of the Skin

3 types of glands can be found in the skin:

Sebaceous glands: Sebaceous glands usually have a connection to a hair follicle, with a few exceptions such as lips and eyelids. The sebum within the sebaceous glands originates from dissolved cells. It is a thin mixture of squalene, triglycerides and waxes and is used to lubricate the skin and hair so that they are protected against dehydration. The production of sebum can be increased or decreased, although this can hardly be influenced by diet, even though this is often indicated.
Olfactory glands: These glands are also associated with hair follicles and are only found in a few areas (axilla, mammary glands, around the navel, auditory canal, genito-anal area). The glands form an odorless secretion, which only gives off an odor through bacterial decomposition on the skin surface.
Sweat glands: There are about 2 million sweat glands distributed over the entire body, which produce an aqueous salt solution. This solution regulates the skin temperature by evaporative cooling on the skin. But sweat can also be produced independently of temperature by emotional stimuli.

Functions of the skin

Our skin has many different functions, it protects our body and gives sensory impressions. Our skin’s appearance not only depends on external and internal reactions, but can also be influenced by the psyche.

Protective and barrier function: On the one hand, our skin gives us mechanical protection against pressure and impacts from outside. On the other hand, it provides physical protection that ensures that deeper tissue is protected from harmful UV radiation. Various protective mechanisms have been developed for this purpose over the course of time. By means of stored vitamin E, radicals produced by radiation can be intercepted. UVA and UVB radiation stimulate increased melanin and lead to a thickening of the skin, which serves as a light filter. The cells on the top layer of skin are dead and keratinized. They serve as a chemical barrier. The surface fat of the sebaceous glands is also distributed on the uppermost layer of skin. This means that there is hardly any possibility for water and other hydrophilic substances to penetrate. This barrier also exists from the inside to the outside and protects us from drying out. Due to sebum and sweat on the skin, a slightly acidic pH value (4.5 – 5.5=) is also reached. This acid mantle serves as a protection against a settlement of germs in and on the skin, which need a slightly alkaline pH value to survive.
Metabolic function: The skin influences many important reactions. It can regulate the body temperature through the strength of the skin’s blood circulation. When it is cold, the skin’s blood circulation is reduced to release less heat to the outside. The reverse is true if the body is too warm. Other metabolic reactions in which the skin is involved are e.g. the excretion of substances through sweat, vitamin D synthesis, storage of energy in the form of fat.
Immune defence: The skin contains many cells that are important for the immune defence (Langerhans cells, mast cells). They protect against the penetration of germs and foreign bodies. An overreaction of the immune system manifests itself in the form of allergic reactions to various substances or cosmetic products.
Sensory organ: The skin has cells and receptors through which sensory impressions are perceived (sense of touch, sense of temperature, pain receptors, itching).
Psycho-social function: The skin can reflect certain feelings. For example, one blushes, pales or gets stress spots. Since these reactions are carried out by the autonomic nervous system, they are difficult to control. Certain psychological stress can lead to diseases such as lip herpes or neurodermatitis for some people. On the other hand, a bad skin condition such as acne or psoriasis can also influence our psychological well-being.

Sources

Sources: Elsässer, S. (2008). Körperpflegekunde und Kosmetik: Ein Lehrbuch für die PTA-Ausbildung und die Beratung in der Apothekenpraxis (1. Aufl.). Berlin, Germany: Springer-Verlag. | Kerscher, M. (2009). Dermakosmetik. Würzburg, Germany: Steinkopff Verlag. | Furter, s. (2007). Crashkurs Dermatologie (1. Aufl.). Munich, Germany: Urban & Fischer Verlag/Elsevier GmbH.