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|The hypodermis is the lower layer of skin shown in the diagram above|
The hypodermis, also called the hypoderm, subcutaneous tissue, subcutis, or superficial fascia, is the lowermost layer of the integumentary system in vertebrates. Hypoderm and subcutaneous are from Greek and Latin words, respectively, for "beneath the skin." Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages. It is derived from the mesoderm, but unlike the dermis, it is not derived from the dermatome region of the mesoderm. In arthropods, the hypodermis is an epidermal layer of cells that secretes the chitinous cuticle. The term also refers to a layer of cells lying immediately below the epidermis of plants.
The hypodermis is used mainly for fat storage.
A layer of tissue lies immediately below the dermis of vertebrate skin. It is often referred to as subcutaneous tissue though this is a less precise and anatomically inaccurate term. The hypodermis consists primarily of loose connective tissue and lobules of fat. It contains larger blood vessels and nerves than those found in the dermis.
Specifically, the hypodermis contains:
- Fibrous bands anchoring the skin to the deep fascia
- Fat, except in the eyelid, scrotum, penis, nipple and areola
- Blood vessels on route to the dermis
- Lymphatic vessels on route from dermis
- Hair follicle roots
- The glandular part of some sudiferous glands
- Nerves: all nerves identified as "cutaneous"
- Nerves: free endings and Pacinian corpuscles
- Bursae, in the space overlying joints in order to facilitate smooth passage of overlying skin
- Fine, flat sheets of muscle, in certain locations, including the scalp, face, hand, nipple, and scrotum, called the panniculus carnosus
In some animals, such as whales and hibernating mammals, the hypodermis forms an important insulating layer and/or food store.
In some plants, the hypodermis is a layer of cells immediately below the epidermis of leaves. It is often mechanically strengthened, for example, in pine leaves, forming an extra protective layer or a water storage tissue.
According to the eighth edition of Developmental Biology by Scott F. Gilbert, the hypodermis of C. elegans is derived from ectoderm.
Subcutaneous fat 
Subcutaneous fat is found just beneath the skin, as opposed to visceral fat, which is found in the peritoneal cavity. Subcutaneous fat can be measured using body fat calipers giving a rough estimate of total body adiposity. This fat aids in the process of homeostasis, by forming a layer of insulation to slow heat loss.
Injection into the subcutaneous tissue is a route of administration used for drugs such as insulin. Subcutaneous injection is believed to be the most effective manner to administer some drugs, such as human growth hormones. Just as the subcutaneous tissue can store fat, it can also provide good storage space for drugs that need to be released gradually because there is limited blood flow. "Skin popping" is a slang term that includes this method of administration, and is usually used in association with recreational drugs.
See also 
- Mosby’s Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book 1994, pp. 998, 774, 1497
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