Adipocytes are fat cells found in the human body. They can also be called lipocytes. One type of fat cell, the white fat cell, is important because it serves as a stockpile of energy. Brown fat cells can also store energy, but their more important function is their ability to create heat. The amount of fat, also referred to as lipids, stored in adipocytes is tightly regulated by the human body.
There are two types of adipocytes present in humans, although in adults the amount of white adipose tissue greatly exceeds the brown adipose tissue. White fat functions to store lipids for future energy use. Additionally, it serves as a component of the endocrine system, and helps to create the hormone estrogen from precursor molecules. Although this function is important in the human body, the excess estrogen found in obese individuals is thought to contribute to a variety of diseases.
The microscopic appearance of white fat cells is unique, and therefore it is easy to distinguish these adipocytes from other types of cells. They are considered to be unilocular, meaning that they have one large fat-filled vacuole, or storage compartment, occupying most of the cell. As a result of the accumulation of lipids, the nucleus is pushed to a peripheral position. These cells typically have a yellow color.
Brown fat cells have a different function than white fat cells. They have thermogenic capabilities, meaning that they have the ability to generate body heat. Adults only have a small amount of brown fat surrounding their kidneys and in their necks. In contrast, up to a quarter of the weight of infants is made of brown fat, which helps to keep them warm after leaving the womb.
The characteristics of brown fat cells as seen under the microscope are different from those of the white fat cells. The brown adipocytes are multilocular, and have numerous tiny vacuoules filled with fat. The brown color of these cells is due to the presence of many mitochondria, which are small structures used to generate energy. The appearance of brown fat cells is described as "foamy" due to the many small vacuoles present within the cells.
In adults, the main purpose of adipocytes is to store energy in the form of fat. The regulation of lipid use and storage within the human body is tightly regulated by hormones. If the body senses that a large amount of glucose is present in the blood stream, a hormone called insulin stimulates fat cells to collect fatty acids from the blood stream and store them as lipids. In contrast, if the body is running low on glucose, hormones can cause the fat cells to release their contents for use as energy.