Adipocytes, also known as lipocytes and fat cells, are the
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
s that primarily compose
adipose tissue
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular e ...
, specialized in storing energy as
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
.
Adipocytes are derived from
mesenchymal stem cell
Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage ce ...
s which give rise to adipocytes through
adipogenesis Adipogenesis is the formation of adipocytes (fat cells) from stem cells. It involves 2 phases, determination, and terminal differentiation. Determination is mesenchymal stem cells committing to the adipocyte precursor cells, also known as preadipoc ...
. In
cell culture
Cell culture or tissue culture is the process by which cells are grown under controlled conditions, generally outside of their natural environment. The term "tissue culture" was coined by American pathologist Montrose Thomas Burrows. This t ...
, adipocyte progenitors can also form
osteoblasts
Osteoblasts (from the Greek combining forms for " bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts functi ...
,
myocytes
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a musc ...
and other cell types.
There are two types of 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 is brown adipose tissue. White adipose tissue is composed of monolocular adipocytes.
In humans, the healthy amount of white adipose ti ...
(WAT) and
brown adipose tissue
Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals.
Classification of brown fat refers to two distinct cell populations with s ...
(BAT), which are also known as white and brown fat, respectively, and comprise two types of fat cells.
Structure
White fat cells
White fat cells contain a single large
lipid droplet surrounded by a layer of
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
, and are known as unilocular. The
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
is flattened and pushed to the periphery. A typical fat cell is 0.1 mm in diameter
with some being twice that size, and others half that size. However, these numerical estimates of fat cell size depend largely on the measurement method and the location of the adipose tissue.
The fat stored is in a semi-liquid state, and is composed primarily of
triglycerides, and
cholesteryl ester
300px, Cholesterol oleate, a member of the cholesteryl ester family
Cholesteryl ester, a dietary lipid, is an ester of cholesterol. The ester bond is formed between the carboxylate group of a fatty acid and the hydroxyl group of cholesterol. Cho ...
. White fat cells secrete many proteins acting as
adipokine
The adipokines, or adipocytokines (Greek ', fat; ', cell; and ', movement) are cytokines (cell signaling proteins) secreted by adipose tissue. Some contribute to an obesity-related low-grade state of inflammation or to the development of metabolic ...
s such as
resistin,
adiponectin
Adiponectin (also referred to as GBP-28, apM1, AdipoQ and Acrp30) is a protein hormone and adipokine, which is involved in regulating glucose levels as well as fatty acid breakdown. In humans it is encoded by the ''ADIPOQ'' gene and it is produced ...
,
leptin and
apelin
Apelin (also known as APLN) is a peptide that in humans is encoded by the ''APLN'' gene. Apelin is one of two endogenous ligands for the G-protein-coupled APJ receptor that is expressed at the surface of some cell types. It is widely expressed i ...
. An average human adult has 30 billion fat cells with a weight of 30 lbs or 13.5 kg. If a child or adolescent gains sufficient excess weight, fat cells may increase in absolute number until age twenty-four.
If an adult (who never was obese as a child or adolescent) gains excess weight, fat cells generally increase in size--but not absolute number--though there is some inconclusive evidence suggesting that the number of fat cells might increase if the existing fat cells become large enough (as in particularly severe levels of obesity).
The absolute number of fat cells is difficult to decrease through dietary intervention, though some evidence suggests that the number of fat cells can decrease if weight loss is maintained for a sufficiently long period of time (>1 year; though it is extremely difficult for people with larger and more numerous fat cells to maintain weight loss for that long a time).
A large meta-analysis has shown that white adipose tissue cell size is dependent on measurement methods, adipose tissue depots, age, and body mass index; for the same degree of obesity, increases in fat cell size were also associated the dysregulations in glucose and lipid metabolism.
Brown fat cells
Brown fat cells are
polyhedral in shape. Brown fat is derived from dermatomyocyte cells. Unlike
white fat cells, these cells have considerable cytoplasm, with several
lipid droplets scattered throughout, and are known as multilocular cells. The nucleus is round and, although eccentrically located, it is not in the periphery of the cell. The brown color comes from the large quantity of
mitochondria. Brown fat, also
known as "baby fat," is used to generate heat.
Marrow fat cells
Marrow adipocytes, are unilocular like white fat cell. The
marrow adipose tissue
Bone marrow adipose tissue (BMAT), sometimes referred to as marrow adipose tissue (MAT), is a type of fat deposit in bone marrow. It increases in states of low bone density -osteoporosis, anorexia nervosa/ caloric restriction, skeletal unweight ...
depot is poorly understood in terms of its physiologic function and relevance to bone health. Marrow adipose tissue expands in states of low bone density but additionally expands in the setting of obesity.
