Brown Adipose Tissue - Anatomy & Physiology
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Introduction
At low ambient temperatures, the body must produce more heat than at warmer temperatures, just to maintain a stable core temperature. With time, the metabolic rate of animals continuously exposed to the cold will increase gradually without any increase in muscle activity. This is termed non-shivering thermogenesis and is primarily attributed to:
- increased activity of the sympathetic nervous system
- increased secretion of thyroid hormones
Physiology
Increased SNS tone leads to increased production and secretion of catecholamines from the adrenal medulla. This increases heat production through stimulation of many tissues including the liver. This is referred to as their calorigenic effect.
The effect of catecholamines is greatest in animals that have brown adipose tissue (BAT) or brown fat. BAT is mainly present in neonates (although not in piglets or birds) and is generally located around the scapulae and the kidneys. The cells of BAT have a much higher density of mitochondria than those of regular adipose tissue, and this is what gives BAT its brown colour. Activated BAT consumes vast amounts of oxygen and releases energy from the oxidation of fatty acids in the form of heat.
Heat production by BAT is particularly important in neonates as there is a significant decrease in the ambient temperature after that experienced in the uterus. Immediately post-partum, the heat loss is increased futher by the evaporation of the amniotic fluid from the body surface and therefore heat production from BAT is of great significance to the survival of the neonate.
BAT is lost with age and is rarely present in adults, although some can be found around the heart and thoracic vessels in adults which undergo hibernation every winter.