Adipose tissue

Adipose tissue
n., plural: adipose tissues
[ˈæd.ɪ.poʊs ˈtɪʃ.u]
Definition: A special type of connective tissue consisting mainly of adipocytes

Adipose Tissue Definition

Adipose tissue, a specialized variety of connective tissue, is composed of lipid-rich cells known as adipocytes. In healthy individuals, the main objective of adipose tissue, which makes up approximately 20–25% of total body weight, is to store energy in the form of lipids (fat). It is possible to classify fat tissue as either parietal, which is found beneath the skin, or visceral, which is found surrounding the organs. There are different forms of adipose tissue and they are distinguished by the morphology of the adipocytes.

1. White adipose tissue is most commonly observed in adults.
2. Brown adipose tissue is most commonly observed in infants.
3. Beige adipose tissue is responsible for the burning of calories and the production of heat.

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Function and Types of Adipose Tissue

In addition to its role as a storage medium for energy, the human body relies on fat tissue for several other crucial tasks. These include insulating the organs from heat, acting as a cushion for them, playing an endocrine role, and producing a wide variety of bioactive substances.

Some of the adipose tissue functions are as below: 

1. The storing and release of potential energy
2. Protection against extremes of heat and cold
3. Providing a cushion around delicate organs
4. Regulating appetite and fullness
5. Preserving a healthy balance of energy
6. Maintaining a normal blood sugar level and cholesterol level
7. Maintaining insulin resistance
8. Producing thermally generated heat
9. Contributing to the immune system
10. Dealing with the breakdown of sexual hormones

The cells that make up adipose tissue are held together by reticular fibers, and connective tissue septa are responsible for separating the tissue into lobules of varying sizes. (Mescher, 2021) White adipose tissue, brown adipose tissue, and beige adipose tissue are the three forms of adipose tissue.

1. White adipose tissue, often known as WAT, is responsible for the storage of energy and contributes to the insulation of the body.
2. Brown adipose tissue (BAT)
3. Beige adipocytes tissue resides in WAT and expends energy to generate heat when exposed to cold. (Physiopedia, 2023)

• White adipose tissue

White adipose tissue is the most common form and can be found in a variety of locations throughout the body, such as subcutaneous fat, visceral fat, and fat in the bone marrow. The structure of white fat cells, also known as adipocytes, is quite straightforward and consists of only a single lipid droplet (a molecule of fat) and a few cellular organelles. They not only assist in energy storage but also insulate the body from dangerously low and high temperatures and offer cushioning for sensitive organs.

SVF cells, which are short for stromal vascular fraction cells, are another type of cell that can be seen in WAT. These cells work together to produce hormones that contribute to the regulation of excessive energy intake balance, appetite and satiety levels, metabolic function, and the inflammatory response. (Clinic, 2023)

• Brown Fat

In humans, brown adipose tissue is most prevalent throughout childhood and gradually disappears as people get older. The primary focus of its presence is in your upper back. White adipocytes are simpler than brown adipocytes because brown adipocytes include multiple lipid droplets and a large number of cellular organelles (particularly, mitochondria).

The brown color of brown fat cells is due to the presence of iron in the organelles that make up brown fat. Because brown adipocytes contain these organelles, they can produce a significant quantity of heat. This is the most important job of the brown adipose tissue (BAT): to generate heat through a mechanism known as non-shivering thermogenesis, which assists in warding off hypothermia in young children. (Clinic, 2023)

• Beige fat

While many references point out the two types of adipose tissues — white (WAT) and brown (BAT), there are other references as well that include beige fat (or brite adipose tissue) as another distinct type of adipose tissue. The name beige is due to the fact that these cells are in beige color (owing to the mix of features from both white and brown adipocytes). Beige adipocytes often develop within the white adipose tissue in response to cold or exposure to certain hormones or signaling molecules. Similar to brown fat, beige fat burns fat to produce heat through thermogenesis.

Below is a table summarizing the distinctive features of the three different types of adipocytes.

Normal Adipose Amounts

20–25% of an adult’s total body weight is typically made up of adipose tissue when that adult human is healthy. The percentage of an individual’s total body weight that is composed of fat can range from less than 10% to more than 40%, depending on the person’s specific body composition and body mass index. An increased amount of human adipose tissue has been linked to a variety of health issues, including obesity, type 2 diabetes, and coronary heart disease, amongst others.

