The adipose tissue is an in various places of the body occurring form of the connective tissue , consisting of fat cells ( adipocytes ) is constructed. The basic task of the fat cell is to store both fat and water in their cell body and to release them again when necessary. After a long time only differentiated between two forms of adipose tissue with different functions - white and brown adipose tissue - a third form of adipose tissue - so-called beige adipose tissue - has now been discovered, which occupies an intermediate position between the two previously mentioned .
White adipose tissue
When one speaks of adipose tissue in the human body , the white adipose tissue is almost always meant, as it is much more common than the brown and beige ones.
Occurrences and tasks
Individual or groups of fat cells can be found almost anywhere in the body, embedded in the loose connective tissue . In the actual fat tissue in certain body regions, on the other hand, numerous fat cells are grouped together by connective tissue in lobules. The adipose tissue is always well supplied with blood vessels.
The white adipose tissue fulfills various functions:
- Storage or depot fat : Lipids are high-energy compounds. Due to the high percentage of fat in the body , people have reserves to go without food for up to 40 days. Depending on gender and nutritional status, the depot fat makes up 10% (athletes, extremely slim people), 15-25% (normal weight), or well over 50% (obese people) of body weight. The function as depot fat is mainly performed by the fatty tissue in the subcutaneous tissue , here mainly the layer of fat on the abdomen and buttocks (pronounced fat deposits), and on the peritoneum .
- Insulating fat: Since fat is a poorer conductor of heat than other tissues, the fat (layer of fat) in the subcutaneous tissue (subcutaneous fat) also protects against rapid heat loss. About 65% of the total fat is in the subcutaneous tissue, the rest is in the abdomen.
- Building fat: Adipose tissue also serves as mechanical protection in certain places in the form of a pressure-elastic pad (fat pad): under the sole of the foot, on the joints (in the knee joint as Hoffa fat body ), on the cheek ( Corpus adiposum buccae , also Bichat fat plug), on the Buttocks and as an organ store in the kidney bed ( capsula adiposa ), in the coronary arteries and under the eyeball ( corpus adiposum orbitae ). When there is a lack of food, the construction fat is only mobilized as the last reserve - this is where the deep, sunken eyes of people after famine disasters move.
- Metabolic organ : The fatty tissue plays an important role in the energy metabolism through the secretion of hormone-like substances.
In some parts of the body, the subcutaneous fatty tissue is usually very weak (back of the hand and foot, nose, eyelid, lip, penis and scrotum , small labia and on the auricle (but not earlobe)). Particularly pronounced fat deposits are located as a layer several centimeters thick on the abdomen and on the buttocks. The thickness of the abdomen as well as the shape, size and weight of the buttocks are determined by the training condition of the muscles and the amount of stored fat (fattening fat), so they also depend on the nutritional status. The amount of fat depot is around 15 kg for men of normal weight and around 15-20 kg for women.
Fat cells (adipocytes) are broken down by the body, but they are also constantly being replaced by new ones, whereby intracellular fat is also subject to constant exchange. Diets can break down the fat stored in adipose tissue, but not the adipose tissue itself.
The fat in human adipose tissue is made up of the following fatty acid proportions : oleic acid 42–51%, palmitic acid 21–30%, palmitoleic acid and stearic acid (both 5–8.5%), myristic acid (2–6%). The ratio of unsaturated to saturated fatty acids is about 60:40.
The adipocytes of white fat are quite large cells (up to 100 µm), the cell body of which is almost completely filled with a large lipid droplet : this is why one speaks of univacuolar fat cells. The nucleus is pressed to the edge by the huge vacuole and flattened; other cell organelles or a lot of cytoplasm are usually not visible (one speaks of a similarity to a signet ring). The fat vacuole is not surrounded by a biomembrane like secret vesicles, but is "free" in the cytosol . However, it is braided with intermediate filaments to keep it together and in shape. Each individual adipocyte is surrounded by a basal lamina and reticular fibers , which keep the cell in shape even under certain force (see construction fat). The white adipose tissue is abundantly supplied by blood capillaries.
The white adipose tissue gets its name from the fact that in standard histological preparations the fat is almost always released and the cells therefore appear completely empty, i.e. white under the microscope.
Human fat is oily in consistency (high oleic acid content) and semi-liquid and intensely yellow at body temperature.
Adipocytes take up fatty acids from the blood and use α-glycerophosphate (activated glycerine ) to synthesize lipids from their metabolism (bypass of glycolysis ), which are stored in the cell ( fatty acid synthesis ). If necessary, the lipids can be split back into their building blocks ( lipolysis ) and released into the blood so that other cells can use them to generate energy. Both processes, lipogenesis and lipolysis, are influenced by the hormones insulin and adrenaline , among other things .
