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 Epidermis
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.
The Dermis
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:

Subcutaneous tissue
Subcutanous tissue is the innermost layer of the skin located under the dermis and consisting mainly of fat. The predominant type of cells in the subcutaneous tissue is adipocytes or fat cells. Subcutaneous fat acts as a shock absorber and heat insulator, protecting underlying tissues from cold and mechanical trauma. Interestingly, most mammals lack subcutaneous tissue because their fur serves as a shock absorber and heat insulator. Sweat glands and minute muscles attached to hair follicles originate in subcutaneous tissue. The loss of subcutaneous tissue, often occuring with age, leads to facial sag and accentuates wrinkles. A common procedure performed by dermatologists to counteract this process is to inject fat (collected elsewhere in the body) under the wrinkles on the face.