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The deepest skin layer, made largely of fat, that cushions, insulates, stores energy, and anchors skin to underlying tissue.
Medically reviewed & updated
The hypodermis, also called the subcutaneous tissue or subcutis, is the deepest of the three main skin layers, lying beneath the dermis. Though sometimes considered separate from the skin proper, it is integral to how skin functions. It is composed mainly of fat (adipose tissue) interspersed with loose areolar connective tissue, and it forms the transition between the skin and the muscles, fascia, and bone underneath.
The hypodermis is built from lobules of adipocytes (fat-storing cells) separated by fibrous septa of connective tissue that carry blood vessels and nerves. Its thickness varies dramatically by body region, sex, and nutritional state: it is thick over the abdomen, buttocks, and thighs and thin over the eyelids and back of the hands. Running through this layer are larger blood vessels, lymphatic vessels, sensory neurons (including Pacinian corpuscles that detect deep pressure and vibration), and the bases of hair follicles and sweat glands that originate higher up.
The hypodermis serves several roles that depend on its fat content. It is a major energy reserve, storing calories as triglycerides. It provides thermal insulation, helping conserve body heat, and acts as a shock absorber that cushions bones and internal organs from impact. Mechanically, it connects the skin to deeper structures while allowing the skin to glide and move freely over muscle. Adipose tissue here is also metabolically active, secreting hormones such as leptin that help regulate appetite and energy balance.
The hypodermis is the target layer for subcutaneous injections, including insulin and many vaccines, because its fat allows slow, steady absorption. Loss of subcutaneous fat (lipoatrophy) and abnormal fat distribution (lipodystrophy) can result from disease or medications. Inflammation of this layer is called panniculitis, while cellulitis can spread through it. Excess subcutaneous fat contributes to obesity-related health risks. This information is educational and not medical advice.