Adipose tissue has a number of functions in the body which include mechanical cushioning (e.g. in the buttocks, and around some internal organs) and thermal insulation, but its main role from a metabolic point of view is that of storing chemical energy in the form of triacylglycerol, and releasing it in the form of non-esterified fatty acids when it is needed by other tissues. However, recent years have seen an explosion of interest in adipose tissue as an endocrine organ as well as a metabolic one. It is now recognised that adipose tissue secretes a number of substances, some of which are true hormones, others that may act locally, and that this capability gives adipose tissue an especially important role in metabolism.
There are several cell types in adipose tissue. We will concentrate upon the cells that store fat, the adipocytes. Other cells include pre-adipocytes (small cells that can differentiate into mature adipocytes when there is a surplus of fat to be stored), endothelial cells (lining blood vessels) and macrophages. Some of these other cells play a role in the secretory activities of the tissue, although not to any great extent in its 'energy metabolism'.
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