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THE ANATOMICAL RECORD 252:600–607 (1998)
Immunohistochemical Study of
Fibronectin and Thyroglobulin in the
Thyroid Gland of Female Rats After
Exposure to Radioactive Iodine
V.S. USENKO,1,2* E.A. LEPEKHIN,2 I.N. KORNILOVSKA,1 V.V. LYZOGUBOV,1
E.O. APOSTOLOV,1 I.S. RALETS,1 AND M. WITT3
1Morphological Laboratory BIONTEC, 320000 Dnepropetrovsk, Ukraine
2Center for Molecular Physiology, National Academy of Science,
320625, Dnepropetrovsk, Ukraine
3Department of Anatomy, Technical University, 01307, Dresden, Germany
ABSTRACT
The aim of this work was to study the effect of a dose of 150 µCi 131I on
the barrier properties of the thyroid epithelium in pregnant female rats.
Thirty-five female Wistar rats were divided into a control and four experimental groups (each distinguished by the time of 131I injection: group I—no less
then 12 days before mating; groups II, III, and IV—on 5th, 10th, and 16th
days of gestation, respectively). The thyroid glands were fixed in Bouin’s
fluid, embedded in paraffin, and stained immunohistochemically for thyroglobulin and fibronectin. In group IV the appearance of follicles with
fibronectin-positive colloid demonstrates the penetration of blood plasma
into the follicular lumen. There are more fibronectin positive follicles in
group III. Regardless of the nature of the follicles’ contents, numerous
thyrocytes with an intensive fibronectin positive reaction begin to appear in
the follicles. In group II the number of fibronectin positive follicles and
thyrocytes is clearly reduced, and in group I only a few remain. In group IV
there is a noticeable reduction in the quantity of colloid inside the follicles
and often an absence of any thyroglobulin positive reaction. There are
thyrocytes in which thyroglobulin positive granules localized in the basal
zone. There is thyroglobulin positive staining in the stroma and blood
vessels. In group II thyroglobulin is no longer found in the stroma. Small
doses of 131I provoke a serious breakdown in the thyroid epithelium’s barrier
properties, although these changes are of a transient nature. The central
zone of the thyroid gland reacts more actively and dynamically to exposure
to radioactive iodine than the peripheral zone. Anat. Rec. 252:600–607,
1998. r 1998 Wiley-Liss, Inc.
Key words: thyroid;
131I; hypothyroidism; fibronectin; thyroglobulin; epithelium; barrier; rat
The study of maternal hypothyroidism induced by small
doses of 131I is a question of particular topical interest in
endocrinology (Cox et al., 1990; De Ona et al., 1991;
Radiobiology, 1997). Radioactive iodine is widely used in
the therapy and diagnostics of thyroid gland pathology.
Furthermore, in view of the threat of various types of
nuclear accidents, it is possible that large areas of land will
be polluted by radioactive iodine isotopes as happened
after the Chernobyl disaster.
There are very few strictly morphological studies on the
influence of radioiodine on the undamaged thyroid gland
r 1998 WILEY-LISS, INC.
(Nunez et al., 1966; Greig et al., 1970; Thurston et al.,
1982; Liu et al., 1986; Artemova et al., 1976; O’Niell et al.,
1968; Trolldenier et al., 1967a,b). A considerable propor-
Grant sponsor: Government of the Free State of Saxonia; Grant
number: AZ 1–6220.3–02/41.
*Correspondence to: Dr. Vasilij S. Usenko, BIONTEC, Komsomolskaja Str. 52A/88, 320000 Dnepropetrovsk, Ukraine.
E-mail: vior@usenko.dp.ua
Received 2 March 1998; Accepted 22 June 1998
THYROID GLAND AFTER RADIOACTIVE IODINE EXPOSURE
tion of the available data on the effect of 131I on the body
are concerned with the purely physiological changes observed in patients or in experimental animals. There is no
information on the effect of 131I on the morphology of the
thyroid gland of pregnant female rats and their offspring.
There is no doubt that the study of the state of the mother
and fetus during a pregnancy, which develops in conditions
of radioactive iodine pollution, is of particular topical
importance.
Of the thyroid gland’s numerous morphological parameters, it is the condition of the follicular epithelium and, in
particular, its barrier function, which is of most importance not only for diagnostic, but also for prognostic
practice.