Location

The amount of adipose tissue located within the human body might differ significantly according to the gender of the individual. In a broad sense, men tend to put on the pounds around their waists, whilst women tend to put on the pounds mostly around their hips. Geneticists have identified unique regions of the human genome that are connected with fat distribution, and it appears that several genes have a bigger influence on the waist-to-hip ratio in females than they do in males. Because these genes govern the activities of fat cells, understanding their functions could throw light on these biological mechanisms. (Rogers, 2023)

Throughout the body, adipose tissue mass can be located in a variety of different regions and locations. White adipose tissue is the type of fat that is found in the greatest quantity in humans. It can be found in the subcutaneous adipose tissue, as well as visceral fat and fat in the bone marrow.

White adipose tissue locations:

• Subcutaneous fat can be found all over the body, namely in the gaps that separate the skin from the muscles that lie beneath it.
• Visceral adipose tissue is most commonly located in and around the organs that reside within the abdominal cavity. These organs include the liver, intestines, and kidneys. Visceral fat can also be found in the peritoneum, which is a serous membrane that surrounds the outside of the abdominal organs.
• There is also white adipose tissue in the bone marrow, which is a tissue that looks like a sponge and is found in the cavity in the center of bones.
• In addition, white adipose tissue can be found in the pericardium, which is the sac that surrounds the heart, as well as cushioning various regions of the body, such as the soles of the feet, the eyeballs, and specific blood vessels. They also occur in yellow bone marrow and breast tissue.

Brown adipose tissue, commonly referred to as brown fat, is mostly present throughout fetal life and in babies, in contrast to white adipose tissue, which is present until adulthood. Brown adipose tissue is predominantly seen on the back of neonates. This tissue can be found throughout the top half of the spine, in between the shoulders, and all around the kidneys. The quantity of brown fat in the body gradually diminishes as people get older. Deposits of brown fat can still be detected in adults around the vertebrae, above the clavicles, in the upper back, and in the mediastinum (the center compartment of the thoracic cavity). Brown fat helps regulate temperature by producing heat.

20–25% of an adult’s total body weight is typically made up of adipose tissue if the adult is healthy. However, the particular proportion of body fat varies quite a little from one person to the next and can be anywhere from less than 10% to more than 40% of an individual’s total body weight. An increased amount of excess adipose tissue has been linked to several different health issues, such as obesity, diabetes type 2, and heart disease. (Hernández, 2023)

Structure

The human body is divided into two sections that are primarily comprised of adipose tissue: (Vidal-Puig et al., 1997) (MD, 2023)

1. Subcutaneous fat, also known as parietal fat, is a type of fat that is located under the skin and is enclosed in connective tissue.
2. Visceral fat is the type of fat that surrounds the internal organs, such as the fat that surrounds the eyeballs (periorbital fat) or the fat capsule that surrounds the kidneys (perirenal fat capsule).

Adipose tissue is made up of cells and the extracellular matrix, just like every other type of tissue. The small amount of external matrix is not as important to the structure of this tissue as the cells are. Adipocytes are the main types of cells that make up fatty tissue. There are also preadipocytes, fibroblast growth factor, capillary endothelial cells, platelet-derived growth factor, macrophages, and adipose-derived stem cells, in addition to adipocytes. The stromal vascular fraction is made up of these non-adipocyte cells. Their primary function is to support and protect the fat tissues.

The peroxisome proliferator-activated receptor (PPAR gamma) is an important part of adipogenesis and adipocyte gene expression. It is also a receptor for insulin-sensitizing drugs in the thiazolidinedione class.

Adipocytes and stromal cells are both liable for the production of the extracellular matrix. It is made up of a very fine network of reticular fibers, which are type III collagen, and its job is to keep the cells in their proper positions. The supply of blood vessels and nerve fibers that are not myelinated is quite abundant in adipose tissue. These structures are typically located within the meshwork that divides neighboring adipocytes when seen on histology slides. In addition to that, mast cells can be found here.

• Lymph nodes in adipose tissue

Lymph nodes are small, bean-shaped structures scattered throughout adipose tissue. They play a vital role in the immune system, filtering lymph fluid and trapping harmful substances, helping to maintain tissue health and overall body defense.

What are the most important facts about fat tissue that you should know?

