Adipocytes

Adipocytes is the term with which scholars identify the cells of the adipose tissue, better known today as the adipose organ.

Main functions of adipocytes

Adipocytes are cells particularly suitable for the accumulation of fats, which store inside large lipid droplets that occupy a large part of the cell volume; to make room for these adipose accumulations, the cytoplasm of the adipocytes is stratified against the cell walls, where the other organelles, such as nuclei and ribosomes, are also gathered.

The first function of the adipocytes therefore consists in the accumulation of fat, to then eventually give it to the organism in case of need.

An overweight individual has adipocytes richer in fat than a normal weight person, while the number of fat cells is comparable. The adipocytic patrimony of the organism is in fact genetically determined by birth (the adipocytes originate from the primitive mesenchyme from which they develop as lipoblasts); the so-called phenomenon of "adipocytic hyperplasia has been demonstrated only in the very obese, for which - especially during" childhood and puberty - the number of adipocytes increases To date, the opposite phenomenon has not been proven: therefore, fat cells can reduce their volume by emptying themselves of fat but not decrease in number.

The lipids stored in the adipocytes derive:

from the dietary supply of triglycerides that circulate in the blood in the form of chylomicrons;

from the hepatic synthesis of triglycerides, transported in the blood within low density lipoproteins;

from the synthesis of triglycerides in the adipocytes through the chemical transformation of other excess substances, mainly glucose.

Adipocytes and adipose tissue in general are also important for the protection of the organism from harsh environmental temperatures (insulating effect), and from external trauma (particularly important, in this sense, the adipose tissue that surrounds the kidneys, helping to keep them in the correct anatomical location).

White adipocytes and brown adipocytes

In the previous chapter we examined the typical structure of the so-called white adipocytes, in which an internal cavity occupied by a large

lipid drips (hence the term unilocular), which pushes the nucleus and cytoplasm against the cell wall, binding the latter to a thin peripheral halo; we have also described the main function of adipose tissue, typical of white adipocytes: that of energy reserve. "human organism it is possible to appreciate a second type of fat cells, numerically much smaller; we are talking about the so-called brown adipocytes. These cells are distinguished from the previous ones by several characteristics:

lipid content distributed in several fat droplets (→ multilocular) rather than in a "single central droplet;

cytoplasm spread throughout the cellular space and richly stuffed with mitochondria;

centrally distributed core;

fat cells more innervated and vascularized;

dark yellow color, hence the term "brown" adipocytes.

The particular amber hue of the brown adipocytes is linked to the generous presence of mitochondrial cytochromes. In the membranes of mitochondria there is a particular protein, called UCP-1, also called decoupling or thermogenin as it is capable of directing the proton flow not to the resynthesis of ATP, but to dissipation in the form of heat (thermogenesis without shivering). The main function of the brown adipose tissue is to produce heat, warming the body to protect it from the external cold. The thermogenic activity of the brown adipose tissue is also induced by excess calories, in an attempt to dissipate superfluous energy avoiding excessive accumulation of fat. The thermogenic activity is subject to the activity of the B-3 adrenergic receptors, possible targets of future anti-obesity drugs.

The fetus and the newborn are provided with large reserves of brown fat, useful for being "burned" at the time of birth in case of unfavorable climatic conditions. In adults, the percentage of brown fat is generally negligible, and increases during exposure to cold temperatures. Conversely, in the normal weight and physically active individual, the white adipose tissue represents 15-20% of the body weight in men and around 25% in women.

Endocrine and immune functions of white adipocytes

Until a few decades ago, adipose tissue was considered an "inert reserve of energy. Today, we know that it acts as a real organ, with a marked endocrine and even immune activity, capable of influencing the metabolic activity of the whole" The white adipocytes, in fact, secrete particular highly active proteins, the so-called adipokines: leptin, adipisin, resistin and adiponectin, which influence the metabolism in synergy with other hormones, such as insulin, to regulate body mass. Furthermore, white adipocytes secrete various cytokines, such as TNFα, IL-6, IL-1 and MCP-1, which act on immune cells by regulating immune processes, and on endothelial cells by modulating the release of nitric oxide. It has been shown that in obese individuals, the overproduction of pro-inflammatory cytokines by white adipocytes is responsible for insulin resistance, metabolic syndrome and related complications.