Subcutaneous tissue

The subcutaneous tissue (Template:Etymology), also called the hypodermis, hypoderm (Template:Etymology), subcutis, or superficial fascia,<ref>Template:Cite web</ref> is the lowermost layer of the integumentary system in vertebrates.<ref>Template:Cite book</ref> The types of cells found in the layer are fibroblasts, adipose cells, and macrophages. The subcutaneous tissue is derived from the mesoderm, but unlike the dermis, it is not derived from the mesoderm's dermatome region. It consists primarily of loose connective tissue and contains larger blood vessels and nerves than those found in the dermis. It is a major site of fat storage in the body.

In arthropods, a hypodermis can refer to 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.

Structure

• Fibrous bands anchoring the skin to the deep fascia<ref name=mcminn2003 />
• Collagen and elastin fibers attaching it to the dermis<ref name=loreal />
• Fat is absent from the eyelids, clitoris, penis, much of pinna, and scrotum<ref name=ta98 />
• Blood vessels on route to the dermis<ref name=tamarkin />
• Lymphatic vessels on route from the dermis<ref name=dartmouth />
• The glandular part of some sweat glands; mammary glands lie entirely within the subcutaneous tissue<ref name=fischer2006 /> (which are modified apocrine sweat glands)<ref name=krstic04>Template:Cite book</ref>
• Cutaneous nerves<ref name=dartmouth /> and free endings
• Hair follicle roots
• Ruffini<ref name=kenneth2007 />Template:Rp and Pacinian corpuscles
• Mast cells<ref name=goldsmith>Template:Cite book</ref>
• 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

The hypodermis forms an important insulating layer and/or food store in some animals, such as whales and hibernating mammals.

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.

Subcutaneous fat

Two computed tomography images of a horizontal section of the mid-abdominal region; one from a normal-weight individual and the other from an obese person. In both, the bony structures and organs appear similar. The primary difference is that in the normal-weight person, there is little subcutaneous fat, and the obese person shows substantially subcutaneous fat.

Cross-sections of the torso of a person of normal weight (left) and an obese person (right), taken by CT scan. Note the 3.6 cm (1.4 inches) of subcutaneous fat on the obese person.

Subcutaneous fat is the most widely distributed subcutaneous tissue layer.<ref name=ta98/> It is composed of adipocytes, which are grouped together in lobules separated by connective tissue.<ref name=loreal /> The number of adipocytes varies among different body areas, while their size varies according to the body's nutritional state.<ref name=mesh /> It acts as padding and as an energy reserve, as well as providing some minor thermoregulation via insulation.<ref name=kenneth2007 /><ref name=loreal /> Subcutaneous fat is found just beneath the skin, as opposed to visceral fat, which is found in the peritoneal cavity,<ref name=harvard2006 /> and can be measured using body fat calipers to give a rough estimate of total body adiposity.<ref name=orphanidou1994 />

Clinical significance

Injection

Template:Main Injection into the subcutaneous tissue is a route of administration used for drugs such as insulin: because it possesses few blood vessels, the tissue absorbs drugs slowly.<ref name=kenneth2007 />Template:Rp 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 of limited blood flow. "Skin popping" is a slang term that includes this method of administration and is usually used in association with recreational drugs.

Disease

• Subcutaneous abscess
• Subcutaneous tumor

See also

• Dermis
• Epidermis