A considerable increase in thyroglobulin penetration
from the follicle cavity into the stroma can have fatal
consequences because thyroglobulin is one of the main
autoantigens produced during the progression of autoimmune disease of the thyroid gland (Berezin et al., 1993;
Scherbaum, 1987). Thus, a change in the localization of
thyroglobulin could be evidence of a change in the properties of the follicular epithelium, although this problem has
not yet been researched.
On the other hand, one of the most prevalent extracellular matrix proteins is fibronectin. Fibronectin is involved
in the organization of the extracellular matrix via its
interactions with the collagens and the proteoglycans and
binds to basal membrane proteins such as the laminins.
The insoluble fibronectin complex of the extracellular
matrix originates either from the exudation of soluble
fibronectin from plasma or from the local synthesis and
secretion of a cellular form (Samuel et al., 1994). Thyrocytes have the capacity to synthesize the cellular form of
fibronectin, whilst at the same time the properties of the
actual cells themselves are determined by the interaction
of thyrocytes with the components of the basal membrane,
fibronectin included (Demeure et al., 1994; Ruoslahti,
1984; Samuel et al., 1994). Soluble fibronectin is known,
together with other plasma proteins, to percolate through
the damaged epithelium in case of a breakdown in the
latter’s barrier properties (Casscells et al., 1990; Jin et al.,
1991). However, there are no available data on the study of
fibronectin in the thyroid gland after exposure to radioactive iodine.
This work is a fragment of an extensive research program devoted to studying different aspects of the effect of
small doses of radioiodine on pregnant female rats and
their offspring. The aim of the present investigation was to
study the effect of a dose of 150 µCi 131I (5.5 MBq) on the
localization of thyroglobulin and fibronectin, and on the
barrier properties of the thyroid epithelium in the thyroid
gland of female rats who had given birth after being
exposed to radioactive iodine either before pregnancy or at
different stages of pregnancy.
MATERIALS AND METHODS
Thirty-five female Wistar rats (170 to 220 g body weight)
were used. The animals were kept at 26°C and 60% air
humidity in a controlled dark/light cycle (10 hr dark/14 hr
light) with food and water available ad libitum. Females
and males were mated from 7 a.m. to 9 a.m. Day 0 of
gestation was calculated from the moment a large amount
of sperm was found in the vaginal smears.
We employed a rat hypothyroidism model using the
minimum effective doses of 131I described by Reilly et al.
601
(1986). One group of animals (seven females) was used as a
control and was not treated at all. The experimental rats
were given intramuscular injections of 0.05 ml of physiological saline containing 150 µCi 131I (5.5 MBq), which corresponds to the absorbed dose of 0.5 Gy (Greig et al., 1970).
Unlike the Reilly model, the rats were not given intraperitoneal injections, but rather intramuscular injections with
the aim of lessening the direct radioactive activity on the
embryos and ovaries, and excluding the possibility of
mechanical damage.
All the treated females were divided into four groups in
accordance with the time of injection of 131I (group I—no
less then 12 days before mating; groups II, III, and IV—on
5th, 10th, and 16th days of gestation, respectively). It is
worth noting that the exposure to radioactive iodine 131I
during pregnancy was effected 1 day before the onset of
certain critical stages in the rat embryonic development.
Group II. The 5th day of embryonic development is the
eve of the implantation of the embryo and when the
embryoblast splits into two layers: the epiblast and the
hypoblast (Edwards, 1968).
Group III. The 10th day of embryonic development is
on the eve of thyroid primordia by means of evagination
from foregut. It is also the stage of neurulation (Edwards,
1968; Phillips et al., 1959).
Group IV. The 16th day of embryonic development is
the eve of the beginning of follicle formation, of iodine
concentration as well as thyroxin production (Phillips et
al., 1959; Luciano et al., 1979; Remy et al., 1980).
Decapitation of all animals was performed under hexenal
anesthesia (30 mg per kg, ip) from 3 to 20 hr after giving
birth. Then, rats were bled and dissected. The right lobes
of the rats’ thyroid glands were fixed in Bouin’s fluid,
routinely dehydrated, and embedded in paraffin. Paraffin
sections (5 µM) were cut and mounted on gelatin-chromealum-coated slides, two or four sections per slide.
