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The thyroid gland of the woodchuck Marmota monaxA morphological study of seasonal variations in the follicular cells.

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The Thyroid Gland of the Woodchuck, Marmota monax:
A Morphological Study of Seasonal Variations
in the Follicular Cells
Department of Anatomy and Metabolic Unit, Department of Medicine,
Uniuersity of Vermont College of Medicine,
Burlington, Vermont 05401
The morphology of the thyroid gland of the woodchuck, Marmota
monax, was studied during the four seasons of the year. In the spring the thyroid
is extremely heterogeneous i n appearance. Some follicular cells appear quite
active. They contain a well defined Golgi apparatus, abundant large colloid
droplets and pseudopodia but few, if any, apical vesicles. Other less active cells
have poorly defined rough surfaced endoplasmic reticulum and lack a well developed Golgi apparatus. They do not contain apical vesicles or colloid droplets.
Summer thyroids have uniformly small follicles which are lined by high cuboidal
cells containing numerous mitochondria, apical vesicles, abundant rough surfaced endoplasmic reticulum, and lipid droplets but few colloid droplets. There
is extensive lateral a n d basal infolding of the cytoplasmic membranes i n these
cells. I n the fall and winter the follicles are larger than in the summer and contain more colloid. Numerous heterogeneous dense bodies appear in the cytoplasm
of the follicular cells i n the fall and increase in number in the winter when there
is a n obvious sparsity of such glycoprotein synthetic organelles as Golgi apparatus and rough surfaced endoplasmic reticulum. These rnorphologic changes are
compared with previous studies of thyroid structure and function i n other animals and are correlated with the seasonal physiologic activities of the woodchuck.
There is an endogenous annual cycle of
thyroid gland activity in mammalian hibernators which is intimately related to
the seasonal activities of these animals
(Kayser, '61; Hoffman, '65). The thyroid
glands appear to be most active in early
spring, gradually become less active
throughout the late spring and summer,
are almost completely inactive by the time
the animals hibernate, and commence activity again in late winter just before arousal from hibernation. Most estimates of
thyroid activity during the four seasons
have been based on histologic criteria, although there have been a few studies of
uptake and release by the thyroid,
blood protein-bound iodine, thyroid secretion rates, and blood thyroid hormone concentrations (Lachiver, '51 ; Vidovic and
Popovic, '54; Wenberg and Holland, '73;
Young, '75; Hulbert and Hudson, '76).
No detailed microscopic studies have
been reported for the woodchuck, Marmota
monax, since the brief (4 animals) histologic study of Cushing and Goetsch in
ANAT. REC., 187: 4 9 5 5 1 4 .
1915 in which they found no striking differences between the thyroid of a single
spring woodchuck and thyroids taken the
previous winter from animals in hibernation. A thorough light microscopic study
of the closely related European marmot,
Mannota marmota, was published by Coninx-Girardet in 1927, in which she reported
seasonal changes in the quantity of colloid
and in the height of the epithelial cells of
the thyroid gland. She concluded that the
gland appeared inactive during the winter
and most active at the time of arousal. In
1970 Olivereau confirmed these observations for the same species using routine
light microscopic and autoradiographic
The only electron microscopic studies of
thyroid tissue during hibernation reported
thus far have been made of bats (Azzali,
'67; Fujita, '71; Nunez and Ekcker, '70;
Nunez et al., '74; Velicky and Titlbach,
Received Jan. 19, '76. Accepted Oct. 5, '76.
1 Present address: Harriet G. Bird Memorial Laboratory, Stow, Massachusetts 01775.
'72). As hibernation almost certainly evolved
separately in the various orders of mammals, it would not be surprising if there
were differences in detail in the annual
cell cycles of the different hibernators.
This report describes both light and electron microscopic variations observed in
the thyroid follicular cells of a rodent hibernator, the woodchuck, throughout the
year. These observations are correlated
with previous findings in other hibernators,
and with the physiologic activities of the
woodchuck during the four seasons of the
Adult woodchucks of either sex were
live-trapped in the wild during the spring
(March and early April), summer (July)
and fall (late September). Animals to be
used for winter studies were captured in
September and allowed to hibernate in an
environmentally controlled cold room at
7°C until December when they were sacrificed. The others were kept in the laboratory for approximately eight days before
sacrifice. They were housed in individual
cages in a room maintained at 16-20°C
and were fed Purina Rabbit Chow and
water ad libitum.
