The skin is more than a convenient layer separating you from the outside world. 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.
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.
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:
Last Reviewed: 30 September 2009