The concentrations of plasma T4 and TSH were measured using the EIA test (Medix Biochemica, Finland).
The staining techniques used were Mallory-Slinchenko’s
method (Slinchenko, 1964), immunohistochemical staining with the use of antibodies to fibronectin (FN) and
thyroglobulin (TG).
Rabbit polyclonal antibody versus bovine thyroglobulin
(Serva, Heidelberg, Germany) was obtained in our laboratory. Western immunoblotting of the SDS-fraction from rat
thyroid gland with TG-antiserum detects one main band of
300 kD polypeptide and one minor band of about 35 kD
polypeptide.
TG rabbit polyclonal antiserum was used at a dilution of
1/8,000, FN polyclonal antibody (Sigma, St. Louis, MO)—
dilution 1/200 and monoclonal antiserum vs. cells’ fibronectin (cFN; Sigma)—dilution 1/200. An indirect avidin-biotinperoxidase method was used for immunochemical staining
(Lacey, 1989). Peroxidase was visualized by incubating the
sections in 0.05% of 3.3-diaminobenzidine tetrahydrochloride solution containing 0.01% hydrogen peroxide. The
specificity of staining was controlled by substituting the
primary antibodies with a buffer as a negative control and
by using skin samples as a positive control. After being
rinsed in water, the sections were lightly counterstained
with hematoxylin. Statistical analysis included descriptive statistics for each group. The significance of the
difference between the mean values of control and treated
602
USENKO ET AL.
Fig. 1. Control rat’s thyroid gland. An intensive FN-positive reaction in
the blood vessels (long arrows) and the basal membranes thyroid
epithelium (short arrows). Connective tissue cell with FN-positive cytoplasm and a sharply positive staining of the cell contour (arrowhead).
Immunohistochemical FN staining with hematoxylin counter staining.
⫻50.
groups was calculated by both the Student’s t-test and the
Mann-Whitney U-test.
RESULTS
A measurement of the hormone level in the female rats’
blood in the first postnatal days showed a significant
reduction in the concentration of thyroxin in all experimental groups (mean 43%) as compared to the control group
(U-test and t-test P ⬍ 0.05). Parallel to this, the level of
TSH in the females of all experimental groups showed a
dramatic increase—about 792% (U-test and t-test P ⬍
0.05).
The Control Group
Fibronectin. It is essential to note that the polyclonal
antiserum vs. human FN used in this study should,
according to the data sheet, connect with both the plasma
and cellular forms of human FN. However, a parallel
staining of sections of the control rats’ thyroid glands using
both polyclonal antiserum vs. human FN and monoclonal
antiserum vs. cellular FN showed that the polyclonal
antiserum did not stain those structures which were
stained by the monoclonal antiserum. This proves that
polyclonal antiserum in the rat thyroid gland connects
only with plasma FN.
The use of polyclonal antiserum vs. FN in the sections of
the thyroid gland of the control animals reveals that the
most intensive presence of FN is found in the blood vessels.
With regard to other structures FN can be found predominantly in the basal membranes of the thyroid epithelium.
There is no expression of FN in the epithelium itself or in
the interfollicular contacts. Furthermore, FN positive staining reveals separate, relatively thin fascicles of fibres in
the organ’s stroma. In some sections one occasionally finds
connective tissue cells with FN-positive cytoplasm and a
sharply positive staining of the cell contour (Fig. 1). The
most intensive FN reaction is in the central zone of the
thyroid gland lobe and as a rule this reaction decreases
Fig. 2. Control rat’s thyroid gland. An intensive TG-positive reaction in
the colloid inside the follicles (arrows) of the thyroid gland, and with the
apices of the thyrocytes (arrowhead). Immunohistochemical TG staining
with hematoxylin counter staining. ⫻50.
sharply as one moves closer to the capsule. The capsule of
the thyroid gland itself may have a highly diverse FN
reaction across its different sections and is stained extremely unevenly.
The visualization of cellular FN using monoclonal antiserum showed that it is found in thyrocytes in the form of
intensively stained structures which are spindle-shaped,
reticular, or granular in form. These structures are located
in the apical part of the perinuclear zone. Examination of
tangential sections which pass through the apical parts of
thyrocytes sometimes reveals a positive staining of cell
contours.
