hipodermis

Hipodermis, more commonly known as the hypodermis, refers to the subcutaneous tissue located beneath the dermis of the skin. In humans and many mammals it is largely composed of adipose (fat) tissue intermingled with loose connective tissue, and it contains larger blood vessels and nerves that supply the skin. The thickness of the hipodermis varies by body region, age, sex, and overall body fat.

The primary functions of the hipodermis are energy storage, insulation, and cushioning. Adipose tissue stores lipids that can be mobilized for energy, while the fat layer helps insulate the body against heat loss and provides mechanical protection for underlying muscles and bones. The hipodermis also anchors the skin to deeper structures via fibrous septa that connect to the deep fascia, allowing some movement of the skin while keeping it in place.

Clinically, the hipodermis is the usual site for subcutaneous injections, which are administered into this fatty layer to deliver medication with slower, longer-lasting absorption than intramuscular routes. Its composition and distribution can affect injection depth and absorption. Conditions affecting the hipodermis, such as obesity, lipodystrophy, or cellulite, reflect variations in adipose tissue arrangement and connective tissue septa within this layer.

Development and histology: the hipodermis is derived from mesoderm, with adipogenesis generating adipocytes within lobules separated by connective tissue septa. Histologically, it comprises adipose tissue, loose connective tissue, blood vessels, and nerve fibers, sometimes referred to as the superficial fascia in some anatomical classifications.