Ultrastructural development of adipose tissue in foetal sheep.

The development of subcutaneous and perirenal adipose tissue in foetal sheep of 40--150 days conceptual age was examined using electromicroscopy. Adipose tissue from both sites developed from pre-adipose cells of similar appearance, with few mitochondria, and commenced accumulating lipid on about day 70 of gestation. In the perirenal fat there was marked proliferation of mitochondria between days 80 and 90 and the tissue developed into brown fat, cell profiles being characterized by many mitochondria with numerous distinct cristae, whereas in the subcutaneous cells which developed into white adipose tissue few mitochondria with fewer cristae were observed. Multilocular and unilocular cells occurred in both tissues near term. Nerves were first seen between brown adipose cells in a 130-day-old foetus and were frequently seen in older foetuses, but very few nerves were seen in the white adipose tissue. In hypophysectomized foetuses, either full- or post-term, the subcutaneous fat and internal fat were shown to be white and brown adipose tissue respectively, as in normal foetuses, although in one 164-day-old foetus the perirenal tissue appeared to be a mixture of brown and white fat.