A slightly positive FN staining gives a heterochromatin
result in the nuclei of some thyrocytes leaving karyolemma and nucleoli unstained. The blood vessel content
was the only substance in the sections of the rat thyroid
gland which connected with both polyclonal and monoclonal antiserum. However, the cellular FN reaction was
extremely weak.
From the above one can conclude that the polyclonal
antiserum we used exposes only plasma FN in the rat
thyroid gland; a finding of particular importance for
successfully realizing the goal of our research.
Thyroglobulin. In the thyroid gland of the control
female rats which had given birth, the antiserum vs. TG
connected with the colloid inside the follicles of the thyroid
gland, and with the apices of the thyrocytes. At the same
time, the most intensive staining was observed in the zone
where the epithelium and colloid contacted (Fig. 2). The
cytoplasm of the apical zone of thyrocytes also often
showed a positive TG result. Individual thyrocytes with
intensively diffusely stained cytoplasm are very rarely
found. These cells have pyknotic nuclei and the structure
of their cytoplasm is seriously damaged. It is obvious that
this staining is caused by TG imbibition of the matrix of
the dead cell. Groups of follicles with a sharply increased
level of resorption in the colloid are sometimes found
where the colloid has a frothy appearance and a sharply
positive TG reaction, whilst in the apical parts of thyrocytes large TG-positive drops of colloid are visible.
THYROID GLAND AFTER RADIOACTIVE IODINE EXPOSURE
603
Fig. 3. Treated rat’s thyroid gland on the 6th day after exposure 131I
(group IV). Connective tissue swelling. An intensive FN-positive reaction
in the stroma. Immunohistochemical FN staining. ⫻50.
Treated Animals: Group IV
Fibronectin. On the 5th/6th day after exposure to
radioactive iodine the connective tissue of the thyroid
gland is swollen and has a sharply FN-positive reaction
(Fig. 3) which in its intensity is comparable with the FN
reaction in the blood vessels. At the same time, in the
connective tissue one finds bubble-like areas of varying
sizes in which there is no FN. In the swollen layers of
connective tissue there are inflamed fascicles of fibres with
an intensive dark-brown staining, this being evidence of a
high concentration of FN in them. No FN positive cells
were found in the stroma. The clear demarcation of zones
in the lobe of the thyroid gland is preserved. The largest
swelling and an FN positive staining is found in the
central zone of the lobe. The organ’s capsule is substantially swollen and loose in appearance. At the same time it
has a relatively uniform FN positive reaction.
With regard to thyroid gland parenchyma, it is predominantly in the lobe’s central zone that one finds follicles, the
total contents of which display a sharply positive diffuse
FN reaction.
Thyroglobulin. In the thyroid gland of the female rats
exposed to radioactive iodine on the 16th day of pregnancy,
i.e., 5–6 days before birth, one found considerable polymorphism in relation to the morphology of TG in the follicles.
The follicles were reduced in size. The contents of a large
number of follicles, predominantly small, do not reveal a
clear TG reaction. Many follicles have colloid which is
reticular or granular in structure. A close analysis of the
sections of follicles which where stained using antibodies
vs. TG and plasma FN showed that the colloid in these
follicles contains both FN and TG (Fig. 4A,B).
A high level of variety was observed in the presence and
nature of TG localization in the thyrocyte cytoplasm. A
substantial number of thyrocytes which show morphological signs of degeneration have cytoplasm with an intensive, uniform TG-positive staining. The nuclei of these
thyrocytes are quite often displaced into the cell’s apical
zone, deformed, or swollen in appearance whilst the cytoplasm is compressed. Those thyrocytes which show no sign
Fig. 4. Treated rat’s thyroid gland on the 6th day after exposure 131I
(group IV). Close, successive sections of the thyroid lobe: A: Immunohistochemical FN staining. FN-positive colloid with reticular structure (arrowheads). Staining of the connective tissue fibre (short arrows) and vessel
contents (long arrows). ⫻100. B: Immunohistochemical TG staining with
hematoxylin counter staining. The colloid is the same as in A, follicles
retain TG-positive staining, have a reticular structure (arrowheads).
⫻100.
of crude degenerative change mostly have TG-positive
granules in the apical zone or are filled with stained
granules. However, there are also thyrocytes in which
clearly distinguishable concentrations of TG positive granules are only evident in the basal zone (Fig. 5).
