Skin Facts
Getting to Know the House (Epidermis)

Skin Facts
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 the 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 a 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 more round-shaped cells and the cornified layer of the skin flat, shingle-like cells.

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.

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).

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.

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 cornified layer derives its name from the Latin word for horn. This layer is horn-like in that it consists of dead cells of the epidermis, averaging about 20 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 1 mm, 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!

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 mostly comprises 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 serves to insulate and protect muscle, bones and internal organs. Fat is also a reserve source of energy.

Building Our House

Just as a bricklayer would build a house from the basement up, the skin is also "built" from the basal layer up. The basal layer is where cells divide, supplying the epidermis with new cells every day. New cells are pushed up through the epidermis, and change from column-shaped cells to rounded-shape cells to flat cells. In the cornified layer, the flat skin cells slough off. The entire cycle of growth takes about 28 days.

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