At the time of sacrifice the woodchucks
were weighed and, with the exception of
the hibernating animals, anesthetized with
35-50 mg/kg of sodium pentobarbitol administered intraperitoneally . The thyroid
glands were then quickly removed.
One lobe was processed in Helley's fixative and embedded in paraffin. Serial sections (6 p ) were taken, and the slides were
stained with hematoxylin and eosin or periodic acid Schiff.
The other lobe was cut into several small
pieces, fixed in cold 4.2% glutaraldehyde
in 0.1 M Millonig's buffer, pH 7.2, for 24
hours, minced, and postfixed in 1 % osmium tetroxide for one hour. The tissue
was subsequently dehydrated and embedded in Epon 812.
Thick sections (1 p ) were stained in 1 %
toluidine blue and examined with the light
microscope. Thin sections (80-100 mp)
were stained in saturated aqueous uranyl
acetate and examined with a Phillips EM
The thyroid of the spring woodchuck is
extremely heterogeneous in appearance.
There is great variability in the height of
the epithelium and in the amount of colloid in the various follicles (fig. 1). Fat cells
are numerous and brown fat is occasionally observed between large colloid filled
follicles in the less active regions of the
glands. The cells of these follicles contain
scanty cytoplasm and a flattened nucleus.
The Golgi apparatus is poorly developed and
there is sparse rough surfaced endoplasmic reticulum. Free ribosomes are scattered throughout the cytoplasm as are bundles of fine filaments. Dense bodies occur
occasionally, although colloid droplets and
apical vesicles are notably absent.
Numerous small follicles which contain
little colloid also occur in other regions of
the spring thyroids. These follicles are
lined with cuboidal cells which contain
large PAS positive droplets (fig. 5). Colloid
droplets frequently occur in pseudopodia
on the apical surfaces of these cells as well
as throughout the cytoplasm. These droplets are occasionally observed in close apposition to dense bodies and may be of
varying electron densities, suggesting various stages of thyroglobulin depletion (Nadler, '74). The amount of rough surfaced
endoplasmic reticulum and of free ribosomes vanes from cell to cell in the smaller
follicles. In some areas free ribosomes appear in clusters rather than being widely
distributed throughout the cytoplasm. Most
cells contain a well defined Golgi apparatus, a moderate number of mitochondria,
and occasional bundles of fine filaments
(fig. 9).
In contrast to the heterogeneous appearance of the spring thyroids, the summer
thyroids contain a rather homogeneous
population of small to medium sized follicles which are lined with high cuboidal
epithelium (fig. 2), although there are
occasional large follicles in the periphery
of the gland. Very few PAS positive droplets occur in any of the cells (fig. 6). Fat
cells are scattered between the central
follicles but occur most frequently in the
regions of the gland which contain the
larger follicles. Characteristics of the summer epithelium include the presence of
apical vesicles, lipid droplets and a large
number of mitochondria. A well developed
Golgi apparatus, abundant rough surfaced
endoplasmic reticulum and dense bodies
are also present. Colloid droplets are seldom
observed in these cells but lipid droplets
are quite common and are usually localized
in the basal region of the cells. They are
frequently found in close association with
mitochondria. In contrast to the fall and
winter thyroids, there are numerous lateral and basal infoldings of the cytoplasmic membranes in the thyroid cells of the
summer animals (fig. 10). These surface
modifications also occur to some extent in
the smaller follicles of the spring animals.
The fall thyroid glands contain follicles
which are generally larger than those observed in the summer glands (fig. 3). There
are, however, localized areas of the gland
that contain very small follicles. A proportionately greater quantity of fat is present
between follicles throughout the glands of
the fall animals and occasionally brown
fat is observed. Fine PAS positive droplets,
much smaller than those observed in the
spring animals, occur in the cytoplasm of
many cells (fig. 7). Colloid droplets, apical
vesicles, and lipid droplets are occasionally
observed. Non-dilated rough surfaced endoplasmic reticulum, a moderate number
of mitochondria and dense bodies are also
present (fig. 11). There appear to be two
kinds of dense body in the fall thyroids.
The heterogeneous ones contain regions of
variable electron density; they are larger
than the darker ones which have a more
homogeneous appearance. The homogeneous dense bodies can be seen in the cytoplasm of thyroid cells in all the seasons,
whereas the heterogeneous ones are not
usually observed in either the spring or
summer animals.
