Answer:
During the third month of gestation, mesenchymal cells differentiate into preadipocytes. These precursor cells can differentiate into mature fat cells or coexist in the connective tissue stroma of mature adipose tissue and accumulate lipid. Mature fat cells cannot replicate. When subjected to periods of ischemia or inadequate nutrition, these cells can de-differentiate into preadipocytes which are capable of replication.
Adipocytes triple in size during the first year of life, continuing to grow and multiply during the first 5 years and again during adolescence. After adolescence, a fixed number is established. Hypertrophic obesity results in an increase in the size of the fat cells, without a change in their number. It is usually a post-adolescent phenomenon, and holds true until total body fat exceeds 40 kg (morbid obesity - greater than 200% ideal body weight), at which point new fat cells are produced to accommodate the enlarging lipid reserves.
There are two regional patterns of fat deposition. Gynoid refers to fat accumulation in the lower abdomen, lateral thighs, iliac crest and buttock giving a “pear shape.” Android refers to accumulation of fat in the upper abdomen and flanks giving an “apple shape.”
The subcutaneous tissue of the trunk consists of distinct superficial and deep adipose layers separated by a discrete fascial membrane. The fat layers of the superficial layer are small and tightly packed within closely spaced septa, whereas those of the deep layer are large, irregular and much less organized.