3.3: The Epidermis - Components and Functions

Keratin and Keratinocytes

The epidermis is the outermost layer of the skin (Figure 3.1) and (Figure 3.2a). It is composed mostly of a thin sheet built up from many layers of flat cells produced continuously in the deepest region of the epidermis. As they accumulate, many of these cells are pushed upward toward the outside of the body. As they move farther away from the favorable internal environment of the body, they begin to produce a protein called keratin. Therefore, these cells are called keratinocytes. Finally, these cells are moved so close to the air that they die, leaving behind their keratin as the outermost layer of the epidermis, the stratum corneum. Starting with the formation of a new cell in the deep region of the epidermis, this process takes about a month.

Figure 3.1 Components of the integumentary system. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Keratinocytes produce several substances that help regulate inflammation and immune responses, which are defense activities. The stratum corneum provides a barrier against microbes; many chemicals, including water; and abrasion. Though it is always being gradually worn away at the outer surface, it is maintained by having new keratin produced at the same rate by the next generation of keratinocytes. The stratum corneum serves well as long as it remains thick enough and is not broken by cuts, tears, scrapes, or burns.

Figure 3.2a Structure of the epidermis and underlying region. Epidermal cells and layers. (Used with permission. Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland)

Figure 3.2b Structure of the epidermis and underlying region. Melanin production and transfer. (Used with permission. Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland)

Figure 3.2c Structure of the epidermis and underlying region. Langerhans cells (microscope view). (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

If the keratin is injured, the keratinocytes reproduce at a faster rate and repair the damage, as occurs when a cut heals. The epidermis can even form a thick pad of keratin a callus) in areas that are regularly subjected to physical abuse. Thus, the keratinocytes provide adaptation to maintain homeostasis.

Melanin and Melanocytes

The keratinocytes are aided by other types of cells. One type is in the innermost layer of the epidermis and produces the brown skin pigment melanin. Cells of this type are called melanocytes (Figure 3.2a )They make up 2 to 3 percent of the cells in the epidermis.

Melanocytes send melanin into keratinocytes that are about to begin migrating toward the outer surface of the skin (Figure 3.2b ). The melanin helps maintain homeostasis by absorbing excess light. Recall that excess light can cause great harm to the body. This is especially true of ultraviolet (UV) light, which contains a great deal of energy and is particularly well absorbed by proteins, RNA, and DNA. UV light is in high concentration in sunlight and in the light from certain bulbs, such as those used for getting a tan.

Excess light is hazardous to the body because it changes the structure of molecules. There are three main undesirable outcomes. One is damage to the protein molecules making up much of the inner layer on the skin (i.e., dermis), which are discussed later in this chapter. This damage may contribute to wrinkling of the skin. The second outcome is injury to the protein molecules inside the cells. The result can be painful and severe sunburn with blistering and peeling of the epidermis. Besides causing pain, such injury detracts from the barrier functions provided by the keratin. The third outcome is alteration of DNA, which can lead to skin cancer.

Fortunately, enough melanin can usually be produced to significantly reduce the incidence of these problems. Like the keratinocytes, the melanocytes are adaptable, and they increase their production of melanin when the skin is exposed to excess light. This is evident as the development of a tan. Melanocytes also decrease melanin production when less light is encountered. Then, as the melanin‑rich keratinocytes die and are sloughed off at the surface of the skin, melanin‑poor keratinocytes replace them and the tan fades.

Immune Function and Langerhans Cells

The third type of important cell in the epidermis is one that starts a body defense strategy called an immune response (Chapter 15). These cells are called Langerhans cells (dendritic cells) and make up less than 1 percent of the epidermal cells (Figure 3.2a and Figure 3.2c ). Their specific function is to monitor substances throughout the epidermis to determine whether they are native to the body or foreign. These substances may be free molecules between the cells, molecules on the surfaces of microbes, or viruses. Of course, being near the surface of the body, Langerhans cells are positioned to guard against the entry of harmful materials and organisms.

When a Langerhans cell determines that a substance is foreign and therefore does not belong in the body, it alters that substance so that other cells in the immune system can attack and eliminate it. This helps maintain proper chemical conditions within the body and greatly reduces the risk of infection and cancer.