10
Adipocytes
Fig. 2
Histological appearance of normal mouse adipose tissue. Shown is a 6-
µ
m
parafFn section stained with hematoxylin and eosin. Note the single large lipid
droplet with numerous interstitial cells.
flash labeling. Over time, the percentage
of labeled mesenchymal cells declines
as the label appears in cells that have
accumulated lipid. In contrast, labeling
of endothelial cells remains relatively
constant. These data suggest a dynamic
process in which mature adipocytes are
derived from committed mesenchymal
progenitors that divide and develop into
adipocytes. Interestingly, nearly 90% of
the initial label is lost by Fve months,
strongly indicating that cellular renewal
occurs throughout life. In this regard, it is
well established that cells can be isolated
from human and rodent adipose tissue that
readily differentiate into mature adipocytes
in vitro
. Indeed, pluripotent progenitors
derived from adult adipose tissue may
have numerous therapeutic applications.
These observations indicate that adipose
tissue contains a signiFcant population of
committed progenitors that are capable
of contributing to tissue renewal and
remodeling under appropriate conditions.
Under normal laboratory conditions, cel-
lular proliferation in rat adipose tissue
drops to very low levels after weaning.
Nonetheless, a variety of physiological
and pharmacological conditions reveal dy-
namic regulation of adipose tissue. ±or
example, Hirsch and colleagues demon-
strated in the 1970s that the obesity pro-
duced by high fat diets in rats involves both
fat cell hyperplasia as well as hypertrophy.
±at cell renewal has also been observed
after partial lipectomy, and elevated fat
cell turnover has been observed in models
of hypothalamic obesity. One of the best
examples of physiological adipose tissue
plasticity occurs in seasonal fat deposition
of hibernators. Although the mechanisms
involved in fat cell proliferation are largely
unknown, adipose tissue itself is a rich
source of growth factors and cytokines
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