Brown adipose tissue
Encyclopedia
Brown adipose tissue or brown fat is one of two types of fat or adipose tissue
(the other being white adipose tissue
) found in mammals.
It is especially abundant in newborns and in hibernating
mammals. Its primary function is to generate body heat in animals or newborns that do not shiver. 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, which contain iron and make it brown. Brown fat also contains more capillaries than white fat, since it has a greater need for oxygen than most tissues.
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 warm-blooded
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 instead 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.
is a major death risk for premature neonates).
Numerous factors make infants more susceptible to cold than adults:
Heat production in brown fat provides a baby with an alternative means of heat regulation.
Further, recent studies using Positron Emission Tomography
scanning of adult humans have shown that it is still present in adults in the upper chest and neck. The remaining deposits become more visible (increasing tracer uptake, that is, more metabolically active) with cold exposure, and less visible if an adrenergic beta blocker
is given before the scan. The recent study could lead to a new method of weight loss
, since brown fat takes calories from normal fat and burns it. Scientists were able to stimulate brown fat growth in mice, but human trials have not yet begun.
In rare cases, brown fat, instead of involuting, continues to grow; this leads to a tumour known as a hibernoma
.
Brown fat cells and muscle cells both come from the middle embryo layer. The three layers of the embryo during the gastrulation stage are ectoderm, mesoderm, endoderm. Mesoderm is the source of myocytes (muscle cells), adipocytes, and chondrocytes (cartilage cells). Adipocytes give rise to white fat cells and brown fat cells.
Researchers found that both muscle and brown fat cells expressed the same muscle factor Myf5, whereas white fat cells did not. This suggested that muscle cells and fat cells were both derived from the same stem cell. Furthermore, muscle cells that were cultured with the transcription factor PRDM16 were converted into brown fat cells, and brown fat cells without PRDM16 were converted into muscle cells.
However, there may be two types of brown fat cells—with and without Myf5. The other type, without Myf5, may share the same origin as white fat cells. They both seem to be derived from pericytes, the cells which surround the blood vessels that run through white fat tissue.
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
(the other being white adipose tissue
White adipose tissue
White adipose tissue or white fat is one of the two types of adipose tissue found in mammals. The other kind of adipose tissue is brown adipose tissue....
) found in mammals.
It is especially abundant in newborns and in hibernating
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
mammals. Its primary function is to generate body heat in animals or newborns that do not shiver. 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, which contain iron and make it brown. 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 eukaryoticEukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
utilize fuels to produce energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
(in the form of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
). This process involves storing energy as a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
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
ATP synthase
right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...
enzyme; this is known as chemiosmosis
Chemiosmosis
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....
.
In warm-blooded
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
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
Uncoupling protein
An uncoupling protein is a mitochondrial inner membrane protein that can dissipate the proton gradient before it can be used to provide the energy for oxidative phosphorylation.There are five types known in mammals:* UCP1, also known as thermogenin* UCP2...
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
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
. This alternative route for protons uncouples oxidative phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...
and the energy in the PMF is instead 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
Thermogenesis
Thermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
. 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 (hypothermiaHypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
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., drying their skin, putting on clothing, moving into warmer areas, 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 and just below the skin). (Note that contracting these blood vessels has disadvantages, such as reducing immunity in the skin which could allow a skin or internal infection to develop, and perhaps reducing the rate at which the skin can heal.)
Heat production in brown fat provides a baby with an alternative means of heat regulation.
Presence in adults
It was believed that after infants grow 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. However, more recent research has shown that brown fat is related not to white fat, but to skeletal muscle.Further, recent studies using Positron Emission Tomography
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
scanning of adult humans have shown that it is still present in adults in the upper chest and neck. The remaining deposits become more visible (increasing tracer uptake, that is, more metabolically active) with cold exposure, and less visible if an adrenergic beta blocker
Beta blocker
Beta blockers or beta-adrenergic blocking agents, beta-adrenergic antagonists, beta-adrenoreceptor antagonists or beta antagonists, are a class of drugs used for various indications. They are particularly for the management of cardiac arrhythmias, cardioprotection after myocardial infarction ,...
is given before the scan. The recent study could lead to a new method of weight loss
Weight loss
Weight loss, in the context of medicine, health or physical fitness, is a reduction of the total body mass, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue...
, since brown fat takes calories from normal fat and burns it. Scientists were able to stimulate brown fat growth in mice, but human trials have not yet begun.
In rare cases, brown fat, instead of involuting, continues to grow; this leads to a tumour known as a hibernoma
Hibernoma
A hibernoma is a benign tumour of brown fat, i.e. adipose tissue , and thus a type of lipoma.They arise when brown fat continue to grow.-Histology:...
.
Embryology
Brown fat cells and muscle cells both seem to be derived from the same stem cells in the embryo. Both have the same marker on their surface (Myf5, myogenic factor), which white fat cells don't have.Brown fat cells and muscle cells both come from the middle embryo layer. The three layers of the embryo during the gastrulation stage are ectoderm, mesoderm, endoderm. Mesoderm is the source of myocytes (muscle cells), adipocytes, and chondrocytes (cartilage cells). Adipocytes give rise to white fat cells and brown fat cells.
Researchers found that both muscle and brown fat cells expressed the same muscle factor Myf5, whereas white fat cells did not. This suggested that muscle cells and fat cells were both derived from the same stem cell. Furthermore, muscle cells that were cultured with the transcription factor PRDM16 were converted into brown fat cells, and brown fat cells without PRDM16 were converted into muscle cells.
However, there may be two types of brown fat cells—with and without Myf5. The other type, without Myf5, may share the same origin as white fat cells. They both seem to be derived from pericytes, the cells which surround the blood vessels that run through white fat tissue.