The thyroid glands of the winter woodchucks contain follicles that are about the
same size as those of the fall animals (fig.
4). Despite the abundance of these follicles, which are lined with low cuboidal
epithelium, very large colloid filled follicles lined with squamous epithelium occur
occasionally. The brown fat in the thyroid
glands of these animals is highly variable
in quantity and location throughout the
gland. The epithelium of the winter thyroid is generally lower than that of the fall
animals and because of this the glands
tend to have a less cellular appearance.
Very fine PAS positive droplets, similar to
those observed in the fall, are present in
the cytoplasm of some of the cells (fig. 8).
The epithelial cells lining the follicles contain a very small Golgi apparatus and little
or no rough surfaced endoplasmic reticulum, although free ribosomes are abundant. Apical vesicles and colloid droplets
are absent but fat droplets may occur occasionally. Both homogeneous and heterogeneous dense bodies are present and obvious in the cytoplasm of these cells, the
heterogeneous bodies being especially
abundant (fig. 12).
The thyroid glands of hibernating animals, quiescent during the winter, are
known to become reactivated at the end of
hibernation or at the beginning of arousal
(Kayser, '61; Hoffman, '65). In the woodchuck, reactivation of the gland appears
to be a gradual phenomenon as the thyroids of spring woodchucks had inactive
regions interspersed randomly among the
more active areas, even as late as one
month after the end of hibernation. The
thyroids of the woodchucks sacrificed during April became progressively more uniform in appearance. In contrast to the
study by Zalesky ('35) of the thirteen-lined
ground squirrel, we did not find that heightened activity progressed uniformly from
the central to the peripheral follicles but
rather that i t occurred in random fashion
similar to that reported by Fujita ('71) in
the bat. The presence of adjacent active
and inactive areas in the same gland leads
one to speculate on the role of TSH in the
random reactivation of the spring thyroid.
Is there a regional difference in thyroid
cell responsiveness to circulating TSH at
this time of year, or is there some control
mechanism, involving regional alterations
in blood flow, which allows certain areas
of the gland to be stimulated at one time
and others at a later date? It is known that
the response of individual thyroid cells to
acute TSH stimulation in other mammals
is highly variable (Neve and Dumont, '70;
Wetzel and Wollman, '72) and the response
of the thyroid gland to endogeneous TSH
levels in the spring woodchuck appears to
be equally variable.
During the early spring, when thyroid
hormone is essential to enable the woodchuck to survive the stresses of the breeding season and the cold, the animals are
without food and thus they lack the dietary
iodine and protein which is necessary for
the synthesis of new hormone. Possibly the
gradual regional reactivation of the gland
permits the secretory process to occur over
a longer period of time than would be possible if all the cells resumed full activity
at the same time under such conditions.
The inactive follicular cells in the spring
thyroids resemble those seen in hibernating woodchucks and appear to lack the
capacity for production andlor secretion of
thyroid hormone. The active cells of the
spring thyroids contain many large colloid
droplets, pseudopodia and dense bodies but
apical vesicles are not apparent. The latter
cells are rapidly reabsorbing stored colloid and secreting thyroid hormones into
the blood as the circulating concentrations
of both triiodothyronine and thyroxine are
elevated at this time of the year (Wenberg
and Holland, '73; Young, '75). The cellular changes observed in the active cells
are similar to those which occur in the
early stages of TSH stimulation (Seljelid,
'67; Wetzel et al., '65), but differ somewhat from the changes normally observed
in spring bats. In the bat, the follicular
cells appear to reabsorb luminal colloid by
means of apical pinocytotic vesicles rather
than by pseudopodia unless the animals
have been treated with TSH (Azzali, '67;
Fujita, '71; Nunez et al., '74).
The thyroids of summer woodchucks appear very active at the light microscopic
level because they contain many uniformly
small follicles. This observation might lead
to the erroneous conclusion that thyroid
secretory activity remains high throughout
the summer; however, very few colloid droplets can be found in the cells. The low
blood thyroid hormone concentrations
present at this time of the year confirm the
fact that little colloid reabsorption is occurring (Wenberg and Holland, '73; Young,
'75). Instead, the follicular cells appear to
be actively synthesizing thyroglobulin for
subsequent storage. They contain abundant mitochondria, many apical secretory
vesicles, extensively dilated rough surfaced
endoplasmic reticulum and numerous lipid
droplets. These lipid droplets are often
found in close association with the mitochondria, suggesting that they may be
serving as an energy source. Similar lipid
droplets have been found in the very active
thyroids of the African basenji dog (Nunez,
'72) and in the guinea pig pancreas which
has a very active intracellular transport
system (Jamieson and Palade, '68). In addition, the lateral and basal infoldings of
the cytoplasmic membranes suggest that
the follicular cells are active in protein
synthesis. Nunez and Becker ('70) have
speculated that similar basal plasma membrane infoldings in follicular cells of the
bat in September provide increased areas
for transport of precursors of thyroglobulin
into the cells.
