Morphology of adipose cells in lambs at birth and during subsequent transition of brown to white adipose tissue in cold and in warm conditions

Abstract Light and electron microscopy were used to study the morphology of adipose cells in newborn Merino lambs ( Ovis aries ). Postnatal changes in morphology were also examined in naturally fed Merino lambs held at 26°C or at 3°C for up to 32 days from the time of birth, and in lambs fasted for up to three days at 26°C or at 3°C since birth. All adipose cells examined in the newborn lambs showed the morphological characteristics of brown adipose cells; no white adipose cells could be found. The brown adipose tissue in normally fed lambs was replaced progressively by white adipose tissue during the first two or three weeks of life, and this replacement was retarded in the lambs held at 3°C. During replacement a continuous spectrum of cells with morphological characteristics between those of brown and white adipose cells were seen. No degenerating brown adipose cells were observed; and, apart from brown adipose cells, no cells were identified that could have been precursors of white adipose cells. This evidence suggests that in various body regions of lambs white adipose cells are derived from brown adipose cells. Lambs fasted at 3°C or under conditions that approached thermoneutrality (26°C) showed rapid depletion of lipid from brown adipose cells; hence brown adipose tissue of the lamb differs from that in the newborn rabbit in which this tissue is not readily depleted of fat during starvation under thermoneutral conditions.