Some adipocytes appear during embryonic development, but their numbers increase greatly following birth. This increased number is due to both mitotic division of the adipocytes and the conversion of preadipocytes (derived from fibroblasts) into new adipocytes. This differentiation (specialization) is promoted by a high circulating level of fatty acids, particularly of saturated fatty acids. This represents a nice example of a negative feedback loop, where a rise in circulating fatty acids promotes processes that ultimately help to convert the fatty acids into stored fat.
The differentiation of adipocytes requires the action of a nuclear receptor protein—in the same family as the receptors for thyroid hormone, vitamin A, and vitamin D—known as PPARy. (PPAR is an acronym for peroxisome proliferator activated receptor, and the y is the Greek letter gamma, indicating the subtype of PPAR.) Just as the thyroid receptor is activated when it is bound to its ligand, PPARy is activated when it is bound to its own specific ligand, a type of prostaglandin. When this ligand binds to the PPARy receptor, it stimulates adipogenesis by promoting the development of preadipocytes into mature adipocytes. This occurs primarily in children, since the development of new adipocytes is more limited in adults.
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