1. Your microbiome may affect the amount of body fat that you have: Researchers are investigating a new field of medicines known as postbiotics, which are the byproducts that bacteria leave behind and which assist the body in producing insulin more efficiently. A new study found that giving mice with obesity postbiotics lowered their insulin sensitivity (without the requirement for the mice to lose any weight), which heralds promising prospective treatments for obesity with type 2 diabetes.
2. A high body fat percentage increases the risk of developing cancer. Adipose tissue also secretes hormones that speed up the growth of cancer cells. When adipose tissue grows, it paves the way for an increase in the number of immune cells that might reside within the tissue. These B and T immune cells release chemicals that contribute to inflammation, such as adipokines. Peptides provide signals to other organs (as well as cytokines), which in turn establish the optimal environment for the formation of tumors.
3. The way fat cells in adipose tissue store and release lipids is a key part of how the energy balance is controlled. During childhood and adolescence, adipose tissue grows by increasing both the size of fat cells and, more importantly, the amount of fat cells. Even though about 10% of fat cells die off every year in adults with stable body weight, the amount of fat cells stays the same over time. When an adult’s weight goes down, only the size of fat cells changes (they get smaller), but when their weight goes up, both the size and amount of fat cells go up. The number of fat cells is mostly set by the time a person is in their teens. This is a big reason why it’s important to prevent your kids from getting fat.
4. Adipose tissue is a crucial component of the adipose organ, acting as a specialized reservoir for free fatty acids (FFAs) and influencing glucose metabolism in diverse cell types. Particularly, high-fat diet-induced alterations in adipose tissue can have profound effects on glucose uptake and adipocyte differentiation, impacting both white and brown adipocytes. During embryonic development, adipose tissue forms, and its distribution determines the regional fat deposition, including abdominal fat. FFAs, released from adipose depots, contribute to systemic free fatty acid levels, affecting energy homeostasis and metabolism. Understanding the intricate interplay between adipose tissue, glucose metabolism, and free fatty acids is critical for unraveling the complexities of metabolic disorders and developing targeted therapeutic interventions.
5. According to the American Diabetes Association, having an inadequate amount of fat can have a comparable effect as having an excessive amount of fat on a person’s chance of developing type 2 diabetes. Obesity can also put a person at risk for developing type 2 diabetes.
6. Research on fat has recently expanded into the field of epigenetics, which refers to all of the various factors that regulate how and when each gene expression happened. Researchers that study fat to gain a better understanding of metabolic disorders have recently shifted their attention to the field of epigenetics. This is because changes in epigenetics are likely to play an important part in the development of chronic conditions such as metabolic syndrome and type 2 diabetes. It is feasible to reverse and edit these elements, which means that we may be able to find new strategies to prevent and treat these prevalent ailments. This is in contrast to the genetic code, which cannot be reversed or altered.
7. Many recently published research has generated interest in adult BAT, which suggests that BAT stimulation provides an interesting prospective approach for the treatment of age-related obesity. (Physiopedia, 2023)
8. Additionally, tumor necrosis factor can influence the endocrine functions of adipose tissue. TNF leads to metabolic imbalances by disrupting both the function of adipose tissue and its capacity to retain excess fuel. (Cawthorn & Sethi, 2008)
9. In addition to its role as the body’s major place for storing extra energy, adipose tissue also functions as an endocrine organ, producing a variety of biologically active substances that help maintain metabolic homeostasis. (Coelho, Oliveira, & Fernandes, 2013)
10. Adipocyte hypertrophy is a reaction to having too many nutrients. It keeps adipose tissue’s ability to buffer nutrients and shields other tissues from lipotoxicity. (Muir et al., 2016)

Frequently Asked Questions on Adipose Tissue

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• In what ways does adipose tissue interact with the body’s other organs?

Through the release of some hormones and the response to the release of other hormones, adipose tissue can communicate not only with the organs located throughout the body but also with the central nervous system. It does this by sending signals of hunger and satiety (when you feel like you’ve had enough to eat) to the brain. It does this in response to insulin by transforming any extra blood sugar into lipids, which are then stored away for later use. Sex hormones (e.g. estrogen and testosterone) play a role in determining the distribution of fat in the body. Additionally, adipose tissue contains its very own active immune cells, which, in response to specific stimuli, can eliminate dead fat cells or produce an inflammatory reaction. A disruption in any one of these functions can lead to a metabolic illness. (Clinic, 2023)

• What are some of the most common diseases and ailments that impact this bodily system?

Adipose tissue dysfunction could lead to a variety of metabolic problems, including but not limited to:

1. Insulin resistance, which can progress to diabetes.
2. Appetite and satiety signals that aren’t working properly, which leads to obesity.
3. Health issues related to the heart and high blood pressure.
4. The accumulation of fat in the organs and the condition known as fatty liver.

• What could go wrong with adipose tissue?

