It is easier to understand the normal appearance and function of the skin if we compare it to a house and the earth on which that house rests. For example, at its most basic level a house can be considered to comprise two parts: the house itself and the earth it rests on. A two-storey house has a basement and two floors. The basement, or foundation, of the house rests on the earth. The house itself consists of bricks and a roof. The bricks are joined together by cement.
The skin, like a house, is also divided into two parts: the outer, thinner portion called the epidermis (which we can think of as our house) and the inner, thicker layer called the dermis (which we can think of as the ground or earth). Just as a house rests on the earth for stability, the epidermis rests on top of the dermis.
If the house consists of bricks, cement, and a roof, the skin also has "bricks," "cement," and a "roof." The outer layer of the skin (epidermis) is composed of so-called "bricks," which we call skin cells or keratinocytes. Just as bricks are joined using cement, skin cells or keratinocytes are joined together by small attachments called desmosomes. As cement keeps the bricks together as a continuous layer, desmosomes join the skin cells as a continuous layer.
Getting to Know the House (Epidermis)
Let's now examine the outer layer of skin a little closer. Just as a house consists of a basement, two floors, and a roof, the epidermis consists of a basement, two floors, and a roof. The "basement" of the skin is called the basal layer. As the basement of the house rests on the earth, the basal layer of the skin rests on the dermis. The two "floors" of the epidermis, in order, are the spinous and granular layers. The "roof" of the epidermis is called the cornified layer.
If we look closely at the house, we will see that the bricks that form the house consist of different shapes on the different floors. In our skin, the cells that form the outer layer of the skin also change shape in the different layers. The basal layer comprises column-shaped cells, the first and second floors have more round-shaped cells, and the cornified layer of the skin has flat, shingle-like cells.
Basal Layer (Basement)
The bottom layer of the epidermis is the basal layer, which sits right on the dermis. It is a single layer of cuboidal-shaped cells that continuously divide to make skin cells (keratinocytes). This is similar to the basement of the house, which rests on the earth.
Spinous Layer (First Floor)
The layer of cells above the basal layer is the spinous layer. The skin cells in this layer are more rounded and are pushed upward to the next layer (granular layer).
Granular Layer (Second Floor)
In the next layer, called the granular layer, skin cells stop dividing, their nuclei (the heart of the cell holding all genetic information) start degenerating, and they die. The granular layer is named for the granules present, which contain a specific substance that helps make the skin waterproof. Lipids (fats) and other proteins are also found in this layer.
Our Skin as a House
Layer of house
- roof
- second floor
- first floor
- basement
- earth
- cornified layer
- granular layer
- spinous layer
- basal layer
- dermis
The final and outermost layer of the skin is the cornified layer, which consists of flat, dead cells that resemble the shape of the shingles on a roof. This is the layer of skin we see. The cornifled layer derives its name from the Latin word for "horn." This layer is horn-like as it consists of dead cells of the epidermis, averaging about twenty cells deep. This creates a tough, protective barrier. The cornified layer sheds every two weeks.
The epidermis is the thinnest layer of the skin, measuring about 0.4 inch (1 millimeter), or the thickness of several sheets of paper. The thickness varies on different parts of the body. The epidermis is the thickest on the palms of the hands and soles of the feet, and thinnest on the eyelids.
One difference between the house and normal skin is that, unlike the basement of the house, the basal layer of the skin is not flat. It has projections downward called rete ridges. (We could think of this as the foundation of a house that has leaked and created lumps in the floor!)
Dermis (Earth)
The epidermis (house) rests on the dermis (earth). Compared to the epidermis, the dermis is thicker and contains nerves, blood vessels, oil (sebaceous) glands, and sweat glands. The dermis is comprised mostly of a strong protein called collagen and a flexible, elastic protein called elastin. These proteins allow the skin to bend and return to its normal shape.
Below the epidermis and dermis is the subcutaneous layer. The main component of this layer is fat, which insulates and protects muscle, bones, and internal organs. Fat is also a reserve source of energy. To find out more, you can check out Psoriasis Skin Condition.