Adipocyte Definition

Adipocyte cells are brown, beige, pink, or white fat cells that store triglyceride droplets, secrete them, and/or assist in the conversion of lipids to energy. Adipocytes constitute the majority of fat tissue. Adipocytes may produce hormones and other effector molecules that are critical for metabolic regulation. Adipocyte tissue is undergoing intensive research to determine its precise function in light of its link with metabolic diseases.

What is an Adipocyte?

Adipocytes, also known as fat cells or lipocytes, are the predominant cells found in brown, beige, and white adipose tissue. These tissues perform distinct roles: brown and beige adipocyte cells generate energy, while white adipocyte cells store it in the form of lipid droplets.

Although adipocytes predominate, adipose tissue also contains non-adipocyte cells such as neurons and immunological cells. Rather than “simple” connective tissue, adipose fat is a sophisticated, high-functioning organ.

A white adipocyte cell is composed of a lipid body surrounded by a cytoplasmic ring. The lipid body swells to push the nucleus toward the cell membrane, flattening it.

While the number of fat cells in the body is largely steady in people who maintain a healthy weight, the size of these cells may increase or decrease. However, the more obese a person is, the higher their white lipocyte count and the greater their likelihood of producing more fat cells.

Brown adipocytes have a greater amount of cytoplasm and are dispersed with tiny lipid droplets. Because the nucleus lacks a bigger lipid globule in the middle, it is not flat. The cytoplasm is densely packed with mitochondria, suggesting that brown fat cells play an important role in energy production.

Beige lipocytes have a structure that is a hybrid of white and brown adipocytes, with medium-sized lipid droplets and a median amount of mitochondria and cytoplasm in comparison to white and brown cells.

Pink adipocytes have a considerably bigger Golgi apparatus and endoplasmic reticulum, as well as milk vesicles rather than lipid bodies. These cells are differentiated white lipocytes that arise in subcutaneous mammary tissue during pregnancy. They resemble a hybrid between a fat cell and an epithelial cell and have been reported exclusively in rats so far.

Adipocytes are present in both red and yellow bone marrow. Adipocytes in the bone marrow (BMAs) contribute to the regulation of blood cell formation, commonly known as hematopoiesis.

Yellow BMAs are big and insensitive to their surroundings; red BMAs, located in the marrow where blood stem cells are found, have a far greater ability to communicate.

All adipose cells have mitochondria, a nucleolus, a Golgi apparatus, a rough and smooth endoplasmic reticulum, a very strong cell membrane, and vacuoles.

Adipocyte Function

When examining adipocyte activity, it is necessary to distinguish between white, pink, beige, bone marrow, and brown cells. All adipocyte types, however, produce chemical signalling molecules known as adipocytokines.

Adipocytokines are proinflammatory chemicals; abnormal fat cell secretion, as seen in obese individuals, leads to chronic inflammatory diseases. Additionally, when we gain weight, the amounts of these substances increase, and the body’s response to them becomes less efficient. For example, excessive fat tissue contributes to insulin resistance.

The most abundant adipocytokines generated by all kinds of fat cells are as follows:

• Leptin is required for the acute inflammatory response and the proliferation of T cells.
• In the presence of hyperglycaemic signals, adipsin promotes insulin secretion;
• Adiponectin: regulates glucose metabolism, fatty acid oxidation, and adipocyte differentiation;
• Omentin is required for insulin sensitivity, angiogenesis, and vasodilation to occur.

All fat cell types contribute to the activity of metabolic and immunological pathways. Additionally, they describe the relationship between obesity and diabetes, cardiovascular disease, infection susceptibility, and metabolic dysfunction.

White Adipocyte Function

Adipocytes are fat-storage cells that are the most abundant kind of cell in white and beige fat tissue. During fasting times, triglyceride droplets are broken down and released to supply the essential components for lipolysis, the conversion of fats to fuel.

The fed condition, which stimulates insulin release, and the fasting state, which stimulates catecholamine release, both influence the amount of fat released from white fat cells.

Brown Adipocyte Function

Brown adipocytes generate energy via thermogenesis. Nonshivering thermogenesis is regulated by norepinephrine and the hypothalamus; it is impossible to achieve without brown fat.

Young infants, who are unable to shiver to generate internal heat, have a high proportion of brown fat. By residing in perpetually cold surroundings, it is feasible to induce brown adipocyte differentiation.

