The short answer to the question is . . . .
But the more technical answer regarding the developmental origin of adipose tissue is one that has eluded scientists for some time now. Exactly where and how adipocytes arise is the subject of a recent article in the journal Science.
Researchers looking at mice found that adipocytes in white adipose tissue (WAT or white fat) arise from progenitor cells located within the walls of blood vessels. Apparently, these progenitor cells commit to producing fat cells just before and/or just after birth. However, this happens only in vessels of tissues that will become fatty--not in the vessels of other tissues.
Exactly how this all plays out is yet to be worked out--but now we have something to work out!
Check out these resources:
White Fat Progenitor Cells Reside in the Adipose Vasculature
Wei Tang, et al. Science 322 (5901), 583. (24 October 2008) [DOI: 10.1126/science.1156232]
Can We Nip Obesity in Its Vascular Bud?
C. Ronald Kahn
Science 322 (5901), 542. (24 October 2008)[DOI: 10.1126/science.1165667]
[Journal editor's summary and perspective on the original article. The full summary has a great image showing the concept.]