Adipocytes

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

Main functions of adipocytes

Adipocytes are cells particularly suited to the accumulation of fat, which store large lipid droplets occupying most 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 are also massed, such as the nucleus and ribosomes.

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

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

The lipids stored in the adipocytes derive:

from the dietary intake of triglycerides circulating in the blood in the form of chylomicrons;

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

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

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

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 one is appreciated.

lipid drip (from which the term uniloculari), which pushes 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. In the human organism it is possible to appreciate a second type of fat cells, much smaller in number; we are talking about the so-called brown adipocytes. These cells are distinguished from the previous ones by various characteristics:

lipid content distributed in several fat droplets (→ multilocular) instead of in a single central droplet;

cytoplasm spread throughout the cell space and richly filled with mitochondria;

centrally distributed core;

most innervated and vascularized fat cells;

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

The particular amber hue of brown adipocytes is linked to the generous presence of mitochondrial cytochromes. In the membranes of mitochondria a particular protein is present, called UCP-1, also called disaccopiante or thermogenin as it is able to direct the proton flow not to ATP resynthesis, but to dissipation in the form of heat (thermogenesis without shudder). The main function of brown adipose tissue is to produce heat, heating the body to protect it from the cold outside. The thermogenic activity of brown adipose tissue is also induced by caloric excesses, in an attempt to dissipate the superfluous energy avoiding the excessive accumulation of fat. Thermogenic activity is subject to the activity of 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 the event of unfavorable weather 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, white adipose tissue represents 15-20% of 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 energy reserve. Today, we know that it acts as a real organ, with a marked endocrine and even immune activity, able to influence the metabolic activity of the whole organism. In fact, white adipocytes secrete particular highly active proteins, the so-called adipokines: leptin, adipisine, resistin and adiponectin, which influence the metabolism in synergy with other hormones, such as insulin, to regulate body mass. Furthermore, white adipocytes secrete several cytokines, such as TNFa, 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, hyperproduction of pro-inflammatory cytokines by white adipocytes is responsible for insulin resistance, metabolic syndrome and related complications.