This MedLibrary.org supplementary page on Brown fat is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:
Related Sponsors
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 hibernating mammals.1 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.
Contents |
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. 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 endothermic 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. First, each cell has a higher number of mitochondria compared to more typical cells. Second, these mitochondria have a higher-than-normal concentration of thermogenin in the inner membrane.
Function in infants
In neonates (newborn 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 toward 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).
The burning of brown fat provides a baby with an alternative means of heat regulation.
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 studies2 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
- ^ Gesta S, Tseng YH, Kahn CR (October 2007). "Developmental origin of fat: tracking obesity to its source". Cell 131 (2): 242–56. doi:. PMID 17956727.
- ^ Nedergaard J, Bengtsson T, Cannon B (August 2007). "Unexpected evidence for active brown adipose tissue in adult humans". Am. J. Physiol. Endocrinol. Metab. 293 (2): E444–52. doi:. PMID 17473055, http://ajpendo.physiology.org/cgi/pmidlookup?view=long&pmid=17473055.
External links
- Histology at BU 04901lob - "Connective Tissue: multilocular (brown) adipocytes"
|
||||||||||||||
Wikipedia content modification information:
- This page was last modified on 3 November 2008, at 18:24.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Brown fat".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.