Vet Pathol29:278-287 (1992)

Age-related Changes in Thyroid Structure and Function in Sprague-Dawley Rats S. RAO-RUPANAGUDI, R. HEYWOOD, AND C. GOPINATH Huntingdon Research Centre, Huntingdon, and St. George’s House, Huntingdon, United Kingdom Abstract. Investigation of thyroid glands from 500 male and 500 female Sprague-Dawley rats, at time points of 8, 17, 30, 56, and 108 weeks of toxicity studies conducted at the Huntingdon Research Centre between 1981 and 1 984, revealed age-related structural and functional changes that have previously not been well documented. The number of ultimobranchial cysts decreased with age, while area(s) of C-cell hyperplasia appeared with age. Beginning at 56 weeks, some of the thyroid follicles were hyperdistended with colloid, had irregular lumens, and were lined by flattened epithelium. These follicles had clumped, granular, and stratified colloid. Follicular tumors were found in 8% of the males and 6% of the females at 108 weeks. There was an increase in absolute thyroid weights (males from 21.8 k 4.0 g to 46.5 f 19.05 g, females from 17.2 4.53 g to 41.7 t 26.92 g) and body weights (males from 382.0 t 70.6 g to 806.0 f 120.7 g, females from 220.0 f 21.0 g to 495.0 t 127.3 g) with age in both sexes, but the relative thyroid weights were not significantlyaffected. Negative allometry was observed. With an increase in the age of the rats, there was a decrease in the height of the follicular epithelium and an increase in the internal follicular diameter and the total number of follicles. No prediction for sex could be detected. Serum T3 and T4 concentrations were constant until 56 weeks of age, but at 108 weeks, the values were markedly reduced (in males, serum T3 concentration decreased from 91.60 f 13.970 ng/100 ml to 32.90 f 10.878 ng/100 ml, and in females, from 90.80 f 11.338 ng/100 ml to 48.10 2 8.875 ng/100 ml; in males, serum T4 concentration decreased from 5.94 t 0,679 pg/lOO ml to 3.04 -I 0.604 pg/IOO ml, and in females, from 4.59 k 0.717 pg/100 ml to 2.77 f 0.786 pd100 ml). The data suggest that the thyroid function ofSpragueDawley rats reduces as the rats age.

*

Key words: Age; hormones; qualitative and quantitative change; Sprague-Dawley rats; rats; thyroid.

Despite the fact that the Sprague-Dawley rat is extensively used in toxicological studies, and the thyroid is known to change with age,2,4Jo.24 there is a paucity of literature on age-related changes of the thyroid in Sprague-Dawley rats. Many reports have described thyroid dysfunction in conjunction with increasing age. Several studies have indicated an age-related decrease in the release of the hormones from the thyroid gland. Conversion of T4 to T3 and the fractional removal rate of T4 were found to decrease as age increased.28 These changes result in only minimal alteration in the concentration of plasma T4 but marked decreases in plasma T3.17 The thyroid gland of the adult human being, like that of animals, decreases in size (weight) between maturity and s e n e ~ c e n c e Histologically, .~~~~~ there are changes in rodents with aging characterized by alterations in the size of the follicles and in the height of the follicular epithelium and in the number of its structural c ~ m p o n e n t s . ~ J ~ J ~ J ~ The purpose of this study was to establish the normal variations in the structure and function of the thyroid that occur at different ages and attempt to provide a basis by which experimentally induced lesions may be better evaluated. We report age-related lesions and

morphometric data obtained from laboratory-maintained Sprague-Dawley rats. Materials and Methods Data for the morphologic studies were obtained from 500 male and 500 female control Sprague-Dawley rats (Crl: CD[SD]BR) that were on randomly selected toxicity studies at the Huntingdon Research Centre between 1981 and 1984 at time points of 8, 17, 30, 56, and 108 weeks. The quantitative measurements were made on five male and five female rats from each subgroup. The rats were bamer-maintained and housed in groups of five in suspended cages with wire mesh floors. Room temperature and relative humidity were maintained at 21 f 2 C and 50 f 5%, respectively. Lighting was provided on a 12-hour light: 12-hour dark cycle. Labsure Laboratory Animal Diet (Special Diet Services Manea, Cambridge) was fed ad libitum, and water was freely available. At the end of the experimental periods, the rats were weighed and euthanatized with carbon dioxide. A full macroscopic postmortem examination was performed. The thyroid gland was dissected free of fat, weighed, fixed in 10% neutral buffered formalin, and embedded in paraffin wax (56 C). Transverse sections were cut through a plane through both thyroid lobes at the level ofthe parathyroid gland. Three 5-pm sections were stained with hematoxylin and eosin. For

