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Adipogenesis occurs during the entire lifespan of an organism. The adipocyte lineage originates from mesenchymal progenitors, which through yet unknown mechanisms form adipocyte precursor cells or preadipocytes, which then differentiate into mature, lipid-containing adipocytes. It is generally thought that adipocyte tissue develops from mesoderm with contributions from the lateral plate mesoderm and somites. Neural crest stem cells have been also been suggested to be a source of subcutaneous adipose.
The notion that WAT depots may be derived from distinct precursors is supported by a number of lines of evidence. Firstly, different WAT depots appear at chronologically distinct periods. In rodents, WAT, which develops mainly after birth, first appears in the perigonadal and subcutaneous depots, and only later in the omental depot. In humans, WAT development begins early in the second trimester of gestation and by birth is well developed in both the visceral and subcutaneous depots. In contrast, BAT emerges earlier than WAT during fetal development and reaches its maximal size relative to body weight at birth, when nonshivering thermogenesis is required. Later, it involutes with aging, in both humans and rodents.
Adipose tissue is an active metabolic tissue which reduces heat loss through skin, serves as an energy reserve, and supports and protects organs. In newborns, brown adipose tissue generates a considerable amount of heat that helps maintain proper body temperature. The cells of adipose tissue, called adipocytes, specialize in triglyceride (fats) storage, in large centrally located droplets. Because each adipocyte fills up with a single, large triglyceride droplet, its cytoplasm and nucleus are pushed to the periphery of the cell. Adipose tissue houses numerous nerve endings and rich vascular networks that help regulate and mobilize the energy stores.
Most adipose tissue in adults is white adipose tissue (WAT). Brown adipose tissue (BAT) is widespread in the fetus and infant, but is present in only small amounts presenting adults. BAT obtains its dark color from a very rich blood supply, along with numerous pigmented mitochondria that participate in aerobic cellular respiration.
In humans, WAT is dispersed throughout the body with major intra-abdominal depots around the omentum, intestines, and perirenal areas, and subcutaneous depots in the buttocks, thighs, and abdomen. WAT can be found in many other areas, including in the retro-orbital space, the face and extremities, and yellow bone marrow. Adult human BAT depots are present in cervical, supraclavicular and paravertebral anatomical locations. In rodents, BAT is most abundant in the neonatal period and is mostly concentrated in the interscapular region.