epithelial cells. From the outside in, these layers are the following:
Stratum corneum epidermis layers (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 epidermis layers (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 epidermis layers 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 epidermis layers(also sometimes called the spinous layer) has ten layers containing prickle cells, named for the spine-like 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 epidermis layers(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’s 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.
Ridges and grooves form on the outer surface of the epidermis to 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.
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