Localization, cellular morphology and respiratory capacity of "brown" adipose tissue in newborn reindeer.

1. The localization and cellular morphology of adipose tissue was studied by light, fluorescence and electron microscopy in reindeer between 2 weeks pre partum and 4.5 months post partum during calving, and the subsequent growth period. The respiratory capacity of the adipose tissue was examined in terms of morphometric mitochondrial volume and cytochrome-c oxidase or succinate dehydrogenase activity. 2. Adipose tissue was located at specific anatomical sites in the newborn reindeer (from 0 to 2 days of age). The perirenal-abdominal depot was the largest location (32%) followed by the inter(pre)scapular (18%) and sternal (12%) depots. Internal depot dominated over external or peripheral depots (66-34%). The locations of adipose tissue were largely similar in foetal, newborn and young reindeer. 3. The adipose tissue of the newborn reindeer had all the typical cell morphological characteristics of brown adipose tissue: abundant mitochondria, multilocular fat, high vascularization and a dense spot-like sympathetic innervation between the adipocytes. In the young reindeer, however, it resembled white adipose tissue, being almost totally unilocular with few mitochondria. 4. There was a significant correlation between morphometric mitochondrial volume and cytochrome-c oxidase activity (r = 0.848) in the adipose tissue. Mitochondrial volume, cytochrome-c oxidase and succinate dehydrogenase activity were highest after birth and decreased to almost an undetectable level during the first month. A parallel decrease occurred in the amount of brown adipose tissue from birth (1-2%) to the age of about one month (0.3%). 5. It is concluded that the distinct cell morphological features and high respiratory capacity of the adipose tissue indicate the presence of brown adipose tissue at specific anatomical locations in newborn reindeer. A marked progression towards the characteristics of white adipose tissue then takes place at the same locations during the first month. The results suggest the fundamental significance of brown adipose tissue for non-shivering thermogenesis in newborn reindeer.