The hypodermis is the lowest of three layers of the skin. It is virtually the supply and support station of the entire skin with a number of other important functions.
What is the hypodermis?
Schematic diagram showing the anatomy and structure of the skin. The skin is a sensitive organ. Daily care and medical precautions help against skin aging and skin diseases. Click to enlarge. The hypodermis, also called the subcutis, is the lowest of three layers that make up the skin. It practically forms the link between the body and the skin layers above it, the dermis and the epidermis. The main components of the subcutis are loose connective tissue and embedded fat cells (lipocytes). The subcutis is clearly demarcated from the body by fascia, which consists of firm collagenous fibers and envelops the entire body.
Anatomy and structure
The hypodermis or subcutis reaches a thickness of 0.5 mm to 30 mm, depending on its location on the body, consists mainly of loose connective tissue, and provides for the displaceability of the skin with respect to the underlying body surface. Connective tissue strands from the overlying dermis pass through the subcutis and form a firm connection with collagenous tissue (fascia) of the body below the subcutis. They provide a limit to the displaceability of the subcutis so that it moves back to its original position after the mechanical pushing stimulus has ended. Arterial and venous blood vessels and nerve cords run in the subcutis, each with a multitude of branches into the dermis. Likewise, the subcutis houses fat cells (adipocytes) as energy storage, “shock absorbers” and cold protection. So-called Vater-Pacinian tactile corpuscles are distributed in large numbers in the subcutis. These are receptors for mechanical stimuli that react particularly sensitively to vibrations and report them back to the brain. Smooth muscle cells are also found in a few areas of the subcutis, such as the scrotum, labia, and nipples.
Function and tasks
The subcutis acts as a link between the body and the dermis above the subcutis and performs a variety of tasks. The arterial blood vessels supply oxygen and nutrients to the overlying layers of skin. The venous vessels carry away carbon dioxide and other “waste products”. The embedded nerve cords collect impulses from the various mechano- and temperature receptors in the dermis and transmit them to the brain. Likewise, stimuli from the Vater-Pacinian tactile corpuscles placed in the subcutis itself are transmitted to the brain via nerve cords. The tactile corpuscles are particularly sensitive to vibrational stimuli. On the one hand, the stored fat in the adipocytes serves as a chemical energy store in the event that no food is available for a longer period of time. On the other hand, it also serves as a mechanical buffer for pressure shocks or blows from outside, but also, for example, as a natural “gel cushion” in the sole of the foot that softens mechanical impacts on the spine during walking and running. The fat also performs the function of thermal insulation and can provide effective protection against cold. It is continuously stored as a reserve “for a rainy day” when excess – unneeded – energy has been extracted from the ingested food in the digestive tract. The connective tissue strands that connect the overlying dermis to the underlying fasciae of the body provide a limit to the displaceability and prevent the dermis and epidermis from easily detaching from the hypodermis. They are, in effect, the tethers of the skin.
Diseases and ailments
The embedded fat cells in the subcutis can suddenly grow and proliferate for reasons that are not yet well understood, forming lipomas (lipomatosis). These are benign solid fat tissue nodules that can range in size from a few millimeters to several centimeters. Lipomas often occur in fatty tissue in the thighs, abdomen, upper arms and neck and are usually harmless and painless. However, if they are pressing on nerves or blood vessels or are a serious cosmetic problem, they can be removed by liposuction (liposection) or with surgery. Lipomas are prone to recurrence, which then requires another procedure. In relatively rare cases, subcutaneous fat cells can be the starting point for a liposarcoma.This is a malignant tumor, the cause of which is not clear. The starting point may be, for example, a previous injury or ionizing radiation. Liposarcoma can be surgically removed and treated with chemotherapy. There is obviously no connection with the harmless lipomas. Mechanical injury to the subcutaneous tissue or damage from intense cold or heat stimuli causes necrosis of the affected fat cells, which can result in inflammation of the subcutaneous tissue. The dead lipocytes then tend to sclerose, which means that hardened connective tissue forms in their place, a kind of internal scar. Edema, accumulations of tissue fluid in the subcutis, may develop from various causes and regress when the cause is removed.
Typical and common skin diseases
- Vitiligo (white spot disease).
- Skin rash
- Skin fungus
- Rosacea (rosacea)
- Systemic lupus erythematosus (SLE)
- Skin cancer
