Stereological assessment of age-related changes in lipid droplet surface area and vascular volume in rat interscapular brown adipose tissue.

The surface area of stored triglyceride and the volume of capillaries in the interscapular brown fat pad of the rat have been adopted as morphological indices of the overall thermogenic capability of the tissue. The present study examines the relationship between the chronological profiles of these parameters and reported senile changes in the biochemical and physiological characteristics of brown adipose tissue. Lipid droplet surface density and vascular volume density were estimated by computer-assisted planimetry with electron micrographs prepared from interscapular brown adipose tissue samples obtained from rats of various ages. The volumes of the fat pad at these ages were also determined and used to calculate droplet surface areas and vascular volumes. Triglyceride surface area showed a 20-fold enhancement in the early postnatal period (0.44 X 10(2) cm2 at birth; 9.43 X 10(2) cm2 at 4 weeks). A further but less remarkable increase occurred in adulthood (11.67 X 10(2) cm2 at 6 months) and, in old age, only a slight fall was noted (9.66 X 10(2) cm2 at 2 years). Intralobular capillary volume also rose sharply early during the first week after birth, reaching a peak at 4 weeks (2.40 X 10(-2) cm3). The values recorded in adulthood and old age (1.90 X 10(-2) cm3 at 6 months; 1.76 X 10(-2) cm3 at 2 years) were not significantly different from that obtained at 4 weeks of age. These results show that the attainment of maximum values of lipid droplet surface area and vascular volume in rat interscapular brown adipose tissue coincides with the period of the tissue's peak metabolic capacity.(ABSTRACT TRUNCATED AT 250 WORDS)