Adipocytes are the cells of adipose tissue. In addition to storing fat, they perform many other functions. Adipose tissue produces many hormones and is the largest endocrine organ in the human body.
What are adipocytes?
Adipocytes are not just fat-storing cells. They participate very actively in overall metabolism. In this process, they unite to form multinucleated cells, with the individual cells of the network connected by so-called gap junctions. There are two types of adipocytes. These are the univacuolar and the plurivacuolar adipocytes. The univacuolar adipocytes represent the white adipose tissue and contain only one vacuole, which has the task of storing fat. The vacuole can occupy up to 95 percent of the cell volume, pushing the other cell organelles and the nucleus to the edge of the cell. Thus, most of the cell is composed of storage fat. The plurivacuolar adipocytes belong to the brown adipose tissue and have several vacuoles that can be filled with storage fat. However, these do not push the other organelles to the edge of the cell. They have many mitochondria, which burn the fat directly inside the cell to generate heat. For example, the brown adipose tissue becomes active when it cools down. By burning fat, the organism ensures the maintenance of body temperature. The ratio of brown to white adipose tissue is decisive for energy consumption. However, in adult humans, brown adipose tissue plays little role, so fat reduction cannot be based on its activation.
Function, effect, and tasks
The most important task of adipocytes is the storage of body fat. The white adipose tissue is mainly responsible for this. To a small extent, energy is produced in the brown adipose tissue by burning fat. Energy production within these cells is independent of the body’s general energy metabolism. They only serve to maintain the body temperature when the outside temperature drops. For this purpose, the fat stored in the adipocyte is burned directly. In humans, this function is normally only relevant in infants. Later, the brown adipose tissue atrophies. However, there may be some people who are unable to gain weight because they still have a relatively large amount of brown adipose tissue. However, research has shown that the role of adipocytes is much more complex than the fat storage function would suggest. Adipose tissue is the largest endocrine organ, which is very active in metabolism. The amount of stored fat plays a very important role. Among other things, adipocytes produce, in addition to many hundreds of active substances, three important hormones that have a regulating effect on metabolism. These are the hormones leptin, resistin and adiponectin. Leptin inhibits the feeling of hunger. The more storage fat the adipocytes contain, the more leptin is secreted. However, additional administration of leptin to produce the feeling of satiety is unsuccessful because the leptin content of an obese person is already high and the additional administration has no effect. Resistin and adiponectin control insulin resistance. The more fat is stored in adipocytes, the lower the concentration of adiponectin. However, adiponectin promotes insulin sensitivity. Conversely, resistin increases insulin resistance. How else these hormones can be used therapeutically in diabetes requires further investigation.
Formation, occurrence, properties, and optimal levels
In general, the number of adipocytes remains the same throughout life. Only the volume of the cells changes when fat is stored or released. An adipocyte can store a maximum of 1 microgram of fat. When the uptake capacity of all adipocytes present in the body is reached and yet more fat continues to be built up than broken down, cell divisions are initiated in the preadipocytes, the so-called steatoblasts. New adipocytes develop from the steatoblasts. The number of fat cells does increase in this case. However, the number of adipocytes remains the same when fat is reduced. The newly formed small fat cells are insulin sensitive, unlike the existing adipocytes. After differentiation of the new fat cells, they also become insulin resistant again.
Diseases and disorders
Obesity has become a common disease.The more fat is stored in the adipocytes, the greater the risk of developing type II diabetes. Diabetes, in turn, is an underlying disease for many degenerative processes in the body. Eventually, the metabolic syndrome may develop with a complex of diseases such as obesity, diabetes, dyslipidemia, atherosclerosis and cardiovascular disease. During the development of obesity, insulin resistance decreases over time. Insulin ensures that blood sugar, fatty acids and amino acids are channeled into the body’s cells to generate energy or to provide for body composition. Excess energy that is not consumed is stored by adipocytes in the form of fat. Hormonal processes in the fat cells, in turn, control insulin resistance to limit the unlimited supply of glucose. This process is actually normal. However, it gets out of control if calories continue to be supplied that cannot actually be stored. Insulin resistance develops into a chronic condition. Insulin is produced in large quantities. However, it becomes increasingly ineffective. The blood glucose level rises. The pancreas is stimulated even more to produce insulin. This continues until there is an exhaustion of production. Now the relative insulin deficiency becomes an absolute insulin deficiency due to insulin resistance. Manifest diabetes with all its consequences has developed.
