The average adult has 40 billion fat cells (aka adipose tissue). These cells protect vital organs, store energy, direct hormones, influence bone growth, and release chemicals that affect the body’s metabolism. An obese person has approximately three times the amount of normal fat cells, which also continue to multiply throughout life and affect body function. Stem cells from adipose tissue are used in the therapeutic treatment of various diseases. Fat cells are beneficial to the human body but can also be disadvantageous; especially in cases of obesity.
Fat Cell Development 
Fat cells develop from stem cells and the body creates a set number of them during the developmental stages that last for life. Infants develop fat cells during the third trimester of pregnancy. As a child grows, these cells divide and multiply. An obese child adds twice the rate of new fat cells than a thin child does, which results in an increase in the total amount of fat cells early in life. The fat cells will continually seek replenishment throughout adulthood, and so this puts obese children at an early weight disadvantage.
When fat cells have reached their maximum storage capacity, they undergo mitosis, or cell division, in which they divide into two new cells that eventually expand and also undergo mitosis. When the amount of calories taken into the body is in excess of what is burned, the existing fat cells produce and store more fat. The body will also recruit primitive fat cells, preadipocytes, to mature, which also increases the capacity to store fat. Fat cells that die off in adulthood, approximately 8 percent each year, are rapidly replaced. Consequently, adults retain a constant number of fat cells that are in various stages of size and development. Body fat also increases with age and women experience a greater percentage of it than men do. Adipose tissue also has the ability to direct the body to store additional fat even though eating habits are not dramatically increased.
Fat Cell Physiology
Fat cells regulate energy and how it is utilized through the release of chemical signals throughout the body. These chemicals affect tissues of the brain, liver, muscles, reproductive organs, and immune system. Fat cell receptors release chemicals that initiate appetite and affect insulin, the immune system, and reproduction. The cells regulate metabolic processes by instructing muscles to burn energy or the liver to refurnish fuel supplies. Fat cells also constrict blood vessels, raise blood pressure, and influence clotting, which correlates with obesity and leads to heart attack and stroke. Fat cells also encourage cell growth, which could provide important information about the relationship between cancer and obesity.
When people overeat, fat cells grow approximately three or four times their size. A higher concentration of body fat means larger fat cells. One study found that the number of fat cells set from childhood and adolescence remained the same in humans, even with drastic weight loss. This indicates that weight gain and loss is actually the growth or shrinkage of fat cell volume and not the quantity of the cells. However, another study found that the turnover rate for fat cells is approximately 10 percent each year and older fat cells are replaced by newer ones in adults of all ages and body weights.
Stem Cells from Fat? Yes!
Fat contains mature adipocytes and immature preadipocytes and is a source of adult stem cells, which are precursor cells with the potential to develop into various specialized cells and tissues. Current research is focusing attention on the potential for fat-derived stem cells in the cure for various diseases. There has been some success in this area, more specifically, in patients with Multiple Sclerosis who have experienced relief from their symptoms after undergoing fat stem cell therapy to treat their disease.
Leptin and Bones 
Researchers use mice to study various human diseases, and have found strains that are obese because of single gene mutations. An example is the ob/ob mouse (ob stands for obese), which was identified in 1950. These mice are obese, resistant to insulin, and always hungry. In 1994, scientists discovered that the mutated ob gene was responsible for producing the hormone leptin (Greek for thin). Leptin is expressed predominantly by adipocytes, with smaller amounts secreted by cells of the stomach and the placenta. Leptin receptors are located in areas of the hypothalamus important in regulating body weight, and in T lymphocytes and vascular endothelial cells. Experiments showed that when the ob/ob mice were given leptin supplements, they lost weight. Since that time, leptin received a lot of attention and many scientific studies were performed on it. It was thought that leptin would have the same effect in obese humans, but clinical trials proved otherwise. Leptin has helped treat rare diseases, however, such as lipodystrophy, a disease in which fat cells atrophy and disappear.
Leptin also drives the ovarian cycle by stimulating the secretion of hormones. Because of this, lack of leptin and a consequent estrogen shortage should cause bone loss. Scientists found that is not the case, and instead, lack of leptin and not excessive weight, is responsible for increased bone mass. Ironically, leptin indirectly restrains but directly stimulates bone formation. It causes osteoblasts (bone forming cells) to make proteins, which in turn stimulates born formation. However, leptin also stimulates the cells of the hypothalamus to make hypothalamic osteoblast inhibitory factor (HOBIF) or it triggers some neural process that restrains the matrix-making ability of osteoblasts, which inhibits bone formation.
The World Health Organization (WHO) reported that 1.6 billion adults are overweight. We also know that excessive fat is disruptive to the body's hormonal balance. To resolve this problem, according to the New York Times, "The best way to get rid of visceral fat and shrink fat cells all at once is diet and exercise. Just a loss of 7 percent of total body weight helps." Although too much fat can work against us, under normal conditions it plays an important role by protecting organs and regulating metabolic processes that affect various systems in the body.
Copyright ©2012 Joyce E.M. Wall