The Integumentary System: The Epidermis
Skin — together with hair, nails, and glands — composes the integumentary system. The epidermis, which contains no blood vessels, is made up of layers of closely packed epithelial cells.
From the outside in, these layers are the following:
Stratum corneum (literally the “horny layer”) is about 20 layers of flat, scaly, dead cells containing a type of water-repellent protein called keratin. These cells, which represent about three-quarters of the thickness of the epidermis, are said to be cornified, which means that they’re tough and horny like the cells that form hair or fingernails.
Humans shed this layer of tough, durable skin at a prodigious rate; in fact, much of household dust consists of these flaked-off cells. Where the skin is rubbed or pressed more often, cell division increases, resulting in calluses and corns.
Stratum lucidum (from the Latin word for “clear”) is found only in the thick skin on the palms of the hands and the soles of the feet. This translucent layer of dead cells contains eleidin, a protein that becomes keratin as the cells migrate into the stratum corneum, and it consists of cells that have lost their nuclei and cytoplasm.
Stratum granulosum is three to five layers of flattened cells containing keratohyalin, a substance that marks the beginning of keratin formation. No nourishment from blood vessels reaches this far into the epidermis, so cells are either dead or dying by the time they reach the stratum granulosum. The nuclei of cells found in this layer are degenerating; when the nuclei break down entirely, the cell can’t metabolize nutrients and dies.
Stratum spinosum (also sometimes called the spinous layer) has five to ten layers containing prickle cells, named for the spinelike projections that connect them with other cells in the layer. Langerhans cells, believed to be involved in the body’s immune response, are prevalent in the upper portions of this layer and sometimes the lower part of the stratum granulosum; they migrate from the skin to the lymph nodes in response to infection.
Some mitosis (cell division) takes place in the stratum spinosum, but the cells lose the ability to divide as they mature.
Stratum basale (or stratum germinativum) is also referred to as the germinal layer because this single layer of mostly columnar stem cells generates all the cells found in the other epidermal layers. It rests on the papillary (rough or bumpy) surface of the dermis, close to the blood supply needed for nourishment and oxygen.
The mitosis that constantly occurs here replenishes the skin; it takes about two weeks for the cells that originate here to migrate up to the stratum corneum, and it’s another two weeks before they’re shed. About a quarter of this layer’s cells are melanocytes, cells that synthesize a pale yellow to black pigment called melanin that contributes to skin color and provides protection against ultraviolet radiation (the kind of radiation found in sunlight).
The remaining cells in this layer become keratinocytes, the primary epithelial cell of the skin. Melanocytes secrete melanin directly into the keratinocytes in a process called cytocrine secretion. Merkel cells, a large oval cell believed to be involved in the sense of touch, occasionally appear amid the keratinocytes.
In addition to melanin, the epidermis contains a yellowish pigment called carotene (the same one found in carrots and sweet potatoes). Found in the stratum corneum and the fatty layers beneath the skin, it produces the yellowish hue associated with Asian ancestry or increased carrot consumption.
The pink to red color of Caucasian skin is caused by hemoglobin, the red pigment of the blood cells. Because Caucasian skin contains relatively less melanin, hemoglobin can be seen more easily through the epidermis. Sometimes the limited melanin in Caucasian skin pools in small patches.
Can you guess the name of those patches of color? Yep, they’re freckles. Albinos, on the other hand, have no melanin in their skin at all, making them particularly sensitive to ultraviolet radiation.
Uplifted portions of the dermis, dermal papillae, cause ridges and grooves to form on the outer surface of the epidermis that increase the friction needed to grasp objects or move across slick surfaces.
On hands and feet, these ridges form patterns of loops and whorls — fingerprints, palm prints, and footprints — that are unique to each person. You leave these imprints on smooth surfaces because of the oily secretions of the sweat glands on the skin’s surface.
In addition to these finer patterns, the areas around joints develop patterns called flexion lines. Deeper and more permanent lines are called flexion creases.