Brown adipocytes synthesise uncoupling protein-1 due to the expression of the gene encoding this protein. This is not the case with fat cells that are white, beige, or pink. This protein is responsible for the conversion of fatty acids to heat energy, which is the principal activity of brown fat tissue.

As recent studies demonstrate, brown fat tissue is also critical for a healthy immune system and protects against metabolic illnesses such as diabetes and non-alcoholic fatty liver disease. The less brown fat in the body, the greater the chance of developing various metabolic diseases.

Beige Adipocyte Function

While it was previously believed that beige fat comprised the same cell types as brown and white fat, it now seems that beige adipocytes are a distinct entity; they lack the uncoupling protein 1 (UPC1) gene and are largely stimulated by a hormone called irisin.

Irisin is released when skeletal muscle contracts. This provides evidence for the association between a sedentary lifestyle and an elevated risk of cardiovascular disease and diabetes. Irisin promotes the “browning” of fat depots by boosting the population of brown adipocytes in beige and brown fat depots.

Beige adipocytes act similarly to brown cells in that they generate heat from lipid fuel, but lack UCP1. While experts are uncertain why, some speculate that they are brown cells in a semi-dormant condition that might raise brown adipose tissue levels when stimulated over a longer period of time.

Bone Marrow Adipocyte Function

Recent research has focused on the yellow and red lipocyte populations in their respective bone marrow sites; low or high fat cell proportions in the bone marrow are related to inadequate or excessive blood cell generation, respectively.

Blood cell malignancies inhibit adipocyte formation in the bone marrow; intriguingly, solid tumours in other organs often interact with cancer-associated adipocytes (CCAs).

Pink Adipocyte Function

Pink adipocytes have been reported only in rats; white, beige, and brown adipocytes may develop into pink cells that do not seem to be stem cells.

Pink lipocytes are glandular cells; their numbers increase during pregnancy and breastfeeding. Due to the fatty content of mammalian milk, this requires the use of nearby triglyceride droplets.

While scientists are currently searching for a human comparable cell, they agree on the adaptability of adipocytes. Additionally, obesity and breast cancer are associated with mammary gland adipocyte populations.

Adipocyte Differentiation

Adipogenesis, the process by which fat cells are produced, starts in the stem cell.

Mesenchymal stem cells may differentiate into a variety of cell types and can also self-renew. They divide to become preadipocytes when activated. Growth factors, the tissue type in which the stem cell is situated, hormones, and competition from chemicals that govern other cell populations all influence stimulation.

A preadipocyte is a developed yet immature adipocyte. Brown preadipocytes differentiate into brown lipocytes, whereas white preadipocytes differentiate into white lipocytes.

Adipocytes, too, undergo transdifferentiation. A differentiated fat cell may divide and transform into another kind. The majority of pink cells began life as white adipocytes; brown adipocytes are formed by brown preadipocyte division or white adipocyte transdifferentiation.

Adipocyte Dysfunction

Adipocyte malfunction may be caused by a variety of factors. Genetic mutations have the potential to affect population structure or the way fat cells react to chemical cues.

Impaired adipogenesis results in a decrease in the production of fat cells, leading to existing white cells growing in size. This growth is referred to as adipocyte hypertrophy.

The bigger the adipocyte, the more inflammation there is and the greater the chance of developing insulin resistance, cancer, or obesity.

Each adipocyte must come into contact with a capillary; lipocyte hypertrophy and increased cell proliferation rates drive the development of new blood vessels (angiogenesis), but the blood volume does not rise appreciably.

This results in decreased oxygen and nutrient delivery to all tissues. To compensate, the heart rate rises; cardiac output is often greater in fat individuals.

Antibodies against adipocytes have been used effectively to remove fat from animals, resulting in leaner meat. This is a method of immunising adipocytes. Monoclonal antibodies may one day be used to develop an anti-obesity vaccine. Bimagrumab given intravenously in recent phase II clinical studies decreased up to 20.5 percent of body fat in obese people with type 2 diabetes after 48 weeks.

References

• Richard AJ, White U, Elks CM, et al. (Updated 2020).Adipose Tissue: Physiologyto Metabolic Dysfunction. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK555602/
• Heymsfield SB, Coleman LA, Miller R, et al. (2021). Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity: A Phase 2 Randomized Clinical Trial. JAMA Network Open. 2021;4(1):e2033457. doi:10.1001/jamanetworkopen.2020.33457
• Szablewski L (Ed.) (2019). Adipose Tissue: An Update. London, IntechOpen.