Saturday, July 2, 2011

Fat Cells (Adipocytes)

Adipocytes

PKGhatak,MD




 Fat gets bad press all the time. It began with epidemiological studies linking the increased rate of heart attacks with elevated blood levels of fat and cholesterol.
If fat is that bad - why do we have fat cells in our body in the first place?

 You must have seen the painting of the smiling face of the Mona Lisa. Now think - if she was skinny like the present day supermodels would her smile be as captivating!

Underneath the skin, there is a layer of fatty tissue that protects the body against the elements - acting as a thermal insulator.  A human body has many empty spaces, big or small, all are nicely packed with fatty tissue. Without packing, the kidneys, liver, spleen, and other abdominal organs will have a hard time staying in their respective places and work.

In starved conditions, Fat cells (adipocytes) release a breakdown product fat called Fatty Acids which provide needed energy to the body. When excess energy (food) is ingested, the excess energy is turned into Triglyceride (fat) and stored in adipocytes.  As fat accumulates, the adipocytes bulge with fat. Fat cells may enlarge four times the original size and then divide; thereby, increase the absolute number of fat cells. On the other hand, in prolonged fasting, fat cells shrink in size but do not decrease in number. Adipocytes, when stretched with loaded fat, release a hormone called Leptin. Leptin acts directly on the appetite center and via the vagus nerve. It decreases appetite and increases the rate of metabolism. Besides leptin, adipocytes also produce a BP regulating chemical called Angiotensinogen, a vascular protective protein called Adiponectin, a blood clotting inhibitory factor known as Plasminogen Activator Inhibitor I, a complement called Adiposin or factor D, and cytokines called Interleukin IL6, and Tumor Necrosis Factor Alpha.  These chemicals help to regulate BP, blood sugar, blood lipids, the integrity of blood vessels, healthy body weight and the immune system.
When body weight increases, leptin production also increases but it does not depress hunger.  Probably because, the secretory products of adipocytes have a rate-limiting production or are made inactive by another substance like Resistin - produced by obese adipocytes. Resistin increases insulin resistance and high blood sugar.

Adipose tissue is a loose connective tissue. It contains adipocytes, preadipocytes, and macrophages and has a rich supply of blood vessels and nerve fibers.
Fat cells (Adipocytes) belong to the Fibroblast Family of Cells of connective tissue origin. Other members of this prominent family are myofibroblasts, smooth muscle cells, osteocytes, osteoblasts, chondrocytes, and fibroblasts. Because of the common origin, these cells share the capacity to differentiate into one another.
An adult has 30 billion adipocytes in the body - about 30 lbs. in weight.
Adipocytes are two different kinds of Adipocytes: White Adipocytes and Brown Adipocytes.

 In adults, most adipocytes are white adipocytes (white fat cells). White fat cells store fat inside the cell as a single large globule; chemically it is a triglyceride. (One molecule of Glycerol combined with 3 molecules of fatty acids). The nucleus of the cells is pushed to one side and the cytoplasm of the cell appears as a thin rim when seen under the microscope. Once loaded with an excessive amount of fat, the white fat cells will divide and increase in numbers.  An adult person turnover 10% of fat cells every year. Preadipocytes, in the connective tissue, generate new adipocytes. In situations of need, the connective tissue fibroblasts transform into adipocytes. And this is not a one-way phenomenon, under appropriate circumstances, adipocytes can transform into fibroblasts and fibrocytes.


The brown fat cells are also known as Baby Fat Cells, they are loaded with mitochondria that give them the brown color.  20% of adipocytes in newborn babies are brown fat cells. Brown fat cell numbers decrease with age. In adults, brown fat cells are found in the lower neck and around the main blood vessels of the chest. Hibernating animals have more brown fat cells. The fat in the brown adipocytes is present as drops of various sizes throughout the cell cytoplasm and the nucleus is somewhat out of the center but not pushed to the periphery.  Brown fat cells generate heat by a process known as Uncoupled Respiration and Thermogenesis. This is achieved due to the presence of a unique mitochondrial protein. Simply, it means when exposed to cold the Norepinephrine stimulates fat breakdown in both the white and brown fat cells. But brown fat cells burn fatty acids and glycerol and generate body heat. While the white fat cells release fatty acids into circulation. Brown fat cells are closer, in origin, to skeletal muscles than to white fat cells. In experimental animals, white fat cells can be switched into brown fat cells. Many studies are currently underway to turn human white fat cells into brown fat cells.

The fatty tissue used to be considered storage and packing material but is now considered an important chemical factory. When the method of turning white fat cells into brown fat cells in humans will be achieved and subsequently marketed as a drug, then the fat will be the darling of the pharmaceutical companies. Such a drug is expected to bring in $30 billion/year and will be the drug of choice for obesity.
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