As well as the changes in the localization of TG in the
parenchyma of the thyroid gland one also finds this protein
in the stroma. In the stromal layers there is localized TG
saturation of the connective tissue in close proximity to the
thyrocytes with an intensive, diffuse TG positive staining.
Furthermore, there are individual intensive TG-positive
granules. The contents of the blood vessels exhibit a
particularly weak uniform TG positive reaction.
Group III
Fibronectin. At day 11–12 after exposure to 131I there
is a weaker indication of swelling in the connective tissue
of the thyroid gland. However, the connective tissue layers
remain loose with thick, clearly visible, fibres and a diffuse
604
USENKO ET AL.
Fig. 5. Treated rat’s thyroid gland on the 6th day after exposure 131I
(group IV). Immunohistochemical TG staining with hematoxylin counter
staining. Distinct TG-positive colloid with signs of resorption (C). Presence
of intensive TG positive reaction in the basal zone of numerous thyrocytes
(arrowheads) and uniform diffuse reaction spread over whole cell (arrow).
⫻250.
Fig. 6. Treated rat’s thyroid gland on the 12th day after exposure 131I
(group III). Loosening of the organ’s connective tissue. FN positive
thyrocytes with damaged structure (long arrows). FN positive vacuoles of
varying size in colloid (short arrows). Immunohistochemical FN staining
with hematoxylin counter staining. ⫻250.
FN positive reaction. The capsule is dense with a uniformly weak positive staining.
There are follicles in all zones of the section the contents
of which show a strongly positive FN reaction. In the
cavity of such follicles FN is either diffusely distributed or
represented in the form of granules or reticular structures.
The follicles with FN positive contents often have thyrocytes with FN saturated cytoplasm. A significant number
of follicles whose contents do not have a total FN positive
reaction also contain thyrocytes with an intensive FN
positive cytoplasm reaction. In the colloid, the apices of
such cells often contain differently sized vacuoles which
show a FN positive reaction or a local diffuse positive
staining of the colloid (Fig. 6). Evidence of zonality in this
group can be seen by the fact that the various forms of
damage do not effect those follicles which are located
directly under the capsule. At the same time, in the center
of the lobe the structures of the epitheliomers and interfollicular contacts are quite seriously damaged and difficult
to identify.
Thyroglobulin. On days 11–12 after exposure, the
follicles in the thyroid gland of the females who received
radioactive iodine have a predominantly granular colloid,
which is stained with varying intensity. The localization of
TG-positive follicles in the section of the thyroid gland’s
lobe matches the localization of FN-positive follicles. A
large number of TG positive granules are found in the
cytoplasm of thyrocytes. Small drops may be uniformly
distributed in the cytoplasm, whilst large ones are mostly
found in the apical zones. Degenerating thyrocytes with an
intensive TG-positive staining are sometimes found. The
presence of TG in the stroma structures is no longer
observed.
pronounced and have a coarse fibrous structure. The
connective tissue on the whole shows a more uniform but
less intensive positive FN reaction than for the animals of
the previous group. The organ’s capsule is FN positive,
fibrous, thickened, and solid. Isolated follicles with FNpositive colloid of reticular structure were preserved mainly
in the lobe’s peripheral zone. Individual FN-positive thyrocytes are still found in the epithelium of follicles.
Thyroglobulin. On days 16–17 after exposure to
radioactive iodine certain differences in the localization of
TG are even more noticeable. In most follicles the colloid is
subject to almost total resorption and the remains of TG
positive material is only preserved in the center of the
cavity. The cube-shaped thyrocytes of these follicles develop a relatively smoothly contoured luminar surface.
Follicles filled with TG-positive colloid can be found only
on the organ’s periphery (Fig. 7). However, dying thyrocytes saturated in TG are still present.
Group II
Fibronectin. On days 16–17 after exposure to radioactive iodine the stroma’s connective tissue looks more
organized. The connective tissue layers and septa are well
Group I
Fibronectin. Within 30 to 40 days after the female rats
were exposed to radioactive iodine the stroma of the
thyroid gland is in many ways similar in structure to that
of the control animals. The thickness of the stromal layers
is close to normal and their positioning clearly shows the
usual zonal division. However, the connective tissue is
distinguished from its normal state by the high density of
fibres with a FN positive reaction. The organ’s capsule is
FN positive and fibrous. (Fig. 8). Isolated follicles with an
FN-positive colloid are still found. There is an insignificant
number of FN-positive thyrocytes. The morphology and
nature of staining on the organ’s periphery is close to that
of the control animals.
