Subcutaneous Tissue: The Innermost Layer of Skin

Diagram of human skin

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Subcutaneous tissue, which is also known as the hypodermis, is the innermost layer of skin. It's made up of fat and connective tissues that house larger blood vessels and nerves, and it acts as an insulator to help regulate body temperature. The thickness of this subcutaneous layer varies throughout the body and also from person to person.

Subcutaneous Tissue Composition

The skin is composed of three layers: the epidermis, the dermis, and subcutaneous tissue. There are several structures and specialized cells that exist within subcutaneous tissue, including:

  • Collagen and elastin fibers (These attach the dermis to muscles and bones.)
  • Fat cells
  • Blood vessels
  • Sebaceous glands
  • Nerve endings
  • Hair follicle roots

The hypodermis is largely composed of adipose tissue (fat tissue), which is made up of adipocytes, or fat cells. The amount of adipose tissue varies throughout the body. It is thickest in the buttocks, the palms of the hands, and the soles of the feet. The size of adipocytes is determined by an individual's nutritional habits. Generally speaking, a person that maintains a healthy diet and exercise habits has smaller adipocytes and is less likely to be overweight.

The location and thickness of subcutaneous tissue differ by gender. Men tend to accumulate more around the abdomen and the shoulders, while women tend to accumulate it around the thighs, the hips, and the buttocks.

Functions of Subcutaneous Tissue

The epidermis' adipose tissue acts as an energy reserve. Once the body uses up energy that's acquired from consuming carbohydrates, it turns to adipose tissue as a fuel source, which can lead to weight loss. Adipocytes can swell or shrink depending on whether the fat is being stored or used. The hormone leptin is secreted by fat cells to help control appetite and signal when you are full.

Additionally, this fat acts as "armor" that protects muscles, bones, organs, and more delicate tissues. Think of subcutaneous tissue as the protective gear that athletes such as football and ice hockey players wear. It's the body's natural padding, cushioning and protecting your bones and muscles if you fall or take a hit.

Subcutaneous tissue also regulates body temperature by making sure that your internal temperature isn't too high or too low. The hypodermis essentially insulates the body, allowing you to go outside on a cold day without getting hypothermia (a dangerous condition that, if allowed to go on for too long, can result in freezing to death).

Subcutaneous Injection

Since subcutaneous tissue contains blood vessels, medications can be immediately absorbed, but its high-fat content also allows medications to be absorbed gradually over time. This makes them an ideal route for many drugs. That's why may medications are injected into the hypodermis. Examples of medications which may be given by subcutaneous (subQ) injection include epinephrine for allergic reactions, some vaccinations, insulin, some fertility drugs, some chemotherapy medications, morphine, growth hormone, and anti-arthritis drugs. Parts of the body that have greater concentrations of subcutaneous tissue make them ideal injection sites. These include:

  • The outer part of the upper arm
  • The middle part of the abdomen
  • The front of the thigh
  • The upper back
  • The upper part of the buttocks

The Effect of Age on Subcutaneous Tissue

As you age, subcutaneous tissue starts to thin out. This weakened layer of insulation makes the body more sensitive to the cold because less tissue makes it harder to stay warm.

The loss of subcutaneous tissue due to aging also causes the body to sweat less, which, in turn, makes it harder to stay cool in warm weather. It can also affect the body's reaction to certain medications that are absorbed by subcutaneous tissue.

While the hypodermis is not visible, it can have a dramatic effect on the appearance of the skin and the way aging impacts the skin, specifically in the area of the face and neck. With aging, the volume of facial fat decreases and there is less supportive tissue to support the normal elasticity of the skin. The facial skin begins to droop and sag; the bones and muscles of the face also lose volume. Some people may choose to receive chemical cosmetic fillers to "plump up" the skin in these areas.

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