The chief role of the keratinocytes is to produce keratin, a tough fibrous protein that gives the epidermis its protective properties. Tightly connected to one another by a
large number of desmosomes, the keratinocytes arise in the deepest part of the epidermis from cells that undergo almost continuous mitosis, or cell division. As these cells are
pushed toward the skin surface by the production of new cells beneath them, they
manufacture the keratin that eventually fills their cytoplasm. The cytoplasm makes up the
bulk of the cell and is located between the outer layer of the cell and the nucleus.
By the time the keratinocyte reaches the skin surface, they are dead, flat sacs completely filled with keratin. Millions of these dead cells rub off every day, giving us an entirely new epidermis every 30 to 45 days—the average time from “birth” of a
keratinocyte to its final wearing away. In the normal healthy epidermis, the production of new cells balances the loss at the surface of the skin. Where the skin experiences friction,
both cell production and keratin formation are accelerated.
Layers of the Epidermis
In thick skin, which covers the palms of the hand and soles of the feet, the
thickened epidermis consists of five layers, or strata. In thin skin, which covers the rest of the body, only four strata are present.
1. Stratum Basale (Basal Layer)
The stratum basale, the deepest layer of the epidermis, is firmly attached to the dermis along a wavy borderline. Also called the germinating layer, this stratum consists of a single row of cells representing the youngest keratinocytes. These cells divide
rapidly. Occasional Merkel cells are seen among the keratinocytes. Each semi-circular Merkel cell is closely associated with a disc-like sensory nerve ending and may serve as a
receptor for touch.
Between 10 percent and 25 percent of the cells in the stratum basale are
melanocytes (“melanin cells”). These make the dark skin pigment melanin.
The spider- shaped melanocytes have many branching processes that reach and touch all of the keratinocytes in the basal layer. Melanin is made in membrane-lined granules and then
transferred through the cell processes to nearby keratinocytes.
Consequently, the basale
keratinocytes contain more melanin than do the melanocytes themselves. The melanin granules accumulate on the surface of each keratinocyte, forming a shield of pigment
over the nucleus.
In Caucasians, the melanin disappears a short distance above the basal
layer, where it is digested by lysosomes in the keratinocytes. In
black skinned individuals, no such digestion occurs, so melanin occupies keratinocytes throughout the
epidermis. Although black skinned individuals have darker melanin and more pigment in each melanocyte, they do not have more melanocytes in their skin. In all but the darkest
people, melanin builds up in response to ultraviolet rays, the response that we know as suntanning.
2. Stratum Spinosum (Spiny Layer)
The stratum spinosum is several cell layers thick. Mitosis, or cell production through division, occurs here, but less frequently than in the basal layer. Under microscopic imaging, the keratinocytes in this layer have many spine-like extensions.
Scattered among the keratinocytes of the stratum spinosum are Langerhans cells. These star-shaped cells are particulate ingesting microphages that help activate the immune system.
3. Stratum Granulosum (Granular Layer)
The thin stratum granulosum consists of three to five layers of flattened
kearatinocytes. These keratinocytes contribute to the formation of keratin in the upper layers of the epidermis. This keratin contains a waterproofing material that is secreted into the areas between the cells and is the major factor for slowing water loss from the
epidermis. Further more, the external wall of the cells thicken, so that they become more resistant to destruction. You might say that the keratinocytes are “toughening up” to make the outer layers of the epidermis the strongest.
4. Stratum Lucida (Clear Layer)
The stratum lucid only occurs in thick skin, not thin skin. This layer consists of a few rows of flat dead keratinocytes. Electron microscopes show that its cells are identical
to cells at the stratum corneum, the next layer.
5. Stratum Corneum (Horny Layer)
The most external part of the epidermis, the stratum corneum, is many cells thick. This layer is far thicker in thick skin than in thin skin. Its dead cells are flat sacs completely filled with keratin, because their nuclei and organelles were digested away by the lysosome enzymes upon cell death. Both the keratin and the thickened plasma membranes of the cells in the stratum corneum protect the skin against abrasion and
penetration. It is amazing that a dead layer of cells can still perform such important functions.
The cells of the stratum corneum are referred to as horny cells. These cells are the dandruff shed from the scalp and the flakes that come off dry skin. The average person sheds about 40 pounds of these flakes in a lifetime. The common saying “Beauty is only
skin deep” is especially interesting in the light of the fact that nearly everything we see when we look at someone is dead!
Three pigments contribute to skin color: melanin, carotene, and hemoglobin. Carotene is a yellow to orange pigment derived from certain plant products, such as carrots. It tends to accumulate in the stratum corneum of the epidermis and in fat tissue of the hypodermis. Color derived from carotene is most obvious in the palms and soles,
where the stratum corneum is thickest.
The pink tone of Caucasian skin reflects the red color of oxygenated hemoglobin in the capillaries of the dermis. Since Caucasian skin contains little melanin, the epidermis is nearly transparent in untanned individuals and allows blood’s color to shoe\w through more predominantly.
Melanin, the most prominent pigment and is made from an amino acid called tyrosine. Melanin ranges in color from yellow to reddish to brown to black. Its production depends on an enzyme in melanocytes called tyrosinase. Freckles and pigmented moles are localized accumulations of melanin.