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Table 1. Body weights of male and female Sprague-Dawley rats from 8 to 108 weeks of age. Body Weight (g) Sex

Group 1 (8 weeks of age)

Group 2 (1 7 weeks of age)

Group 3 (30 weeks of age)

Group 4 (56 weeks of age)

Group 5 (108 weeks of age)

Male Female

382.0 t 70.6* 220.0 f 21.0

523.0 f 58.0 273.0 k 24.9

632.0 -t 87.3 347.0 k 44.6

730.0 t 89.6 407.0 f 74.9

806.0 +. 120.7 495.0 k 127.3

* Data are expressed as mean + standard deviation; n = 100 per sex per group.

the quantitative estimations, a section with the largest surface area was chosen.

immunoassay in serum samples using Amersham Test Kits No. 74 and 80 (Amersham International, Amersham, U.K.).

Body and thyroid weights At each time interval, body weights and thyroid weights, both absolute and relative, were evaluated from 100 male and 100 female rats selected at random.

Macroscopic and microscopic findings The macroscopic findings, characterizing the thyroid glands as being enlarged or having a mass, were tabulated in association with histopathologic lesions classified as nonneoplastic or neoplastic.

Morphometric analysis of thyroid follicular epithelium To measure follicular epithelial height, 50 follicles from each lobe falling on a straight line were examined using the 4 x objective (Nikon microscope) with a micrometer eyepiece. The epithelial height was measured at two points on the same axis, and the mean of these two measurements was taken as the height of the epithelium. The internal follicular diameters were measured on the major and minor axes, at right angles to each other. The mean of these two readings was taken as the internal diameter of the follicle. All the follicles in both lobes of the section were counted. The number of follicular cells per 10 mm3, mean follicular cell volume, and relative volume of the various components were calculated as percentage using the method of Philip et al. 1.5.21

Allometric growth Allometric analysis of the thyroid was carried out using the equation y = xb.20This equation is based on the assumption that the relative growth rate of an organ is proportional to the growth rate of the whole body. The variable x represents body weight, to which the allometrically growing organ weight y is correlated. The constant b, which is the ratio of the specific growth rate x and y, was estimated statistically by a logarithmic linearization of observed absolute organ (y) and body (x)weights.

Statistical analyses Body weights, thyroid weights (both absolute and relative), quantitative estimations, and serum thyroid hormone concentrations are presented as mean k standard deviation. Analyses of variance were carried out on the mean number of follicularcells, mean cell volume, follicularepithelial height, internal follicular diameter, and total number of follicles. Slope of regression line trends across time (age) were also carried out on these parameters. In addition Student’s t-test was carried out on the mean number of follicular epithelial cells and the mean cell volume. Standard error of difference was calculated on mean relative cell volumes.

Results Body and thyroid weights

Serum thyroid hormone concentrations

Body weights and absolute thyroid weights increased as the rats’ ages increased in both sexes, but relative ley rats on toxicological studies at the Huntingdon Research thyroid weights were unaffected (Tables 1-3). A sigCentre between 198 1 and 1984 was undertaken to establish nificant linear, positive regression was found between normal serum thyroid hormone concentrations. Ten male the thyroid and body weights; this increased over the and ten female rats were randomly selected at each time point. T3 and T4 concentrations were measured by radio- whole observation period. The allometric coefficient A retrospective survey of control data from Sprague-Daw-

Table 2. Thyroid weights of male and female Sprague-Dawley rats from 8 to 108 weeks of age. Thyroid Weight (g) Sex

Group 1 (8 weeks of age)

Group 3 (30 weeks of age)

24.1 & 4.94 29.1 k 7.01 19.1 f 4.27 24.5 k 6.70 * Data are expressed as mean & standard deviation; n = 100 per sex per group.

Male Female

21.8 rt 4.0* 17.2 f 4.53

Group 2 (17 weeks of age)

Group 4 (56 weeks of age)

Group 5 (108 weeks of age)

32.7 k 7.94 27.6 t 5.77

46.5 k 19.05 41.7 k 26.92

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. Table 3. Relative thyroid weights (thyroid weight/body weight) of male and female Sprague-Dawley rats from 8 to 108 weeks of age. Relative Thyroid Weight (Oh) Sex

Group 1 (8 weeks of age)

Group 2

(1 7 weeks of age)

Group 3 (30 weeks of age)

4.67 k 1.20 4.63 -t 0.94 7.08 & 1.81 7.02 zk 1.57 * Data are expressed as mean k standard deviation; n = 100 per sex per group.

Male Female

5.80 k 1.44* 7.90 k 2.18

for the thyroid was b females.

