The skin is the largest body organ and an important one. It protects against microbes, chemicals, ultraviolet rays, cuts, and tears. It has sensory functions, which is another way to say it "feels." The skin also helps control body temperature through blood flow and sweat.
The skin has two layers. The outer layer is called the epidermis. ("Epi" means "above.") The inner layer is called the dermis.
The epidermis is made up of several different types of cells. Two of these cell types are important in studying skin cancer: keratinocytes and melanocytes.
The main cells that make up the epidermis are called keratinocytes. These cells are arranged into five layers, or "strata."
The basal layer is the deepest layer. Cells in this layer can divide and reproduce themselves. They are the most immature, or least specialized, of the keratinocytes. These cells mature and become more specialized as they move toward the surface of the skin.
Cells in the stratum spinosum are larger and more flattened. [Interesting tidbit: When scientists prepare these cells to look under the microscope, they look like prickly, hence the name of this layer.]
Cells in the stratum granulosum are still larger and more mature than cells of the stratum spinosum. They contain the ingredients for the protein keratin.
Lucid (clear) is how this layer looks under the microscope. This layer is present only in thickened areas such as the sole of the feet.
The stratum corneum is the outermost (top) layer. It has the flattest cells, arranged in a basket weave pattern. Keratin in these cells helps the cornified layer protect against moisture, light, and infection. The keratin works together with lipids (fats) and tight inter-cellular connections called "desmosomes." The cells of the stratum corneum are the most specialized of the keratinocytes. These cells are continually sloughed off the surface of the skin and replaced by cells that mature from the basal layer.
Melanocytes are another kind of cell found in the epidermis. These cells are in the basal layer. Melanocytes make the pigment called melanin. The melanin is then transferred to nearby keratinocytes. Some of this melanin then works its way up with the keratinocytes as they move to the top layer.
Skin color depends on how much melanin is made and how much is carried toward the surface. All people have about the same number of melanocytes, no matter what their skin color. Those with darker skin have more melanin in the upper layers of the skin. Sunlight raises the rate at which melanin is made and transferred. This is what causes tanning.
Melanin can protect the cells from ultraviolet (UV) radiation and its harmful effects. It does this by absorbing harmful UV rays. It also cleans up toxins that come from UV damage to skin cells. Within cells, melanin tends to form caps above the nuclei. The caps protect the cells' genetic material from UV damage.
The dermis lies under the epidermis and is much thicker than the epidermis. It is made of loosely arranged connective tissue. The surface of the dermis is covered with rows of projections much like fingers. These are called dermal papillae. They help hold the epidermis and dermis together by forming ridges and grooves.
The lower part of the dermis is a network of interlacing fibers. These fibers give the skin strength and allow it to stretch. The dermis contains a lot of blood vessels, nerve endings, muscle fibers, sweat and oil (sebaceous) glands, and hair follicles. The roots of hairs are located deep in follicles. Hair shafts stick out from the follicles. Small muscles raise and lower the hairs. These muscles extend from the dermis to the side of the hair follicles. Sebaceous glands open into hair follicles and secrete oil called sebum. Sebum oils the skin and hair.
Subcutaneous tissue lies under the dermis. It is a thick layer of connective tissue and fat. The fat helps control body temperature and stores energy from food.
tissue also acts as a shock absorber. It protects the tissues below from
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skin under a microscope go to: http://www.meddean.luc.edu/lumen/MedEd/
Click on "Anatomy and Histology of the Skin."
Step 2: On the list at the left, click on "II. Epidermis."
Step 3: Click on each layer of the epidermis. Note where it is on a real microscopic section of skin.
Step 4: Continue through the layers of the skin. Look at the slides to learn more about the epidermis, dermis, and skin appendages.
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This page last reviewed April 24, 2007
United States Department of Health Human Services