|Yellow adipose tissue in paraffin section|
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. Most recently presence of beige adipocytes with gene expression pattern distinct from either white or brown adipocytes has been described.
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, leptin and Apelin. 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.
Although the lineage of adipocytes is still unclear, pre-adipocytes are undifferentiated fibroblasts that can be stimulated to form adipocytes.
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.
Adult rats of various strains became obese when they were fed a highly palatable diet for several months. Analysis of their adipose tissue morphology revealed increases in both adipocyte size and number in most depots. Reintroduction of an ordinary chow diet to such animals precipitated a period of weight loss during which only mean adipocyte size returned to normal. Adipocyte number remained at the elevated level achieved during the period of weight gain.
However, in some reports and textbooks, the number of fat cell (adipocytes) increased in childhood and adolescence. The total number is constant in both obese and lean adult. Individuals who become obese as adults have no more fat cell than they had before.
People who have been fat since childhood generally have an inflated number of fat cells. People who become fat as adults may have no more fat cells than their lean peers, but their fat cells are larger. In general, people with an excess of fat cells find it harder to lose weight and keep it off than the obese who simply have enlarged fat cells.
According to a research by Tchoukalova et al., 2010, it has been reported that the body fat cells could have regional responses to the overfeeding studied in adult subjects. In upper body, an increasing of the adipocyte size was correlated with upper-body fat gain; however, the total fat cells were not significantly changed. In contrast to the upper body fat cell responses, a number of lower-body adipocytes were significantly increased during the course of experiment but there was no change in the cell size.
Approximately 10% of fat cells are renewed annually at all adult ages and levels of body mass index without a significant increase in the overall number of adipocytes in adulthood.
Endocrine functions 
Adipocytes can synthesize estrogens from androgens, potentially being the reason why being underweight or overweight are risk factors for infertility. Additionally, adipocytes are responsible for the production of the hormone leptin. Leptin is important in regulation of appetite and acts as a satiety factor.
- Wu J, Bostrom P, Sparks LM, Ye L, Choi JH, Giang AH, et al. Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human. Cell 2012; 150:366-76.
- Pool, Robert (2001). Fat: fighting the obesity epidemic. Oxford [Oxfordshire]: Oxford University Press. ISBN 0-19-511853-7.[page needed]
- Hong, Ran; Choi, Dong-Youl; Do, Nam-Yong; Lim, Sung-Chul (2008). "Fine-needle aspiration cytology of a lipoblastoma: A case report". Diagnostic Cytopathology 36 (7): 508–11. doi:10.1002/dc.20826. PMID 18528880.
- Faust, IM.; Johnson, PR; Stern, JR; Hirsch, J (1978). "Diet-induced adipocyte number increase in adult rats: a new model of obesity". AM J Physiol 235 (3): 279–96. PMID 696822.
- Spalding, Kirsty L.; Arner, Erik; Westermark, Pål O.; Bernard, Samuel; Buchholz, Bruce A.; Bergmann, Olaf; Blomqvist, Lennart; Hoffstedt, Johan et al. (2008). "Dynamics of fat cell turnover in humans". Nature 453 (7196): 783–7. doi:10.1038/nature06902. PMID 18454136.
- Pool, Robert (2001). Fat: fighting the obesity epidemic. Oxford [Oxfordshire]: Oxford University Press. p. 68. ISBN 0-19-511853-7.
- Tchoukalova, YD.; Votruba, SB; Tchkonia, T.; Giorgadze, N.; Kirkland, JL.; Jensen, MD. (2010). "Regional differences in cellular mechanisms of adipose tissue gain with overfeeding". PNAS 107 (42): 18226–31. doi:10.1073/pnas.1005259107. PMC 2964201. PMID 20921416.
- Nelson, Linda R.; Bulun, Serdar E. (2001). "Estrogen production and action☆". Journal of the American Academy of Dermatology 45 (3): S116–24. doi:10.1067/mjd.2001.117432. PMID 11511861.
- http://www.protectyourfertility.com/femalerisks.html[dead link] FERTILITY FACT > Female Risks By the American Society for Reproductive Medicine (ASRM). Retrieved on Jan 4, 2009
- Klok, M.D.; Jakobsdottir, S.; Drent, M.L. (2006). "The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review". The International Association for the Study of Obesity. obesity reviews 8 (1): 12–34. doi:10.1111/j.1467-789X.2006.00270.x. PMID 17212793.
- BU Histology Learning System: 08201loa - "Connective Tissue: unilocular (white) adipocytes "
- BU Histology Learning System: 04901lob - "Connective Tissue: multilocular (brown) adipocytes"