Similar to fibroblasts, chondroblasts, osteoblasts and myoblasts, white fat and brown fat adipose cells derive from a mesenchymal stem cells in a process known as adipogenesis.

White Adipose Tissue

Adipose tissue is the storage center for fat. White (unilocular) adipose tissue consists of densely packed adipose cells. Each adipose cell contains a single large drop of fat (hence the "unilocular" name). The fat is dissolved out by alcohol during the preparation procedure, leaving a large vacuole. The adipose cell has a flattened nucleus and sheetlike cytoplasm, which has been pushed into a rim surrounding the fat drop. Between adipose cells there are capillaries and a very small amount of connective tissue with reticular fibers, which can be demonstrated by silver impregnation. In this tissue type, adipose cells are usually separated into lobules by a connective tissue septum containing blood vessels and nerve fibers.

Under the influence of the hormone insulin, the adipose cells can synthesize fat from fatty acids or carbohydrate. The vital balance between deposits and withdrawals of the fat is affected by hormones and by the autonomic nervous system. Except for the eyelid, penis, and scrotum, adipose tissue is present throughout the body, and is especially abundant in the hypodermis, mesenteries, omenta, and around the kidneys. The white adipose tissue, which constitutes as much as 20% of total body weight in a normal male adult and up to 25% in female, acts as a storage site and metabolic source of neutral fat, a cushion against mechanical shock, and a insulator to prevent excessive heat loss or gain through the skin.

Brown Adipose Tissue

Brown (multilocular) adipose tissue differs from white adipose tissue by both the organization of its lipid and its characteristic color. Brown adipose tissue is abundant in hibernating mammals. In human beings, it is common in the fetus, but almost absent in the adult. Brown adipose tissue is distributed mainly in the shoulder girdle and some other areas. It is composed of densely packed brown adipose cells. The cytoplasm contains a number of lipid droplets of various sizes; thus, this tissue type is also called multilocular adipose tissue. In addition, abundant in the cytoplasm are the mitochondria and lysosomes, which give the tissue its brown color. The nucleus of the multilocular adipose cell is spherical, located centrally or eccentrically. Between the adipose cells is a rich supply of capillaries. In hibernating animals and the human fetus, the brown adipose tissue generates heat to keep the body warm during cold temperatures.