Brown adipose tissue

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Brown fat is a type of adipose tissue present in many newborn or hibernating mammals. In contrast to white adipocytes (fat cells), which contain a single, large fat vacuole, brown adipocytes contain several smaller vacuoles and a much higher number of mitochondria. Brown fat also contains more capillaries since it has a greater need for oxygen than most tissues.

Biochemistry

The mitochondria in a eukaryotic cell utilize fuels to produce energy (in the form of ATP). This process involves storing energy as a proton gradient, also known as the proton motive force (PMF), across the mitochondrial inner membrane and the energy is used to synthesis ATP when the protons return across the membrane through the ATP synthase enzyme. This model is known as the chemiosmotic hypothesis.

In endothermic animals, body heat is maintained by signalling the mitochondria to let protons run back along the gradient without producing ATP. This can occur since an alternative return route for the protons is through an uncoupling protein in the inner membrane. This protein, known as thermogenin, or uncoupling protein 1, facilitates the return of the protons after they have been actively pumped out of the mitochondria by the electron transport chain. This alternative route for protons uncouples oxidative phosphorylation and the energy in the PMF is released as heat.

To some degree all cells of endotherms give off heat, especially when body temperature is below a regulatory threshold, however, brown adiopose tissue is highly specialized for this non-shivering thermogenesis. Firstly, each cell has a higher number of mitochondria compared to more typical cells. Secondly, these mitochondria have a higher than normal concentration of thermogenin in the inner membrane.

Function in babies

In neonates (new born babies), brown fat, which then makes up about 5% of the body mass and is located on the back, along the upper half of the spine and towards the shoulders, is of great importance to avoid lethal cold (hypothermia is a major death risk for premature neonates). Numerous factors make infants more susceptible to cold than adults:

The higher ratio of body surface (proportional to heat loss) to body volume (proportional to heat production)
The higher proportional surface area of the head (which is seldom cloth-covered)
The low amount of musculature and the inability or reluctance to shiver
A lack of thermal insulation, e.g. subcutaneous fat and fine body hair (especially in prematurely born children)
The inability to move away from cold areas, air currents or heat-draining materials
The inability to use additional ways of keeping warm (e.g. turning up a heater, drying their skin, changing clothes or performing physical exercise)
The nervous system is not fully developed and does not respond quickly and/or properly to cold (e.g. by contracting blood vessels in the skin)

When growing up, most of the mitochondria (which are responsible for the brown color) in brown fat disappear, and the tissue becomes similar in function and appearance to white fat, as a mere fat deposit, though some adults do retain their brown fat.

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