The adipose tissue originates from mesenchymaL cells and is considered a subtype of the connective tissue.
The main cells of this tissue are the adipocytes, also called adipose cells or fat cells. They may be found scattered singly within the connective tissue or more commonly organized as clusters of diverse sizes. In addition to adipocytes, the adipose tissue also contains blood vessels, nerves and a small number of resident and transient connective tissue cells.
The adipose cells are separated from each other by a small amount of extracellular matrix. Part of this matrix consists of a network of thin reticular fibers formed primarily by type III collagen, usually not seen with a light microscope in routine stained sections.
Adipocytes are characterized by the collection of lipid in their cytoplasm. The lipid may be in the form of droplets or one large drop. They are not surrounded by a membrane and are therefore considered as cell inclusions. Triglycerides (neutral fats) are the main component the lipid.
The number and size of lipid droplets per adipocyte can vary considerably. Many small lipid droplets are found while adipose cells are developing. As the cells mature and form yellow fat the droplets merge into one large droplet or if they form brown fat they remain as multiple droplets.
Thus, two varieties of adipose tissue are recognized: unilocular tissue also called yellow or white fat and multilocular adipose tissue or brown fat. They show significant differences in their morphology, functions, and distribution in the body.