Stratum spinosum is the fourth of the five layers that make up the epidermis — the outermost layer of the skin. It is also called the spinous or prickle cell layer because of the presence of cells with spiny arms diverging outward and interconnecting with other prickle cells. The stratum spinosum also contains keratinocytes and Langerhans cells. Its main function is to protect against foreign materials, and to produce and retain layers of lipids that prevent moisture loss from the skin.

The five layers of the epidermis from inside out are the stratum basale, stratum spinosum, stratum granulosum, stratum licidum, and stratum corneum. Overall, the entire epidermis is quite thin and varies in thickness from about .002 to .059 inches (about 0.05 to 1.5 mm). The stratum spinosum contains five to ten layers of cells.

Generally, the stratum spinosum is composed mainly of basal cells that are produced in the stratum basale and pushed upward to form prickle cells. These cells manufacture bipolar lipids that are organized into layers. The layers provide a structure that prevents evaporation of water and allows the skin to retain moisture. Prickle cells also provide the superstructure of the stratum spinosum. The prickles, or desmosomes, radiating from each cell are the points of attachment that join cell to cell; mitosis, or cell division, occurs infrequently in this layer.

Column-shaped keratinocytes move into the stratum spinosum from the stratum basale where they are produced. The keratinocytes then become polygon-shaped and begin to synthesize keratin. Keratin is a strong fibrous protein that forms a mesh that holds water and aids in retaining moisture within the skin. As fresh keratinocytes move into the stratum spinosum, they push older keratinocytes upward into the next epidermal layer where the cells begin to dry out and die.

Langerhans cells are macrophages — white cells that eat foreign matter. They are part of the immune system and work with T helper cells to protect the skin against foreign substances. Langerhans cells contain Birbeck granules, which are the hearts of their immune detection system. It is the job of these cells to detect skin penetration by foreign matter and then to trap and transport the invaders to the lymph nodes to be destroyed. Langerhans cells are made in the bone marrow, travel to the epidermis, and intermingle with keratinocytes.