Adipose tissues have long been considered to derive from a common origin. Even the functionally different brown and white adipose tissues were generalized to share a common origin. Brown adipose tissue is a highly innervated and vascularised tissue containing multilocular and multimitochondrial brown adipocytes. Brown adipose tissue expends energy through sympathetic nervous system-mediated non-shivering thermogenesis, where uncoupling protein 1 (UCP1) is the key player. In contrast, white adipose tissue consists of unilocular white adipocytes with a main role to store energy in the form of the lipid droplet. We know today that this generalisation is exaggerated since adipocytes can derive from more than one origin and not only be brown or white. We and others have demonstrated that the brown adipocyte has a dermomyotomal origin and derives from the adipomyocyte, the precursor cell that can also become a myocyte, whereas white adipocytes are suggested to derive from pericytes, cells that are embedded within the vascular vessel walls. For a long time there has been evidence that energy-expending adipocytes reside within certain white adipose tissues, based on the fact that cold exposure, by switching on the sympathetic nervous system, leads to levels of UCP1 that are not detectable in mice housed at thermoneutrality. We demonstrated that these cells have a molecular signature that is distinct from brown and white adipocytes. Since these energy-expending cells reside within certain white adipose tissues, we chose to name them brite (brown in white) adipocytes. Moreover, we also identified regulatory factors that were specifically expressed in each adipocyte type, thus, facilitating the possibility to identify the three adipocytes: the brown, the white and the brite. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.