A change in the amount of fat stored occurs mainly by increasing the amount stored in the individual cell. However, new fat cells can also form from stem cells.
Brown adipose tissue
The task of the brown adipose tissue is the direct generation of heat ( thermogenesis ) from the stored fat. It is found only in a few places in the adult human body, but is still found in infants or in animals that hibernate .
The cell of brown adipose tissue has many smaller lipid droplets and is therefore called plurivacuolar . It is also rich in mitochondria , which convert most of the energy from fatty acid breakdown with the help of UCP-1 (uncoupling protein 1) directly into heat instead of into ATP synthesis. The brown color comes from the mitochondrial cytochromes and lipochromes . The latter are dissolved in the fat droplets. Brown adipose tissue is also heavily capillary and is densely innervated by sympathetic nerve fibers. The sympathetic stimuli stimulate lipolysis and the breakdown of fatty acids.
In newborns, the brown fat makes up about five percent of the body weight and is concentrated on the back and along the large blood vessels in the chest. Infants are more sensitive to hypothermia for various reasons: They cannot react with tremors of the skeletal muscles like adults and have an unfavorable surface-to-volume ratio, so that the generation of heat in brown fat can be vital. In adults, the brown fat has largely receded, only around the large arteries, in the mediastinum , on the kidneys and under the armpits can still be found.
Beige fatty tissue
In 2009, various research groups reported that additional proportions of brown fat cells were discovered in the body of adult test subjects. In 2012, genetic analyzes revealed that this was a third previously unknown type of fat cell. The adipocytes known as "beige fat cells" are diffusely distributed in the white adipose tissue and, like the brown cells, are rich in mitochondria . The main difference to brown tissue is the significantly lower concentration of an activation protein ( UCP1 ), which is needed to convert energy. It is assumed that beige fat cells can be activated by certain hormones (e.g. irisin ) and external influences (e.g. cold) and then, like brown adipose tissue, contribute to the body's heat production. External activation of the beige cells is therefore of interest for the future treatment of obesity.
Extraction of stem cells from adipose tissue
For decades, stem cells have been extracted primarily from bone marrow for medical purposes. The basic research for this new generation of stem cells can still be expanded, but mesenchymal stem cells can be isolated from the adipose tissue and reused. The fat stem cells ("adipose-derived stem cells" ASC) contain many multipotent stem cells and the extraction method is gentler compared to bone marrow donation. This method is already used in several European countries (e.g. the Czech Republic). Because this procedure falls under the Tissue Act in Germany and the extraction and further use method is considered a medicinal product, the official approval has not yet been completed (as of 2018).
- Baby fat
- Liposuction (liposuction)
- Johannes Sobotta, Ulrich Welsch: Textbook histology. Cytology, histology, microscopic anatomy. 2nd Edition. Urban & Fischer, 2005, ISBN 3-437-42421-1 .
- ↑ a b Jun Wu, Pontus Boström u. a .: Beige Adipocytes Are a Distinct Type of Thermogenic Fat Cell in Mouse and Human. In: Cell. 150, 2012, p. 366, doi : 10.1016 / j.cell.2012.05.016 .
- ↑ a b c Sabine Kurz: The new fat is beige ; Bild der Wissenschaft, July 13, 2012 , last accessed September 9, 2019.
- ↑ J. Fanghänel, F. Pera, F. Anderhuber and a. (Ed.): Waldeyer Anatomie des Menschen . 17th edition. Verlag Walter de Gruyter , Berlin 2003, ISBN 3-11-016561-9 , p. 1222 ( preview [accessed November 20, 2011]).
- ↑ The Persistence of Adipocytes, or Why All Diets Fail. In: Deutsches Ärzteblatt. May 7, 2008. (online) ( Memento of the original from June 27, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ KJ KINGSBURY, S. PAUL, A. CROSSLEY, DM MORGAN: The Fatty Acid Composition of Human Depot Fat . In: Biochemical Journal . tape 78 , March 1961, p. 543 , PMC 1205373 (free full text) - (English).
- ↑ D. Drenkhahn (Ed.): Anatomie. 16th edition. Volume 1, Urban & Fisher, Munich 2003, pp. 127–128.
- ↑ Kuhbier, JW; Weyand, B .; Sorg, H .; Radtke, C .; Vogt: Stem cells from adipose tissue: a new resource for regenerative medicine? Ed .: The surgeon; Journal for all areas of operative medicine. 81st edition. Hanover 2010, p. 826-832 .
- ^ J. Jobst: stem cells from bone marrow, umbilical cord blood or adipose tissue. In: Kigorosa. Roman Safreider, October 3, 2018, accessed March 13, 2019 .