Marrow adipose tissue response to exercise approximates that of
white adipose tissue
White adipose tissue or white fat is one of the two types of adipose tissue found in mammals. The other kind is brown adipose tissue. White adipose tissue is composed of monolocular adipocytes.
In humans, the healthy amount of white adipose ti ...
.
Exercise reduces both adipocyte size as well as marrow adipose tissue volume, as quantified by
MRI
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...
or
μCT imaging of bone stained with the lipid binder
osmium. Exercise stimulate transdifferentiation of white fat into brown fat (browning phenomena)
Development
Pre-adipocytes are undifferentiated
fibroblasts that can be stimulated to form adipocytes. Studies have shed light into potential molecular mechanisms in the fate determination of pre-adipocytes although the exact lineage of adipocyte is still unclear.
[
*] The variation of body fat distribution resulting from normal growth is influenced by nutritional and hormonal status dependent on intrinsic differences in cells found in each adipose depot.
Mesenchymal stem cell
Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage ce ...
s can differentiate into adipocytes,
connective tissue, muscle or
bone.
The precursor of the adult cell is termed a
lipoblast
A lipoblast is a precursor cell for an adipocyte.
Alternate terms include adipoblast and preadipocyte.
Early stages are almost indistinguishable from fibroblasts.
File:Lipoblasts and lipocytes.jpg, Lipoblasts (white arrow) and lipocytes (black ...
, and a tumor of this cell type is known as a
lipoblastoma
Lipoblastoma is a type of rare, subcutaneous, benign, fatty tumor, found in infants, and children, more common in males with tendency of local recurrence. Local recurrence can happen in up to 80% of incompletely resected tumours. Therefore, compl ...
.
Function
Cell turnover
Fat cells in some mice have been shown to drop in count due to fasting and other properties were observed when exposed to cold.
If the adipocytes in the body reach their maximum capacity of fat, they may replicate to allow additional fat storage.
According to some reports and textbooks, the number of adipocytes can increase in childhood and adolescence, though the amount is usually constant in adults. Individuals who become obese as adults, rather than as adolescents, have no more adipocytes than they had before.
Body fat cells have regional responses to the overfeeding that was studied in adult subjects. In the upper body, an increase of adipocyte size correlated with upper-body fat gain; however, the number of fat cells was not significantly changed. In contrast to the upper body fat cell response, the number of lower-body adipocytes did significantly increase during the course of experiment. Notably, there was no change in the size of the lower-body adipocytes.
Approximately 10% of fat cells are renewed annually at all adult ages and levels of body mass index without a significant increase in the overall number of adipocytes in adulthood.
Adaptation
Obesity is characterized by the expansion of fat mass, through adipocyte size increase (
hypertrophy) and, to a lesser extent, cell proliferation (
hyperplasia
Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of organic tissue that results from cell proliferatio ...
).
In the
fatty tissue
Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular en ...
of obese individuals, there is increased production of metabolism modulators, such as
glycerol,
hormones
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required fo ...
,
macrophage-stimulating
chemokines
Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In additi ...
, and
pro-inflammatory cytokines, leading to the development of
insulin resistance
Insulin resistance (IR) is a pathological condition in which cells fail to respond normally to the hormone insulin.
Insulin is a hormone that facilitates the transport of glucose from blood into cells, thereby reducing blood glucose (blood sugar ...
. Production of these modulators and the resulting
pathogenesis of insulin resistance are probably caused by adipocytes as well as
immune system macrophages that infiltrate the tissue.
Fat production in adipocytes is strongly stimulated by
insulin. By controlling the activity of the
pyruvate dehydrogenase
Pyruvate dehydrogenase is an enzyme that catalyzes the reaction of pyruvate and a lipoamide to give the acetylated dihydrolipoamide and carbon dioxide. The conversion requires the coenzyme thiamine pyrophosphate.
Pyruvate dehydrogenase is u ...
and the
acetyl-CoA carboxylase
Acetyl-CoA carboxylase (ACC) is a biotin-dependent enzyme () that catalyzes the irreversible carboxylation of acetyl-CoA to produce malonyl-CoA through its two catalytic activities, biotin carboxylase (BC) and carboxyltransferase (CT). ACC is ...
enzymes, insulin promotes unsaturated
fatty acid synthesis
In biochemistry, fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the cell. Most of the acetyl-CoA which is co ...
