The integumentary system is composed of the integument (skin) and its derivatives, including the hairs, sweat glands, and oil glands. As the largest organ of the body, the skin provides protection to the body and performs other functions.
On a structural basis, the skin is composed of three parts. At its surface the epidermis (A), which is outlined by a bracket in the diagram. The next deep layer contains connective tissue and is called the dermis (B). Then comes a subcutaneous layer called the hypodermis (C).
Epidermis
Returning to the epidermis, we note the detailed view and identify five layers of tissue. Moving from the surface downward, the most superficial (close to the surface) layer is the stratum corneum (A1). This is a layer of flat, dead cells filled with the protein keratin. The layer protects against heat, pathogenic microorganisms, chemicals, and light. Next comes the stratum lucidum (A2). Clear, flat cells with a prekeratin substance called eleidin are found here. The layer is found primarily in the palms of the hands and soles of the feet.
The next lower layer of the epidermis is the stratum granulosum (A3), which contains flattened cells containing the substance keratohyalin. Later, this material will become keratin. The next deeper layer is a very large layer called the stratum spinosum (A4). Keratin is produced in many of these cells.
The deepest layer is the stratum basale (A5). It is a single layer of cuboidal and columnar cells that undergo mitosis and become the cells of the more superficial (those above) layers. The layer is also called the stratum germinativum.