Thyroglobulin. The most significant differences in
comparison with the control are observed in this group.
There is virtually no TG-positive colloid, except for a few
individual follicles. At the same time, there is a large
number of colloidal drops with intensive TG staining in the
THYROID GLAND AFTER RADIOACTIVE IODINE EXPOSURE
Fig. 7. Treated rat’s thyroid gland on the 16th day after exposure 131I
(group II). Very rarely follicles with intensive TG positive colloid can be
found in the lobe’s peripheral zone. Immunohistochemical TG staining
with hematoxylin counter staining. ⫻100.
Fig. 8. Treated rat’s thyroid gland on 30th day after exposure 131I
(group I). Intensive FN positive reaction in the stroma of central zone of
lobe. Follicles rarely contain homogeneous FN positive colloid (arrowhead). Thyroid capsule is loose. Nature of staining of some areas in
periphery of thyroid lobe is similar to that of control. Immunohistochemical
FN staining with hematoxylin counter staining. ⫻100.
apical zones of the thyrocytes. The same picture, irrespective of zone, is repeated all over the parenchyma.
DISCUSSION
The application of a dose of 150 µCi 131I (5.5 MBq) to
pregnant female rats at different stages of pregnancy led
to the onset of a hyperthyroid state which was defined on
the first day after birth. The level of T4 in these animals is
reduced by almost half whilst the content of TSH in the
antiserum of the blood increases approximately 10-fold.
The dose used in the experiment is equal to 0.5 Gy of the
absorbed dose (Greig et al., 1970), which is five times less
than the minimum dose needed to cause the development
of clinical symptoms of radiation damage in rats (Bajenov
605
et al., 1990). This dose is close to that received by those
Soviet citizens living on the territories polluted by 131I as a
result of the Chernobyl nuclear accident (ÑEC Report
EUR 15248, 1993).
It is obvious that the localization of plasma FN in the
thyroid gland of the control animals is exclusively restricted to stromal structures such as blood vessels, basal
membranes, fibres of the connective tissue, and its individual cells. The ground substance of the connective tissue,
contains the normal minimum level of plasma FN. TG is
found in the follicle lumen and in the cytoplasm of thyrocytes. Thus, the immunohistochemically exposed localization of plasma FN and TG in the control animals fully
corresponds to earlier published data (Birembaut et al.,
1980; Rosai et al., 1992).
On the 5 to 6 days after exposure to radioactive iodine
the nature of the distribution of plasma FN changes
sharply. The connective tissue is swollen, enlarged, and all
its components are saturated with plasma FN. It is
obvious that such a change is the result of radiation
damage to the organ’s exchange vessels, a sharp increase
in the permeability of their endothelium, and the filtration
of a significant amount of blood plasma. At the same time,
the appearance of FN positive follicles demonstrates the
penetration of blood plasma into the follicular lumen, and
consequently of a breakdown in the barrier function of the
thyroid epithelium on the 5th to 6th day after exposure to a
small dose of radioactive iodine. There are even more FN
positive follicles by day 11–12. At the same time, regardless of the nature of the follicle contents, numerous
thyrocytes with an intensive FN positive reaction begin to
appear in the follicles. As a rule, they all show signs of
degeneration. A similar picture is observed in heart myocytes subjected to experimental ischemia. (Casscells et al.,
1990; Froen et al., 1995). The colloid can remain intact
above the apices of these thyrocytes, but more often the
colloid is saturated in FN, or resorbent vacuoles with
fibronectin positive contours are situated here. However,
by day 16–17 the number of FN positive follicles and
thyrocytes is sharply reduced, and by day 30–40 only a few
remain, thus giving clear evidence of the restoration of the
thyroid epithelium’s barrier functions.
By day 5–6 after exposure there is already a noticeable
reduction in the quantity of colloid inside the follicles and
often an absence of any TG-positive reaction. On the one
hand, its production could be suppressed by the influence
of radioactive iodine. On the other hand, a substantial
number of thyrocytes in the apical and other parts of the
cytoplasm contain numerous TG-positive granules, thus
providing evidence of active colloid resorption by these
cells. However, TG resorption in thyrocytes is a normal
phenomenon.
