What Is the Hypodermis?

The hypodermis is the bottom layer of skin. Also known as subcutaneous tissue, the hypodermis insulates and protects the body, stores energy (in the form of fat), helps to regulate body temperature, and connects the skin to muscles and bones.

The hypodermis is one of the three layers of human skin, the others being the epidermis (outer layer) and dermis (middle layer). Together, these layers provide a barrier against fluids, infection, and trauma.

This article discusses the hypodermis layer of the skin. It explains the anatomy and function of the subcutaneous tissue. It also covers potential health conditions and the effect of aging on the hypodermis.

Anatomy of the Hypodermis

The hypodermis is the innermost or subcutaneous layer of the skin. Most of the body's fat is stored in this layer. It provides insulation, protection, temperature regulation, and connection between the bones and muscles.

Location

The hypodermis is the innermost layer of the skin located under the dermis (outer layer) and the epidermis (middle layer). The thickness of the hypodermis varies in different regions of the body and can vary considerably between different people.

The hypodermis layer also provides shaping and contouring. For those assigned male at birth, the hypodermis is thickest in the abdomen and shoulders. Whereas the hypodermis for those assigned female at birth is generally thickest in the hips, thighs, and buttocks.

Structure

The hypodermis is a complex structure composed of different cells, tissues, glands, and vessels that work together to protect the body and ensure that it functions normally.

The components of the hypodermis include:

• Fibroblasts: This is a type of cell that produces collagen, the primary building block of skin, muscles, bones, tendons, ligaments, cartilage, and hair.
• Adipose tissue: Also known as body fat, these are fatty tissues found under the skin (subcutaneous fat), around organs (visceral fat), in the breasts, and between muscles.
• Connective tissue: These are dense, fibrous tissues made up of collagen and elastin that supports, protects, and gives structure to other tissues and organs in the body. 
• Blood vessels: These are the arteries, capillaries, and veins that deliver blood and oxygen to vital organs and remove waste products.
• Lymphatic vessels: These vessels help regulate fluid levels in the body, receive waste products from tissues, and transport a fluid called lymph that defends the body against infection.
• Hair follicles: These tube-like structures house each hair strand and extend into the hypodermis where the hair root is located.
• Sweat glands: These tiny organs secrete sweat to keep the body at a normal temperature (98.6 F) whenever it is overheated.
• Nerves: Large nerves pass through the hypodermis to the surface of the skin, including sensory nerves that register pain, temperature, and pressure and enable proprioception (our perception of our body's position in space).
  

Function of the Hypodermis

The hypodermis serves several vital functions in the human body. These include:

• Fat and energy storage: Fat cells (adipocytes) that make up the adipose tissue store energy for the body. The hypodermis also helps to create hormones such as estrogen and leptin.
• Protecting the body: The fat in the hypodermis acts like padding or a shock absorber that protects the bones, muscles, and organs from cold, trauma, or impact.
• Regulating body temperature: The hypodermis acts as an insulator by trapping or conserving heat, offering protection against the cold. It also protects against heat through sweating.
• Attaching the skin to muscle and bone: The hypodermis contains connective tissue which connects the skin to bones, muscles, and organs.
  

Connective tissues in the hypodermis also support structures such as nerves and blood vessels.

Associated Conditions

The following are medical disorders and procedures related to this unique layer of the skin.

Hypothermia and Overheating

The hypodermis is essential for body temperature regulation. It traps heat, protects you from the cold, and causes sweating, protecting you from the heat.

With age, the hypodermis thins. This is one of the reasons that older people are more prone to hypothermia. Thinning of the hypodermis may also mean that you sweat less. Lack of sweating can lead to health conditions such as heat exhaustion and heatstroke.

Obesity

Body fat in the hypodermis layer is called subcutaneous adipose tissue (SAT). This is different than visceral adipose tissue (VAT) that lines internal organs. Excess fat in either area leads to obesity.

Both types of fat have received a lot of attention in recent years due to the growing rate of obesity. Studies note that not all body fat is equal, at least with respect to its role in metabolic syndrome and heart disease.

Bedsores

Bedsores, otherwise known as pressure ulcers, occur when sustained pressure on the skin causes open sores that can extend deep into the hypodermis. Bedsores are most common in people who are bedbound or consistently use a wheelchair.

Bedsores are difficult to treat and can lead to complications if not treated aggressively. This includes bone infection, autonomic dysreflexia (overactive involuntary nerves), and sepsis (a potentially life-threatening response to a whole-body infection).

Panniculitis

Panniculitis refers to inflammation of subcutaneous fat. It is an uncommon condition that causes hardened nodules or plaques that you can feel and sometimes see.

Panniculitis commonly affects the shins and calves before spreading to the thighs and upper body. It usually clears within six weeks, leaving no scars. Swelling, redness, bruising, and joint pain are common.

The causes of panniculitis are many and include:

• Severe viral or bacterial infections
• Inflammatory diseases like inflammatory bowel disease (IBD), lupus, and rheumatoid arthritis
• Certain antibiotics or birth control pills
• Sarcoidosis (a rare disease that causes granular lumps in tissues)
• Blood cancers like leukemia or lymphoma 

Third-Degree Burns

Third-degree burns are those that destroy the epidermis and dermis and expose the hypodermis. This can lead to the potentially life-threatening loss of fluid and heat, depending on how extensive the burn is. It also increases the risk of severe bacterial infection and permanent nerve damage.

Third-degree burns are not only caused by fire or highly heated fluids or objects but also by the sun. Third-degree sunburns are no less serious than third-degree burns from fire.

Third-degree burns can appear dry and leathery with a combination of red, white, or blackened tissues. You may also see the yellowish adipose tissues peeking through. Because many nerve endings will have been destroyed, third-degree burns aren't usually painful to the touch.

Skin Cancer

Skin cancers (including melanoma, squamous cell carcinoma, and basal cell carcinoma ) typically start in the epidermis where sun exposure is greatest but can migrate to the underlying dermis and hypodermis.

One exception is soft tissue sarcomas which originate in the body's soft tissues. These include tissues like muscle, fat, blood vessels, nerves, tendons, and cartilage. Soft tissue sarcoma can happen anywhere in the body but most often involves the arms, legs, and abdomen.

The Hypodermis and Aging

While the hypodermis is not visible, it can dramatically affect the appearance of the skin. This is due to 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 turgor or elasticity of the skin.

As a result, the facial skin begins to droop and sag, resulting in a look that can be interpreted as appearing tired. The bones and muscles of the face also lose volume.

The Hypodermis and Injections

Medications can be injected into different areas of the body, such as into a vein (intravenous injections), into a muscle (intramuscular injections), under the dermis (intradermal injections), or into fatty tissue (subcutaneous injections).

Medications given by subcutaneous injection are absorbed more slowly than drugs given by intravenous injection. This makes them ideal for many drugs, especially those that are self-administered.

Examples of medications that may be given by subcutaneous injection include:

• Insulin
• Blood thinners
• Some vaccinations
• Some fertility drugs
• Some chemotherapy medications
• Human growth hormone (HGH)
• Some anti-arthritis drugs such as Enbrel (etanercept)

Some medications can only be given through one route, while others can be given through multiple routes. It depends on the type and goal of the medication and how it's best absorbed in the body.

Summary

The hypodermis is the innermost layer of the skin. It stores fat and energy, pads and protects the body, attaches skin to the bones and muscles, and is very important in maintaining body temperature. This layer can be used for injections with some types of medication.

The hypodermis provides shaping and contour. The thickness varies per person, with excess fat in this layer leading to obesity. This layer of the skin thins with age, increasing the risk of hypothermia or heat exhaustion.