The woodchuck may double in weight
from spring to fall and most, if not all, of
the accumulated weight is in the form of
fat (Young, '75). It is apparent that if thyroid hormone secretion were decreased
during the summer, the metabolic rate of
the animal would be lowered and weight
gain would be facilitated. This seems to
be the case in the woodchuck. The blood
thyroid hormone concentrations are quite
low (Wenberg and Holland, '73; Young,
'75), there is a decrease in carbon dioxide
production (Bailey, '65) and the thyroids,
though having very well developed secretory organelles, contain very few colloid
droplets during this time of the year.
In the fall, the presence of a well developed Golgi apparatus and a moderate
amount of rough surfaced endoplasmic reticulum indicates that some synthetic activity is still occurring, although not to
the same extent as in the summer. There
is a notable decrease in the number of mitochondria and lipid droplets and in the
number of lateral and basal infoldings of
the plasmalemma. Colloid droplets and
homogeneous dense bodies are present indicating release of the hormone into the
blood stream. These morphological observations are in agreement with serum T4
levels (Wenberg and Holland, '73) and
thyroid secretion rates (Hulbert and Hudson, '76) which have been reported to be
elevated in the fall as compared to the
summer in the woodchuck and the ground
squirrel respectively. Nunez and Becker
('70) reported that as the bat prepares to
enter hibernation, the follicular cells enter
a period of intense synthetic activity, usually in late September and October. Perhaps the intense synthetic activity of the
bat thyroid in autumn corresponds to that
seen in the woodchuck in mid-summer,
the differences in timing reflecting species
and geographical differences.
We observed two types of dense bodies
in the woodchuck. The first type, a homogeneous dense body, was observed i n animals sacrificed throughout the year although they were more numerous in the
spring. These bodies are similar to those
described in other mammals which provide
the hydrolytic enzymes responsible for the
degradation of thyroglobulin (Wissig, ’63;
Wollman et al., ’64; Wetzel et al., ’65; Ekholm and Smeds, ’66; and Seljelid, ’67).
The second type, which contained regions
of varying electron density, was very nonuniform in appearance. Few vacuoles, organelle fragments or membranes were contained in these heterogeneous dense bodies
which became evident in the fall and increased i n number during winter. These
bodies may be involved in the degradation
of old cell organelles or they may be colloid droplet-dense body complexes which
have not completed enzymatic degradation
of thyroglobulin in the droplets prior to
hibernation. They are similar to the “complex” dense bodies described in the bat
during hibernation (Nunez and Becker,
’70) and to the complexes which are described as the result of fusion of the dense
bodies with colloid droplets in nodular
goiter (Lupulescu and Petrovici, ’68).
The large PAS positive droplets observed
in the more active regions of the spring
thyroid glands correspond well with the
numerous colloid droplets which are evident in the cytoplasm of these cells during
the spring. The finer PAS positive droplets
observed in the fall and winter animals a t
the light microscopic level could be very
small colloid droplets i n the cytoplasm of
the cells, homogeneous dense bodies or the
“undigested” colloid portion of the heterogeneous dense bodies. Colloid droplets,
apical vesicles and structures similar in
size and location to dense bodies have been
reported to be PAS positive (Van Heyningen,
’65). Because of the lack of visible colloid
droplets and apical vesicles and because of
the sparsity of homogeneous dense bodies
in the winter animals, we believe that the
fine PAS positive droplets present in the
fall and winter animals are probably the
numerous heterogeneous dense bodies that
are present i n the cells at this time of year
and that these bodies may be colloid droplet dense body complexes that have not
completed the enzymatic degradation of
thyro globulin .