Far more important is the fact that very shortly after
exposure to radioactive iodine, numerous thyrocytes appear with signs of imminent death and a sharply positive
homogenous TG reaction. The discovery of diffuse TGpositive staining of the connective tissue close to the basal
surface of these thyrocytes provides evidence of the possibility of TG filtration into the stroma through the matrix of
dead cells or of its direct penetration through the damaged
contacts between thyrocytes. Furthermore, in the thyroid
stroma TG is often present in the form of individual drops.
The discovery of thyrocytes in which TG-positive granules
are localized in the basal zone strongly suggests that a
possibility that those thyrocytes with damaged polariza-
606
USENKO ET AL.
tion secrete TG not into the follicle lumen, but rather
through the basal membrane into the connective tissue;
although according to available data, a breakdown in the
polarity of thyrocytes in vivo has not been described whilst
a change in the polarity of follicular cells has been found to
be possible under the influence of various factors in vitro
(Nilsson et al., 1988).
In any case, the discovery of a weak TG positive reaction
in blood vessels on the 5th to 6th day after exposure
indicates that the above mentioned forms of damage
combine to ensure the entry of a significant amount of TG
into the blood. These data give morphological confirmation
of the assumption that the increased production of autoantibodies after 131I induced thyroid gland damage is the
result of a massive input of iodized thyroglobulin into the
blood (Nevstrueva et al., 1972).
With regard to the distribution of TG in the thyroid
gland in the later stages, one should note that after 16 to
17 days TG is no longer found in the stroma or in the vessel
contents. There is a sharp decrease in the number of cells
which have TG saturated cytoplasm. This shows the
noticeable restoration of the thyroid epithelium’s barrier
functions. Thus, even small doses of 131I provoke a serious
breakdown in the thyroid epithelium’s barrier properties,
although these changes are of a transient nature. The
study of the change in the localization of TG and FN as a
result of exposure to radioactive iodine or other damaging
factors can serve as a method for evaluating the influence
of these factors on the barrier properties of the thyroid
epithelium.
However, in this period the quantity of colloid continues
to decrease and in the majority of follicles colloid remains
are only preserved in the center of the cavity. After 30 to 40
days, remains of colloid are found only in isolated follicles,
this being evidence of the significant predominance of
colloid resorption over colloid production.
131I is cleared with a half-life of 3.38 ⫾ 0.01 days (Reilly
et al., 1986). Taking into account the fact that female rats
of this experimental group were exposed to the radioactive
iodine no less than twelve days before mating (more than
the 10-fold time of effective half-life of the 131I up to
beginning of embryo’s thyroid iodine intake), the development of fetuses in this group took place in a state of
131I-induced maternal hypothyroidism without the influence of direct effects of radioactive iodine. This group is the
second control group for the evaluation of direct effects 131I
on embryogenesis.
It is important to note that colloid resorption and the
appearance and disappearance of TG-positive and FNpositive thyrocytes and follicles occurs considerably earlier
in the central zone of the lobe of the thyroid gland than it
does in the peripheral zone. Furthermore, the changes in
the peripheral zone are of a far less pronounced nature.
This type of phenomenon is a result which obviously stems
from the existence of different zones in the thyroid gland,
the fundamental basis of which lies in the morphogenetic
(Low et al., 1982, 1985) and functional (Nunez et al., 1966)
differences between the central and peripheral zones of
every lobe. Thus, the central zone of the thyroid gland
reacts more actively and dynamically to exposure to radioactive iodine.
ACKNOWLEDGMENTS
This study was supported in part from ANIKO MARINE
GROUP (Cyprus, Russia, Ukraine). We acknowledge the
all-around support and help of the director of the private
firm 7VIOR8 (Dnepropetrovsk, Ukraine), Mrs. Irina Filatova, during our research.
LITERATURE CITED
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in experimental litters obtained from mothers subjected to immunization and the action of 131I. Probl. Endokrinol. Mosk. 1976;22:
66–70.
Bajenov VA, Vasilenko IA. Iodine. In Ilyin LA, Filov VA, eds. Harmful
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