Epidermis

Epidermis has four layers of cells. From the outermost to the innermost, they are the , granular, spinous, and basal layers. The layer consists of between about 15 and 40 rows of dying cells. These cells are filled with a tough, waterproof protein called keratin. The granular layer consists of one or two rows of dying cells that contain small grains of a substance called keratohyaline. The spinous layer is composed of between about 4 and 10 rows of living cells that have spinelike projections where the cells touch one another. The basal layer is also made up of living cells. It consists mainly of a single row of tall, narrow basal cells. The basal layer also includes cells called melanocytes. These cells produce a brown pigment called melanin.

The basal cells divide continually and form daughter cells. Some daughter cells remain in the basal layer. Others move toward the outer surface of the skin and eventually form the upper layers of the epidermis. These cells are called keratinocytes, and they produce keratin. Keratin is found only in the epidermis, hair, and nails. Keratin makes the skin tough. It also prevents fluids and certain substances from passing through the skin. As the keratinocytes move upward through the epidermis, they become filled with more and more keratin. On reaching the surface of the skin, they have died and become flat and dry. Eventually, they are shed as thin flakes.