Skin biology and structure
The skin is the human body’s largest organ, with a range of functions that support survival.
A view through the microscope reveals the layered structure of the skin, and the many smaller elements within these layers that help the skin to perform its mainly protective role.
The skin has two main layers, the epidermis and dermis. Below these is a layer of subcutaneous (‘under the skin’) fat.
The outer surface of the skin is the epidermis, which itself contains several layers — the basal cell layer, the spinous cell layer, the granular cell layer, and the stratum corneum. The cells in the epidermis are called keratinocytes.
The deepest layer of the epidermis is the basal cell layer. Here cells are continually dividing to produce plump new skin cells (millions daily). These cells move towards the skin surface, pushed upward by the dividing cells below them.
Blood vessels in the dermis — which is below the basal cell layer — supply nutrients to support this active growth of new skin cells. As the basal cells move upwards and away from their blood supply, their cell content and shape change, as follows.
Cells above the basal cell layer become more irregular in shape and form the spinous layer. Above this, cells move into the granular layer. Being distant from the blood supply in the dermis, the cells begin to flatten and die and accumulate a substance called keratin. Keratin is a protein that is also found in hair and nails.
The stratum corneum (‘horny layer’) is the top layer of the epidermis — it is the layer of the skin that we see from the outside. Cells here are flat and scale-like (‘squamous’) in shape. These cells are dead, contain a lot of keratin and are arranged in overlapping layers that impart a tough and waterproof character to the skin’s surface.
Dead skin cells are continually shed from the skin’s surface. This is balanced by the dividing cells in the basal cell layer to produce a state of constant renewal. Also in the basal cell layer are cells called melanocytes that produce melanin. Melanin is a pigment that is absorbed into the dividing skin cells to help protect them against damage from sunlight (ultraviolet light). The amount of melanin in your skin is determined by your genes and by how much exposure to sunlight you have. The more melanin pigment present, the darker the colour of your skin.
The epidermis also contains dendritic (Langerhans) cells, which are part of the immune system and help protect the body from foreign substances.
Below the epidermis is the layer called the dermis. The top layer of the dermis — the one directly below the epidermis — has many ridges called papillae. On the fingertips, the skin’s surface follows this pattern of ridges to create our individual fingerprints. So the ridges are not on the outermost layer of skin, as it might appear.
The dermis contains a variable amount of fat, and also collagen and elastin fibres which provide strength and flexibility to the skin. In an older person the elastin fibres fragment and much of the skin’s elastic quality is lost. This, along with the loss of subcutaneous fat, results in wrinkles.
When the skin is exposed to sunlight, modified cholesterol in the dermis produces vitamin D, which helps the body to absorb calcium for healthy bones.
Here are some of the other structures within the dermis that enhance the skin’s function.
- Blood vessels supply nutrients to the dividing cells in the basal layer and remove any waste products. They also help maintain body temperature by dilating and carrying more blood when the body needs to lose heat from its surface; they narrow and carry less blood when the body needs to limit the amount of heat lost at its surface.
- Specialised nerves in the dermis detect heat, cold, pain, pressure and touch and relay this information to the brain. In this way the body senses changes in the environment that may potentially harm the body.
- Hair follicles are embedded in the dermis and occur all over the body, except on the soles, palms and lips. Each hair follicle has a layer of cells at its base that continually divides, pushing overlying cells upwards inside the follicle. These cells become keratinised and die, like the cells in the epidermis, but here form the hair shaft that is visible above the skin. The colour of the hair is determined by the amount and type of melanin in the outer layer of the hair shaft.
- A sebaceous (‘oil’) gland opens into each hair follicle and produces sebum, a lubricant for the hair and skin that helps repel water, damaging chemicals and microorganisms (‘germs’).
- Attached to each hair follicle are small erector pili muscle fibres. These muscle fibres contract in cold weather and sometimes in fright — this pulls the hair up which pulls on the skin with the result being ‘goosebumps’.
- Sweat glands occur on all skin areas — each person has more than 2 million. When the body needs to lose heat these glands produce sweat (a mix of water, salts and some waste material such as urea). Sweat moves to the skin’s surface via the sweat duct, and evaporation of this water from the skin has a cooling effect on the body.
The skin varies in thickness and the number of hair follicles, sebaceous glands and sweat glands in different areas of the body. The thickest skin is on the soles of the feet and the palms of the hands. A large number of hair follicles are on the top of the head.
The innermost layer of the skin is the layer of subcutaneous fat, and its thickness varies in different regions of the body. The fat stored in this layer represents an energy source for the body and helps to insulate the body against changes in the outside temperature.
Functions of the skin
As you can see, there are many different structures within the skin. Together these structures impart many protective properties to the skin that help avoid damage to the body from outside influences. In this way, the skin:
- protects the body from water loss and from injury due to bumps, chemicals, sunlight or microorganisms (‘germs’);
- helps to control body temperature;
- is a sensor to inform the brain of changes in the immediate environment; and
- synthesises vitamin D.
2. DermNet NZ. Structure of the epidermis (updated 10 Nov 2014). http://www.dermnetnz.org/doctors/principles/epidermis.html (accessed Jul 2015).