Most authors agree that thyroid secre-
tion in the hibernator is either at a minimal level or non-existent during the winter (Zelesky, ’ 3 5 ; Nunez and Becker, ’70;
Olivereau, ’70; Hulbert and Hudson, ’76)
and our electron microscopic study supports these findings in the woodchuck as
the cells of the thyroid gland contain poor
Golgi apparatus, little rough surfaced endoplasmic reticulum and no visible apical
vesicles or colloid droplets. The most characteristic feature of the winter thyroid
gland is the presence of the heterogeneous
dense bodies discussed above.
There are marked differences in thyroid
structure and function in the woodchuck
throughout the four seasons of the year.
The spring thyroids are heterogeneous in
appearance, many of the follicular cells
are actively resorbing colloid, and blood
hormone levels are high. The summer thyroids contain uniformly small follicles,
little if any colloid resorption is occurring
and blood thyroid hormone concentrations
are low. In the fall the thyroid gland completes its synthetic activities for the year
and little new hormone is released into the
blood. There appears to be little or no production or secretion of thyroid hormones
by the thyroid gland during the winter
months, and the animals must rely on hormones circulating in the blood i n order to
maintain their basal metabolic rate and to
permit occasional periods of arousal.
This research was supported in part by
PHS 05429-13-5 (P. P. Krupp, Anatomy
Department) and PHS R01 10254 (E. A. H.
Sims, Metabolic Unit, Department of Medicine).
Azzali, G. 1967 Sulle modificazioni cicliche stagionali e sperimentali dell’epitelio follicolare
della tiroide di animal ibernant. Ateneo Parmense. Acta Bio-Medica, 33: 5 4 2 .
Bailey, E. D. 1965 Seasonal changes in metabolic activity of nonhibernating woodchucks.
Can. J. Zool., 43: 905-909.
Coninx-Girardet, B . 1927 Beitrage zur kenntnis
innersekretorischer Organe des Murmeltieres
(Arctomys marmota L.) und ihrer Beziehungen
zurn Problem des Winterschlafes. Acta Zool.
(Stockholm), 8 : 161-224.
Cushing, H . , and E. Goetsch 1915 Hibernation
and the pituitary body. J. Exper. Med., 22: 2547.
Ekholm, R . , and S. Smeds 1966 On dense bodies
and droplets in the follicular cells of the guinea
pig thyroid. J. Ultra. Res., 16: 71-82.
Fujita, H. 1971 Some observations on the fine
structure of thyroids of hibernating and aroused
bats. Z. Zellforsch., 121: 301-318.
Hoffman, R. A. 1965 Terrestrial animals in cold:
hibernation. In: Handbook of Physiology. Section 4, Adaptation to the Environment. h e r ican Physiological Society, Washington, D.C.,
pp. 3 7 9 4 0 3 .
Hulbert, A. J . , and J. W . Hudson 1976 Thyroid
function i n a hibernator, Spermophilus tridecemlineatus. Am. J. Physiol., 230: 1211-1216.
Jamieson, J. D.. and G. E. Palade 1968 Intracellular transport of secretory proteins in the
pancreatic exocrine cell. J. Cell Biol., 39: 589.
Kayser, C. 1961 The physiology of Natural Hibernation. Pergammon Press, New York.
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(Mtrrmotn marnzota L.), Congr. SOC.Savantes,
77: 133-138.
Lupulescu, A., and A. Petrovici 1968 In: Ultrastructure of the Thyroid Gland. Chap. VI. Williams and Wilkins Co., Baltimore, Maryland,
pp. 131-134.
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Handbook of Physiology, Vol. 111. Endocrinology. Section 7, Thyroid. American Physiological
Society, Washington, D.C., pp. 39-54.
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of ultrastructural changes in the stimulated dog
thyroid. Z. Zellforsch., 103: 61-74.
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processes in follicular cells of the bat thyroid.
I. Ultrastructural changes during the pre-, early
and mid-hibernation periods with some comments on the origin of autophagic vacuoles. Am.
J . Anat., 129: 369-398.
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1972 A fine structural study of the highly active thyroid follicular cell of the African basenji
dog. Am. J . Anat., 1 3 3 : 4 6 3 4 8 2 .
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Secretory processes in follicular cells of the bat
thyroid. 111. The occurrence of extracellular
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Olivereau, M. 1970 Donnees cytologiques et autoradiographiques sur la glande thym'ide de
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cells. 11. A microinjection study of the origin of
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electron microscopic observations on glycoprotein containing globules in the follicular cells of
the thyroid gland of the rat. J. Histochem. Cytochem., 13: 286-295.
