Nation of Strong blog post: Basics: Structure and physiology of the skin

Basics: Structure and physiology of the skin

The human skin is our largest organ in terms of surface area, measuring approximately 1.8 m2, and the most visible, accounting for almost one sixth of our body weight.

It can be divided into three main components or layers. A distinction is made between the epidermis (upper skin), the dermis (leather skin) and the subcutis (hypodermis or lower skin). The epidermis and dermis together make up the cutis.

Epidermis (top layer of skin, also called horny layer)

The epidermis is the top layer of the skin. It is capable of keratinisation and can therefore have very variable thicknesses in different parts of the body. With the help of this keratinisation, the epidermis can adapt to increased mechanical stress on the body (for example on the feet or hands). With its very dense cell structure, the epidermis provides protection against external influences such as microorganisms or fluid loss. It consists almost exclusively (over 90 %) of the cell type keratinocytes (horny cells). These cells are specialised and form the horny substance keratin, which gives the skin stability and protection. The epidermis in turn consists of five different cell layers. These layers acquire their specific properties through the process of keratinisation of the keratinocytes.

  1. Basal layer (stratum basale): The lowest layer of the epidermis.
  2. This is where the horny cells (keratinocytes) are formed.
  3. Spiny cell layer (stratum spinosum): The keratinocytes transform into spindle-shaped cells and produce keratin precursors.
  4. Granule cell layer (stratum granulosum): The skin cells in this layer produce granules, which mature into epidermal lipids and keratin through layers four and five.
  5. Shiny layer (stratum lucidium): The skin cells are flattened, crowded together and cannot be distinguished from each other.
  6. Horny layer (stratum corneum): The horny layer is the outermost layer of the epidermis and consists of the flattened, dead skin cells mentioned in layer four. The dead skin scales are shed and replaced at regular intervals (about four weeks). The horny layer also contains the pores of the sebaceous and sweat glands.

The epidermis is covered with a hydrolipid film, which, as the name suggests, consists of a mixture of water and fat. The hydrolipid film consists of the secretions of the sebaceous and sweat glands and protects the skin from the penetration of fungi and bacteria. The film also keeps the skin supple and moisture bound in the skin cannot escape to the outside. The watery part of the hydrolipid film is called the protective acid mantle and has a pH value between 5.4 and 5.9. The slightly acidic environment is extremely important for preserving
helpful and skin-friendly microorganisms while at the same time killing harmful microorganisms.

Dermis (middle layer of skin, also called dermis)

The dermis is the second of the three skin layers and is traversed by a fine blood capillary network, which in this way supplies the epidermis with nutrients. At the same time, waste products are also transported away via the capillary network. It is mainly composed of connective tissue fibres (collagen and elastin). The connective tissue fibres are embedded in a base of gel-like hyaluronic acid, which helps the skin to achieve its volume through its extremely high moisture-binding capacity. The hair follicles and the associated sebaceous glands as well as the sweat glands are also located within the dermis.
More precisely, these glands are located in our skin pores, which are small openings in the skin and form the excretory ducts for the glands described (see epidermis / horny layer). In this way, sebum, which consists of triglycerides, fatty acids and wax esters, and sweat can be released into the epidermis. Follicles, sebaceous and sweat glands are also called skin appendages. The dermis also contains the lymphatic vessels and sensory receptors.

Subcutis (lowest layer of skin, also called hypodermis)

The subcutis, which lies under the dermis and epidermis, is the third layer of skin mentioned here and consists mainly of fat and loose connective tissue. The collagen fibres hold the fat cells together. The subcutis is traversed by larger blood vessels and nerves. Its functions include insulating the body and storing energy. Under the subcutis lies the muscle tissue.

In a Nutshell: 

  • The skin is the largest organ of the human body
  • The skin consists of three layers
  • The skin layers are, from the outside inwards, the epidermis, the dermis and the subcutis.
  • The epidermis is composed of five layers and consists mainly of keratinocytes
  • The dermis consists mainly of connective tissue fibres and hyaluronic acid
  • The subcutis consists mainly of fat and connective tissue

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