1. Seasonal Changes In Brown Fat And Pelage In Southern Short-Tailed Shrews By Elisa M. Dew, Keith A. Carson, and Robert K. Rose Presentation by Neema Chandel

2. Background Small non-hibernating mammals must adapt to survive winter conditions They rapidly lose heat due to their large ratio of surface area to body weight To survive the winter, they rely in part on an increase in cold-induced thermogenesis  nonshivering thermogenesis.

3. Background Cont. These mammals have a specialised thermogenic tissue (brown fat) for producing bursts of heat in response to cold stress. –The tissue is strategically localized in the neck and thoracic regions in relation to major blood vessels so that its heat is quickly transported to those organs (brain and heart) whose continuous high temperatures are vital. The amount of brown adipose tissue in small mammals varies inversely with ambient temperature.

4. Background Cont. Small mammals can minimize their energy needs and expenditures by increasing both density and length of hairs during winter, thus minimizing conductance –Hair follicles become closer together, lengthen, and increase in number of follicle groups.

5. Objectives Quantify microanatomical changes in brown adipose tissue throughout the year To determine if the pelage changes to cope with temperature changes.

6. Hypothesis There will be a seasonal change in mitochondria volume in adipocytes of brown adipose tissue. –Because mitochondria are needed in winter to produce large amounts of heat by nonshivering thermogenesis. –Expected to see the greatest differences betweens winter and summer samples, with intermediate values from the samples collected in spring and autumn.

7. Hypothesis Cont. Expected to observe significantly higher densities of hairs during molting in autumn and spring. Expected to find both longer hairs and an increase in the number of segments of hairs from samples collected in winter. Southern short-tailed shrews were used because they are small non-hibernating mammals with short pelage (fur coat).

8. Materials and Methods 74 shrews of both sexes were collected at monthly intervals between January and December, 1991 in eastern Virginia. Shrews that were found alive were used for the study of brown adipose tissue, whereas those that died in the traps were used for hair analysis. Interscapular brown adipose tissue was dissected out and its cellular components examined with a TEM.

9. Materials and Methods Cont. Hairs were seperated into three width categories: –Type I guard –Type II guard –Woolly 10 hairs of each time from shrews were chosen randomly and measured from each season and the mean length for each hair type from each individual was calculated. Analyzed hair density, hair length, number of segments.

10. Results Mitochondria of brown adipose tissue of shrews collected in winter were significantly larger and occupied a significantly greater cellular volume. Mitochondria of shrews in summer appeared to be more electon-dense with cristae that were less well-defined than those in other seasons. Mitochondria of shrews in both spring and autumn occupied volumes intermediate to those of summer and winter. –Maximal size of mitochondria was slightly, but not significantly larger in spring.

11. Results Cont. Mitochondria of Brown Adipose Tissue

12. Results Cont. Lipid droplets occupied significantly greater cellular volumes in summer than winter. –Due to an increase in droplet size and not an increase in number of droplets. In the summer the lipid droplets were somewhat fewer by significnatly larger. In the winter the lipd droplets were greater in multitude but were smaller. Both spring and autumn had lipid droplets that were intermediate in size and occupied intermediate cellular volumes to those from winter and summer

13. Results Cont. Lipid Droplets Size Winter Spring Summer Autumn

14. Lots of mitochondria with well-defined cristae Small lipid droplets

15. Decrease in mitochondria volume Increase in lipid droplet size

16. Decrease in mitochondria volume Large lipid droplets

17. Increase in mitochondria volume Mitochondria volume is similar spring Lipid droplets are intermediate in size between those in summer and winter

18. Results Cont. There were no seasonal differences in hair density. Type I and Type II guard hairs showed seasonal differences. –In both cases, winter hairs were 1.3 times longer than summer hair. Differences in lengths of woolly hairs were marginally nonsignificant between seasons.

19. Results Cont. Type I guard hairs collected in winter had significantly more hair segments. Neither Type II guard hairs nor woolly hairs showed significant differences in numbers of hair segments across seasons.

20. Discussion Cytoplasmic features of brown adipose cells changed in a manner consistent with seasonal responses in non-shivering thermogensis in the shrews. In the winter, cells had almost 4 times greater mitochondria volume than in summer. –This gives cells greater thermogenic potential. Mitochondria of winter adipocytes were 1.6 times larger than as in summer.

21. Discussion Cont. Interscapular brown adipose tissue showed an inverse relationship between cellular volume occupied by mitochondria and cellular volume occupied by lipid droplets. The volume occupied by lipid droplets nearly doubled from winter to summer. Volume occupied by mitochondria decreased by a factor of nearly 4 from winter to summer. In transitional seasons the values were intermediate.

22. Discussion Cont. Significant changes in sizes of lipid droplets and changes in relative cellular volume suggest that lipid content of brown adipose tissue also is directly related to ambient temperature. There was no significant change in hair density in response to season in the shrews from easter Virginia.

23. Discussion Cont. Non-hibernating mammals rely heavily on nonshivering thermogenesis for winter survival As summer approaches, the need for nonshivering thermogensis decreases –Fewer mitochondria are needed for heat production. –Amount of lipid increases because energy that had been required for maintenance can now be stored. Small lipid droplets seen in the winter merge into larger droplets in summer and then break apart again for easier metabolism as winter approaches.

24. Discussion Cont. Winters are mild in eastern Virginia and very mild in relations to winter in northern areas. Lowest temperature during the study period was – 2.2 0 C in February and there was no measureable snow. Predications that the seasonal contract in mitochondrial and lipid cellular volumes would be greater in shrews from regions wit more extreme temperatures.

25. References Dew, E. M., K. A. Carson, and R. K. Rose. 1998. Seasonal Changes In Brown Fat And Pelage In Southern Short-Tailed Shrews. Journal of Mammalogy. 79:271- 278. Thompson, Adrian. 1997. “Heat Generation.” http://www.cogs.susx.ac.uk/users/adrianth/ecal97/node1 1.html (Oct 7, 1997). http://www.cogs.susx.ac.uk/users/adrianth/ecal97/node1 1.html