Adipocyte

Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat.

There are two types of adipose tissue, white adipose tissue (WAT) and brown adipose tissue (BAT), which are also known as white fat and brown fat, respectively, and comprise two types of fat cells.

White fat cells (unilocular cells)
White fat cells or monovacuolar cells contain a large lipid droplet surrounded by a layer of cytoplasm. The nucleus is flattened and located on the periphery. A typical fat cell is 0.1mm in diameter with some being twice that size and others half that size. The fat stored is in a semi-liquid state, and is composed primarily of triglycerides and cholesteryl ester. White fat cells secrete many proteins acting as adipokines such as resistin, adiponectin and leptin. An average adult has 30 billion fat cells with a weight of 30 lbs or 13.5 kg. If excess weight is gained as an adult, fat cells increase in size about fourfold before dividing and increasing the absolute number of fat cells present.

Brown fat cells (multilocular cells)
Brown fat cells or plurivacuolar cells are polygonal in shape. Unlike white fat cells, these cells have considerable cytoplasm, with lipid droplets scattered throughout. The nucleus is round, and, although eccentrically located, it is not in the periphery of the cell. The brown color comes from the large quantity of mitochondria. Brown fat, also known as "baby fat," is used to generate heat.

Lineage
adipocytes is still unclear, preadipocytes are undifferentiated fibroblasts that can be stimulated to form adipocytes.

Mesenchymal stem cells can differentiate into adipocytes, connective tissue, muscle or bone.

Areolar connective tissue is composed of adipocytes.

The term "lipoblast" is used to describe the precursor of the adult cell. The term "lipoblastoma" is used to describe a tumor of this cell type.

Cell turnover
After marked weight loss, the number of fat cells does not decrease (the cells contain less fat). Fat cells swell or shrink but remain constant in number. However, the number of fat cells may increase once existing fat cells are sufficiently full.

Approximately 10% of fat cells are renewed annually at all adult ages and levels of body mass index.

Endocrine functions
Adipocytes produce estrogen, potentially being the reason why underweight or overweight are risk factors for infertility.