Fat Cells (Adipocytes)

         P.K.Ghatak.MD                                        

Fat is getting a bad press for several decades. It began with a convincing epidemiological study which linked increased rate of heart attacks with  elevated blood levels of fat and cholesterol. The present generation is repeatedly warned that their bulging waistlines will make them susceptible to many diseases from diabetes to cancer as they grow older.

If fat is that bad, why should we have fat cells in the first place?

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

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

In starved conditions, adipocytes release a breakdown product of fat, called Fatty Acids, which provides needed energy. When excess energy is taken in (food) over and above the body’s immediate need the excess energy is turned into triglyceride and stored in adipocytes.  As fat accumulates, the adipocytes bulge with fat. When a person gains weight, the cells increase four times the original size before dividing and thereby increasing the absolute number of fat cells. In weight loss fat cells shrink in size but do not decrease in number. Adipocytes, when stretched with loaded fat, release a hormone called Leptin.  Leptin acts on the appetite center directly and via the vagus nerve. It decreases appetite and increases energy expenditure. Besides leptin, adipocytes produce blood pressure regulating Angiotensinogen, a vascular protective protein called Adiponectin, a blood clotting inhibitor known as Plasminogen activator inhibitor I, complement called Adiposin or factor D, and cytokines - Interleukin IL6 and Tumor necrosis factor alpha. The net effect of these chemicals on the body is to help to regulate BP, blood sugar, blood lipids, blood vessels’ health, a healthy body weight and a competent immune system.

When body weight increases the leptin production also increases but for one reason or other leptin fails to shut down hunger sensation in the appetite center located in the hypothalamus. Other secretary products of adipocytes either 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, 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. Because of the common origin, these cells share the capacity to differentiate into one another.

An adult has 30 billion adipocytes in the body with a weight of about 30 lbs. Adipocytes are two different kinds: 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.  Once loaded with an excessive amount of fat, the white fat cells can divide and increase the total number.  An adult person turns over 10% of fat cells every year. Preadipocytes in the fat connective tissue generate new adipocytes. Connective tissue fibroblasts transform into adipocytes when required. And this is not a one-way phenomenon. Under appropriate circumstances, adipocytes can transform into fibroblasts/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 population decreases with age. In adults, brown fat cells are found in the neck and chest, around main blood vessels. Hibernating mammals have relatively more brown fat cells.  Brown fat cells generate heat without shivering by a process known as uncoupled respiration and thermogenesis because they contain a unique mitochondrial protein. The simple meaning of this high sounding process is that when exposed to cold, the norepinephrine stimulates fat breakdown in both white and brown fat cells. But brown fat cells burn fatty acids and glycerol and generate body heat. The white fat cells, on the other hand, release fatty acids into circulation. Brown fat cells are closer, in origin, to skeletal muscle cells than to white fat cells. In experimental animals, white fat cells can be switched into brown fat cells. Many studies are underway currently to turn human white fat cells into brown fat cells.

The fatty tissue used to be considered as storage and packing material but now is considered as an endocrine organ. When the method of turning white fat cells into brown fat cells in the human will be developed and 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.