Brown fat was recognized in hibernating animals 300 years ago and was thought at first to be an organ like the thymus gland, later an endocrine organ, and still later was identified as an adipose tissue. It has also been found in nonhibernating rodents, in rats and mice, and is recognized microscopically in these animals by having the fat divided into numerous tiny droplets within the cells, instead of large oily cetaceous masses. It is grossly brown in color and is found in characteristic areas—in the neck, about the kidneys, and in mediastinal masses running along sympathetic nerve chains.
Brown fat also occurs in the human embryo at an early stage but is later converted or transformed into or overrun by the usual white or yellow variety.
Evidence has been increasing that the brown fat of the human embryo actually persists in the newborn baby. In a recent study of
THE VIRTUES OF HIBERNATION. JAMA. 1966;198(13):1363. doi:10.1001/jama.1966.03110260075024