Brown fat

Appearance, Structure and Distribution

Brown adipose tissue is sometimes mistaken for a type of gland, which it resembles more than white adipose tissue. It varies in color from dark red to tan, reflecting lipid content. Its lipid reserves are depleted when the animal is exposed to a cold environment, and the color darkens. In contrast to white fat, brown fat is richly vascularized and has numerous unmyelinated nerves which provide sympathetic stimulation to the adipocytes.

Brown fat is most prominent in newborn animals. In human infants it comprises up to 5% of body weight, then diminishes with age. Substantial quantities of brown adipose tissue can be detected in adult humans using positron-emission tomography, especially when the individuals are exposed to cold temperatures. Most of this tissue in adults is located in the lower neck and supraclavicular region. Intriguingly, there is an inverse correlation between the amount of brown adipose tissue and body mass index, with obese individuals having significantly less of the tissue than lean individuals; this suggests that brown fat may be an important factor in maintaining a lean phenotype.

A good place to observe brown fat is in mice, where it persists into adulthood. Dissection of a mouse will reveal two large, lobulated masses of brown fat on the dorsal aspect of the thorax, between the scapulae. Masses of brown fat are also