. It also promotes
glucose uptake and induces
SREBF1
Sterol regulatory element-binding transcription factor 1 (SREBF1) also known as sterol regulatory element-binding protein 1 (SREBP-1) is a protein that in humans is encoded by the ''SREBF1'' gene.
This gene is located within the Smith–Magenis s ...
, which activates the transcription of genes that stimulate
lipogenesis
In biochemistry, lipogenesis is the conversion of fatty acids and glycerol into fats, or a metabolic process through which acetyl-CoA is converted to triglyceride for storage in fat. Lipogenesis encompasses both fatty acid and triglyceride s ...
.
SREBF1
Sterol regulatory element-binding transcription factor 1 (SREBF1) also known as sterol regulatory element-binding protein 1 (SREBP-1) is a protein that in humans is encoded by the ''SREBF1'' gene.
This gene is located within the Smith–Magenis s ...
(
sterol
Sterol is an organic compound with formula , whose molecule is derived from that of gonane by replacement of a hydrogen atom in position 3 by a hydroxyl group. It is therefore an alcohol of gonane. More generally, any compounds that contain the gon ...
regulatory element-binding
transcription factor 1) is a transcription factor synthesized as an inactive
precursor protein
A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein (or peptide) that can be turned into an active form by post-translational modification, such as breaking off a piece of the molecule or adding on another molecul ...
inserted into the
endoplasmic reticulum (ER) membrane by two membrane-spanning
helices. Also anchored in the ER membrane is
SCAP (SREBF-cleavage activating protein), which binds SREBF1. The SREBF1-SCAP complex is retained in the ER membrane by
INSIG1
Insulin induced gene 1, also known as INSIG1, is a protein which in humans is encoded by the ''INSIG1'' gene.
INSIG1 is short for insulin-induced gene 1; it is located on chromosome 7 (7q36). This human gene encodes for a transmembrane protein ...
(insulin-induced gene 1 protein). When sterol levels are depleted, INSIG1 releases SCAP and the SREBF1-SCAP complex can be sorted into transport
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
coated by the
coatomer
The coatomer is a protein complex that coats membrane-bound transport vesicles. Two types of coatomers are known:
*COPI (retrograde transport from trans-Golgi network to cis-Golgi network and endoplasmic reticulum)
*COPII (anterograde transpor ...
COPII
The Coat Protein Complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This ...
that are exported to the
Golgi apparatus. In the Golgi apparatus, SREBF1 is cleaved and released as a transcriptionally active mature protein. It is then free to
translocate to the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
and activate the expression of its target genes.
Clinical studies have repeatedly shown that even though insulin resistance is usually associated with obesity, the membrane
phospholipids
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids ...
of the adipocytes of obese patients generally still show an increased degree of fatty acid unsaturation.
This seems to point to an adaptive mechanism that allows the adipocyte to maintain its functionality, despite the increased storage demands associated with obesity and insulin resistance.
A study conducted in 2013
found that, while INSIG1 and SREBF1
mRNA expression was decreased in the adipose tissue of obese mice and humans, the amount of active SREBF1 was increased in comparison with normal mice and non-obese patients. This downregulation of INSIG1 expression combined with the increase of mature SREBF1 was also correlated with the maintenance of SREBF1-target gene expression. Hence, it appears that, by downregulating INSIG1, there is a resetting of the INSIG1/SREBF1 loop, allowing for the maintenance of active SREBF1 levels. This seems to help compensate for the anti-lipogenic effects of insulin resistance and thus preserve adipocyte fat storage abilities and availability of appropriate levels of fatty acid unsaturation in face of the nutritional pressures of obesity.
Endocrine role
Adipocytes can synthesize
estrogens from
androgen
An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This incl ...
s, potentially being the reason why being
underweight
An underweight person is a person whose body weight is considered too low to be healthy. A person who is underweight is malnourished.
Assessment
The body mass index, a ratio of a person's weight to their height, has traditionally been used t ...
or
overweight are risk factors for
infertility.
Additionally, adipocytes are responsible for the production of the hormone
leptin. Leptin is important in regulation of appetite and acts as a satiety factor.
See also
*
List of human cell types derived from the germ layers
References
External links
* – "Connective Tissue: unilocular (white) adipocytes "
* – "Connective Tissue: multilocular (brown) adipocytes"
{{Authority control
Sequestering cells
Human cells
Connective tissue cells