Both having an excessive amount of adipose tissue and not having enough of it can have severe negative effects on one’s health. The accumulation of excessive amounts of adipose tissue is the primary cause of obesity. Obesity can result in a variety of health complications, particularly if there is an excessive amount of visceral fat. Because it leads the body to grow resistant to the usual glucose-lowering impact of insulin, obesity raises the chance of developing type 2 diabetes. Type 2 diabetes is characterized by higher blood glucose levels. Because of this resistance to the action of insulin, excessive amounts of blood sugar are produced, which is detrimental to a person’s health. The risk of acquiring high blood pressure, elevated cholesterol levels, and an increased propensity for the blood to clot is also enhanced when a person is obese. These variables contribute to an increased likelihood of suffering a heart attack, a stroke, or an abnormal clotting of blood in the legs or lungs (known medically as venous thromboembolism).

Problems of a similar nature can also be caused by lipodystrophy, which is an abnormal accumulation of fatty tissue. This can be observed as a result of extremely uncommon genetic disorders known as inherited lipodystrophy, or it can be seen as a result of the usage of drugs for the treatment of HIV known as acquired lipodystrophy.

Patients who suffer from eating disorders, such as anorexia nervosa, do not consume enough food daily to keep their levels of adipose tissue stable. This can also produce low levels of essential sex hormones such as estrogen, which can result in osteoporosis (bone thinning) and amenorrhea (the absence of menstrual cycles).

• How do we deal with adipose tissue disorders?

Adipose tissue diseases are caused by both undernutrition and overnutrition in addition to genetics. When someone is undernourished, they are given nutritional supplements, often known as “refeeding”. Diet and exercise are the first lines of defense against overnutrition. Class III obesity often requires medical intervention or surgical correction. Many metabolic diseases are linked to obesity, however, not all obese people are metabolically unhealthy. Some issues, such as insulin resistance, may call for specific therapy.

• Which hormones are secreted by adipose tissue?

A variety of hormones are secreted by adipose tissue, each of which performs a specific function in the body. Here are a few examples:

1. Sex hormone metabolism involves aromatase.
2. TNF alpha, IL-6, and leptin, which are referred to as ‘cytokines’ and play a role as messengers between cells.
3. Plasminogen activator inhibitor-1, an inhibitor of plasminogen activation that contributes to blood coagulation
4. Angiotensin, a hormone important in maintaining healthy blood pressure
5. Adiponectin, which helps in maintaining insulin sensitivity and so protects against type 2 diabetes by lowering the risk of acquiring the disease
6. Apolipoprotein E and lipoprotein lipase, which help store fat and metabolize it down into energy.

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References

1. Cawthorn, W. P., & Sethi, J. K. (2008). TNF-α and adipocyte biology. FEBS letters, 582(1), 117-131.
2. Clevelandclinic.org. (2023). Adipose Tissue. Retrieved 27 July, 2023, from https://my.clevelandclinic.org/health/body/24052-adipose-tissue-body-fat
3. Coelho, M., Oliveira, T., & Fernandes, R. (2013). State of the art paper Biochemistry of adipose tissue: an endocrine organ. Archives of medical science, 9(2), 191-200.
4. yourhormones.info. (2021). Adipose tissue. Retrieved 26 July, 2023, from https://www.yourhormones.info/glands/adipose-tissue/
5. Hernández, A. (2023). Adipose Tissue. Retrieved 26 July, 2023, from https://www.osmosis.org/answers/adipose-tissue
6. kenhub.com ( 2023). Adipose tissue. Retrieved 27 July, 2023, from https://www.kenhub.com/en/library/anatomy/adipose-tissue
7. Mescher, A. L. (2021). Adipose Tissue Junqueira’s Basic Histology Text and Atlas, 16e. New York, NY: McGraw Hill.
8. Muir, L. A., Neeley, C. K., Meyer, K. A., Baker, N. A., Brosius, A. M., Washabaugh, A. R., Flesher, C. G., etc. (2016). Adipose tissue fibrosis, hypertrophy, and hyperplasia: Correlations with diabetes in human obesity. Obesity, 24(3), 597-605.
9. Physiopedia. (2023). Adipose Tissue. Retrieved 26 July, 2023, from https://www.physio-pedia.com/Adipose_Tissue
10. Rogers, K. (2023). adipose tissue. Retrieved 26 July, 2023, from https://www.britannica.com/science/adipose-tissue
11. Vidal-Puig, A. J., Considine, R. V., Jimenez-Liñan, M., Werman, A., Pories, W. J., Caro, J. F., & Flier, J. S. (1997). Peroxisome proliferator-activated receptor gene expression in human tissues. Effects of obesity, weight loss, and regulation by insulin and glucocorticoids. The Journal of clinical investigation, 99(10), 2416-2422.

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