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Anatomy of the Skin

The three layers of your skin differ in function, thickness, and strength. The main layers of the skin are the Epidermis, and the Dermis or Corium, and the Subcutaneous Layer or Hypodermis.

The 1st Layer - Epidermis
The first layer of your skin, the outer layer, is called the epidermis. The epidermis is a tough protective layer of skin, protecting the body from the environment. This outermost layer is especially thick on the palms of your hands and the soles of your feet, being 1.5 mm thick (0.0591 inches), but only 0.05 mm (0.00197 inches) thick on the eyelids. The epidermis helps to prevent evaporation of water from the body and to protect the internal layers from harm. The epidermis contains the melanin-producing melanocytes. The epidermis is constantly being renewed, about every 26 to 42 days.

The epidermis is made up of several different types of cells. Two of these cell types are important in the study of skin cancer, keratinocytes and melanocytes.

• Keratinocytes
  The main cells that make up the epidermis are called keratinocytes. These cells are arranged into five sub-layers, or "strata."
  • 1) Stratum Corneum - cornified layer or horny layer - the part of your skin that you can see and feel. 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 stratum basale layer. Because skin cells are constantly being produced by the body, the stratum corneum is replaced by the body approximately every two weeks.
  • 2) Stratum Lucidum - translucent layer. 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.
  • 3) Stratum Granulosum - thin granular 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.
  • 4) Squamous Cell Layer - called stratum spinosum or "prickle-cell" layer. Cells in the stratum spinosum are larger and more flattened. When scientists prepare these cells to look under the microscope, they look like prickly, hence the name of this layer.
  • 5) Stratum Basale - the basal cell layer. 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.
• Melanocytes
  
  • 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. More about Ultraviolet Radiation

The 2nd Layer - Dermis or Corium
The second layer of your skin, the middle layer under the epidermis, is called the dermis. The dermis is much thicker than the epidermis. The dermis layer is made up of two sublayers, the papillary layer, and the reticular layer. The dermis contains:

• Blood vessels
• Lymph vessels
• Nerve endings
• Sweat glands
• Sebaceous glands (oil glands) - Sebaceous glands open into hair follicles and secrete oil called sebum. Sebum oils the skin and hair.
• Collagen and elastin
• Arrectores pilorum or arrector pilli - involuntary muscles are sometimes activated in cold weather to give you goose bumps
• Hair follicles, hair bulbs and hair roots
  • Papillary Layer - upper layer of the dermis.
    Regulates body temperature.
    Supplies epidermis with nutrient-filled blood.
  • A papillary/reticular dermal interface separates the papillary layer and the reticular layer.
  • Reticular Layer - lower layer of the dermis.
    In the reticular layer, hair follicles and sweat glands begin, and continue through the remaining layers to the surface.
    The reticular layer is made up of collagen that provides structure and elasticity to the skin.
    The reticular layer supports components of skin.

The 3rd Layer - Subcutaneous Layer or Hypodermis
The third layer of your skin, the deepest layer under the epidermis and dermis, is made up of fat cells. It is a fatty layer of subcutaneous tissue, known as the subcutis or hypodermis. It contains fats and connective tissue along with large blood vessels and nerves. It connects or binds the dermis above it to the underlying organs. Females have a hypodermis that is generally about 8 percent thicker than in males. The subcutaneous tissue also acts as a shock absorber. It protects the tissues below from injury. The functions of the hypodermis include storing of lipids, insulation, cushioning of the body and temperature regulation.

References:
1) SEER's Training Web Site - Surveillance, Epidemiology and End Results (SEER) - training.seer.cancer.gov
2) The Library of Congress - Research Centers - Science Reference Services - March 2007 - www.loc.gov
3) Centers for Disease Control and Prevention (CDC) - Division of Cancer Prevention and Control - National Center for Chronic Disease Prevention and Health Promotion
April 2007 - www.cdc.gov