Brown adipose tissue
315620
222789799
2008-07-01T03:20:37Z
StN
733580
'''Brown adipose tissue''' ('''BAT''') or '''brown fat''' is one of the two types of [[adipose tissue]] (the other being [[white adipose tissue]]) that is present in many newborn or [[hibernation|hibernating]] mammals.<ref name="pmid17956727">{{cite journal |author=Gesta S, Tseng YH, Kahn CR |title=Developmental origin of fat: tracking obesity to its source|journal=Cell |volume=131 |issue=2 |pages=242-56|year=2007 |url=http://www.ncbi.nlm.nih.gov/pubmed/17956727?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum}}</ref> Its primary function is to generate body heat. In contrast to white [[adipocytes]] (fat cells), which contain a single lipid droplet, brown adipocytes contain numerous smaller droplets and a much higher number of [[mitochondria]]. Brown fat also contains more [[capillaries]] than white fat, since it has a greater need for oxygen than most tissues.
==Biochemistry==
The mitochondria in a [[Eukaryote|eukaryotic]] [[cell (biology)|cell]] utilize fuels to produce [[energy]] (in the form of [[adenosine triphosphate|ATP]]). This process involves storing energy as a [[proton]] gradient, also known as the [[proton motive force]] (PMF), across the mitochondrial inner membrane. This energy is used to synthesise ATP when the protons flow across the membrane (down their concentration gradient) through the [[ATP synthase]] enzyme; this is known as [[chemiosmosis]].
In [[endotherm]]ic animals, body heat is maintained by signaling the mitochondria to allow protons to run back along the gradient without producing ATP. This can occur since an alternative return route for the protons exists through an [[uncoupling protein]] in the inner membrane. This protein, known as [[uncoupling protein 1]] (thermogenin), 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 adipose 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 infants==
In [[neonate]]s (new born infants), 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
* 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)
However, the burning of the brown fat will, in fact, provides a baby with an alternative form of heat induction. This form is an adequate substitute to any and all other unconscious means of an adult.
==Presence in adults==
When growing up, most of the mitochondria (which are responsible for the brown color) in brown adipose tissue disappear, and the tissue becomes similar in function and appearance to white fat - as a mere fat deposit. However, recent studies<ref name="pmid17473055">{{cite journal |author=Nedergaard J, Bengtsson T, Cannon B |title=Unexpected evidence for active brown adipose tissue in adult humans |journal=Am. J. Physiol. Endocrinol. Metab. |volume=293 |issue=2 |pages=E444–52 |year=2007 |month=August |pmid=17473055 |doi=10.1152/ajpendo.00691.2006 |url=http://ajpendo.physiology.org/cgi/pmidlookup?view=long&pmid=17473055}}</ref> using PET scanning of adult humans have shown that it is still present in many adults in the upper chest and neck. The remaining deposits become more visible (increasing tracer uptake) with cold exposure, and less visible if an adrenergic [[beta blocker]] is given before the scan.
==References==
{{reflist}}
==External links==
* {{BUHistology|04901lob}} - "Connective Tissue: multilocular (brown) adipocytes"
{{Connective tissue}}
[[Category:Tissues]]
[[ca:Teixit adipós marró]]
[[de:Braunes Fettgewebe]]
[[sv:Brunt fett]]