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the bat thyroid gland i n winter and early spring.
11. Electron microscopic observations. Folia Morphologica, 20: 4 0 6 4 1 5 .
Vidovic, V. L., and V. Popovic 1954 Studies on
the adrenal and thyroid glands of the ground
squirrel during hibernation. J. Endocrin., 11 :
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circannual variations of thyroid activity in the
woodchuck (Mnrmotcc m o n n x ) . Comp. Biochem.
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1965 Changes in the fine structure and acid
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the follicular cells of the thyroid gland. 11. The
effect of acute thyrotropic hormone stimulation
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1964 Localization of esterase and acid phosphatase in granules and colloid droplets in rat
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Young, R . A. 1975 The Woodchuck, Murmotcr
monax, as a Biomedical Model for the Study of
Obesity. Ph.D. Dissertation, University of Vermont, May, 1975.
Zalesky, M. 1935 A study of the seasonal changes
i n the thyroid gland of the thirteen-lined ground
squirrel (Citellus tridecemlineatus),with particular reference to its sexual cycle. Anat. Rec., 62:
109-1 37.
Spl-ing woodchuck. Small irregular follicles lined with high cuboidal
cells next to larger follicles lined with squamous epithelium. H & E.
x 200.
Sirmrner u.oodckzick. A relatively homogeneous population of small to
medium sized follicles lined with high cuboidal epithelium. H & E.
x 200.
Fall w~oodchuck. Medium to large colloid filled follicles lined with
cuboidal epithelium. H & E. X 200.
Wiizler woodchuck. Follicles similar in size to those i n figure 3 but
lined with low cuboidal epithelium. H & E. X 200.
P. P . Krupp, R. A. Young and R. Frink
Spring woodchuck. Region of thyroid containing numerous irregular
follicles lined with high cuboidal epithelium. The cells contain abundant PAS positive droplets (arrow). PAS. X 800.
S u m m e r woodchuck. Relatively sparse numbers of PAS positive droplets (arrow). PAS. X 800.
Fall woodchurk. Note the absence of larger PAS positive droplets. Very
fine PAS positive droplets can be seen in some areas (arrow). PAS.
X 800.
Winter 7uoodchzick. Very fine PAS positive droplets are observed similar to those in the fall thyroid (arrow). PAS. X 800.
P . P . Krupp, R. A. Young and R . Frink
Spring uwodchiick. An epithelial cell with a pseudopod (P) bulging
into the colloid (C). There are numerous colloid droplets (CDj and
dense bodies (DB). The rough surfaced endoplasmic reticulum (rER)
vanes in dilation from cell to cell. Golgi apparatus ( G ) and bundles of
fine filaments (F) are also observed. Note the presence of membranous
remnants (MRj suggestive of previous interaction of a colloid droplet
with another intracellular structure prior to thyroglobular degradation.
(Nadler, '74). X 18,000.
P. P . Krupp, R. A. Young and R. Frink
S i ~ m m e rwoodclzuck. Colloid containing lumen of the follicle (C) can
be seen in the upper right area of the electron micrograph. There are
numerous mitochondria (M), dense bodies (DB), and apical vesicles
(AV). The rough surfaced endoplasmic reticulum (rER) is well developed.
Lipid droplets (LD) are found i n the basal regions of the cells and there
are extensive lateral (LI)and basal infoldings (BI) of the cytoplasmic
membrane. X 18,000.
P. P. Krupp, R . A. Young and R . Frink
Fall woodchuch. Portion of a thyroid follicle containing colloid (C).
Homogeneous dense bodies (DB) as well as heterogeneous dense bodies
(HDB) are observed in these cells. Non-dilated rough surfaced endoplasmic reticulum (rER) and a moderate number of mitochondria (M)
are also observed. Electron micrograph. X 18,000.
P . P. Krupp, R. A. Young and R . Frink
W7ntci- woodrhuck. Note absence of rough surfaced endoplasmic reticulum a n d the presence of numerous heterogeneous dense bodies
(HDB) and free ribosomes (R).Bundles of fine filaments (F) a n d poorly
developed Golgi apparatus (G) are also observed. Colloid (C). Electron
micrograph. x 18,000.
P. P. Krupp, R. A. Young and R. Frink
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marmota, variation, follicular, stud, gland, thyroid, morphological, woodchuck, seasonal, monaxa, cells
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