The Skin: 7 Most Important Layers and Functions

The skin serves as the first line of defense against infectious agents and substances that may be harmful to the body. There are seven layers of skin and each layer serves different functions.

The skin is the largest organ in the body and it covers the body's entire external surface. It is made up of seven layers ([starting from the top layer down to the bottom [deepest] layer):

1. Stratum corneum
2. Stratum lucidum
3. Stratum granulosum
4. Stratum spinosum
5. Stratum basale 
6. Dermis
7. Hypodermis

The first five layers form the epidermis, which is the outermost, thick layer of the skin.

All seven layers vary significantly in their anatomy and function.

The skin serves various functions that include

• acting as the body’s initial barrier against germs,
• UV light,
• chemicals, and
• mechanical injury.
• It also maintains body temperature and prevents water loss from the body.

The first five layers form the epidermis, which is the outermost, thick layer of the skin.

The functions of the five layers of the epidermis are:

Stratum corneum:

• This is the topmost layer of the skin and is made up of keratin.
• This layer varies greatly in thickness in various regions of the body when compared to the other layers.
• This layer plays the first line of defense against external agents.

Stratum lucidum: This thin clear layer is only present in the thicker skin (palms and soles).

Stratum granulosum: The chemicals (glycolipids) secreted in this layer keep the skin cells glued to each other.

Stratum spinosum (also known as a prickle cell layer): This layer contains dendritic cells, which are the skin’s first line defenders.

Stratum basale (Stratum germinativum):

• This is the deepest layer of the epidermis.
• The cells found in this layer are constantly producing keratinocytes, which play an important role in the formation of Vitamin D with exposure to sunlight. Keratinocytes also produce protein, keratin, and lipids; these act as a protective barrier.
• This layer also contains melanocytes. Melanocytes produce melanin, which is a natural pigment responsible for the color of the skin.

The dermis is connected to the epidermis and is made of collagen (a type of connective tissue), which gives the skin its flexibility and strength. It also houses the sweat glands, oil glands (sebaceous glands), hair, hair follicles, muscles, nerve endings, blood vessels, and dendritic cells. The function of each of these components are as follows

• The nerve endings sense pain, touch, pressure, and temperature.
• The sweat glands produce sweat with exposure to heat and stress. As sweat evaporates off the skin, it helps cool the body.
• The sebaceous glands secrete sebum into the skin. Sebum is an oily substance that keeps the skin moist and soft and acts as a barrier against foreign substances.
• The hair follicles produce the various types of hair found throughout the body. Hair regulates body temperature and provides protection against external injury.
• The blood vessels of the dermis provide nutrients to the skin and help regulate body temperature.

The hypodermis is the deepest layer of skin situated below the dermis. It is also called the subcutaneous fascia or subcutaneous layer.

• It contains fat along with some structures like hair follicles, nerve endings, and blood vessels.
• The presence of fat helps insulate the body from heat and cold and serves as an energy storage area.

Skin thickness varies considerably all over the body.

• The palms of the hands and soles of the feet have the thickest skin because the epidermis contains an extra layer, the stratum lucidum, that is absent in other regions.
• The thinnest skin is found on the eyelids and behind the ear (postauricular) region (0.05 mm thick).
• Male skin is characteristically thicker than female skin.
• Children have relatively thin skin, which progressively thickens until 40 to 50 years of age, and then it begins to get thinner.