Three layers under the skin

Lying three layers deep under the skin, the adipose tissue is composed of a loose collection of specialized cells, called adipocytes, embedded in a mesh of collagen fibers.  Its main role in the body is function as a fuel tank for the storage of lipids and triglycerides.

There are two kinds of adipose tissue; white and brown.  The distribution of both these kinds is not uniform, but white adipose tissue is the most common.

Storing lipids

The main role, or function, of white adipose tissue is to collect, store and then release lipids.  However, because of the properties of the lipids being stored, the adipose tissue also acts as a protective cushion (resists knocks) and also as a layer of insulation against excessive heat loss.

Lipids conduct heat very poorly (only about a third of the rate of other materials), so a even a small layer of adipose cells (about 2 mm) will keep a person warm at 15 degrees centigrade, whereas a person with only a 1 mm layer of protection will be feeling quite uncomfortable.

About 80% of average white adipose tissue is lipid, and of that about 90% is made up of the six triglycerides: stearic, oleic, linoleic, palmitic, palmitoleic and myristic acid.  Also stored are free fatty acids, cholesterol, mono- and di-glycerides.

Each adipocyte cell has a large, central, uniform, lipid packed central vacuole which, as it enlarges, pushes all the cytoplasm, the nucleus and all the other organelles to the edge of the cell, making it look a bit like a band or ring under the microscope.

These cells can vary in size from about 30 microns to over 230 microns, and, despite their distorted appearance, contain all the necessary biochemical machinery of other cells.

Every adipose cell must touch at least one capillary or blood vessel (an artery or vein).  From this the cells draw all their needed supplies, including lipids.

Fatty foods, with high lipid content, often provide more lipids than can be digested and used right away.  The excess is stored in the adipose tissue.  Excess carbohydrate and protein taken in with meals can also be converted to fat (usually in the liver) and then moved to the adipose tissue for longer-term storage.

Lipids are the major fuel reserve for humans and most mammals.  These molecules are very efficient at storing needed energy.  One gram of fat stores about 9 kcal per gram, compared to carbohydrate or protein (4 kcal per gram).  For mobile animals, this means that less bulk has to be carried around and a normal sized body that is about 20% fat has enough stored energy to last about 20 – 30 days without eating!