Seasonal%20Changes%20In%20Brown%20Fat%20And%20Pelage%20In%20Southern%20Short-Tailed%20Shrews

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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

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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.

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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.

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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.

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Objectives

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

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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.

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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).

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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.

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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.

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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.

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Results Cont.
Mitochondria of Brown Adipose Tissue
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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

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Results Cont.
Lipid Droplets Size
Winter Spring Summer Autumn
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• Lots of mitochondria with well-defined cristae
• Small lipid droplets

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• Decrease in mitochondria volume
• Increase in lipid droplet size

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• Decrease in mitochondria volume
• Large lipid droplets

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

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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.

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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.

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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.

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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.

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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.

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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.

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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
  2.20C 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.

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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. 79271-278. Thompson, Adrian. 1997.
Heat Generation. http//www.cogs.susx.ac.uk/user
s/adrianth/ecal97/node11.html (Oct 7, 1997).