=

0.77 in males and 0.89 in

Macroscopic findings

At 56 weeks of age, 5% of male rats had enlarged thyroids; a similar incidence (6%) was recorded at 108 weeks (Table 4). From week 56, occasional male and female animals (1Yo) were found with masses indicative of tumors. Microscopic findings

When rats were 8 weeks of age, the epithelium of the thyroid follicle was generally cuboidal; a few animals had low columnar epithelium (Table 4). The follicular cells had spheroidal vesicular nuclei. The active follicles that were lined by tall epithelial cells showing colloid endocytosis were seen toward the center of the gland. Most of the follicles were small. Ultimobranchial cysts (Fig. 1) were found in one-third of all animals examined. Occasionally, animals had focal hyperplasia of the intrafollicular epithelium (Fig. 2). Ten percent of male rats and 20% of female rats were found to have foci of C-cell hyperplasia, as evidenced by a collection of a few C-cells between the follicles. Ectopic thymic tissue was found in occasional animals. By 17 weeks of age, there was a greater incidence of inactive large follicles when compared to the thyroids of 8-week-old rats. At this time, the large follicles were not only present in the periphery of the gland but were seen deeper in the gland. Some of the follicles had columnar epithelium (Fig. 3), but the majority had cuboidal epithelium. Almost all follicles contained colloid. The incidence of ultimobranchial cysts and foci of C-cell hyperplasia were the same as those seen at 8 weeks. Essentially similar findings were recorded at 30 weeks. At 56 weeks, the entire gland was occupied by follicles larger than those seen in younger rats. The follicles were distended with colloid and lined by a low cuboidal epithelium (Fig. 4). Several of the follicles contained colloid, which was eosinophilic and homogenous, and 17% of male and 10% of female animals had clumped, granular colloid. The high inci-

Group 4 (56 weeks of age)

Group 5 (108 weeks of age)

4.51 f 1.14 6.93 f 1.69

5.82 k 2.22 8.58 k 4.08

dence of ultimobranchial cysts persisted; at this time one-third of the animals were found with foci of C-cell hyperplasia. At 108 weeks, there were marked differences in the morphologic appearance of thyroid, characterized by a further decrease in epithelial height, an increase in interstitial fibrous connective tissue, and an increase in numbers and internal diameter of the follicles. Ten percent of the males and 28% of the females showed hyperdistension and irregular follicles. These follicles had flattened epithelium with irregular, shrunken, hyperchromatic nuclei, and the lumen was distended with colloid. Though all the follicles contained colloid, 25% of the males and 11Yo of the females had colloid, which occurred in small clumps or appeared as layered granular material. In some animals the colloid was stratified. Occasionally, brownish pigment was present in the colloid. The incidence of ultimobranchial cysts declined to lo%, but the high incidence (about a quarter of males and half of females) of foci of C-cell hyperplasia persisted. In addition, 8% of the male and 3% of the female rats showed discrete area(s) of C-cell hyperplasia (Fig. 5). These areas contained increased numbers of C-cells in clusters or scattered in the interfollicular space with no apparent compression of surrounding tissue. The first follicular adenoma was seen at 56 weeks of age. At 108 weeks of age, 8% of the males and 6% of the females had follicular tumors. The follicular adenomas (Fig. 6 ) were solitary nodules that were well demarcated from glandular tissue and compressed the surrounding tissue. There was variability in the size and the shape of the follicles. In the follicles, the number of follicular epithelial cells increased, they were closely packed, and they overlapped each other. The cavities were lined by a single layer of hyperchromatic epithelial cells. Follicular carcinomas (Fig. 7) were diagnosed on the basis of invasion into the surrounding connective and intrafolliculartissue. C-cell carcinomas (Fig. 8) were found in 1% of rats. They consisted of variably sized nests of cells with vesicular nuclei and pale eosinophilic cytoplasm. Tumor cells invaded the capsule and the blood vessels. Mitotic figures were present.

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Table 4. Incidence summary of macroscopic and microscopic findings in the thyroids of rats from 8 to 108 weeks of age.* Incidence Findings

Group 1 (8 weeks of age)

Group 2 (17 weeks of age) F M

Mt

F

0 0

0 0

0 0

37

34

32

5

2

10

Group 3 (30 weeks of age) M F

Group 4 (56 weeks of age) M F

Group 5 (108 weeks of age) M F

Macroscopic Enlarged Mass Nonneoplastic Ultimobranchial cysts Focal hyperplasia of intrafollicular epithelium Foci of C-cell hyperplasia Area(s) of C-cell hyperplasia Follicular epithelium columnar Hyperdistension and irregular follicular lumen Increased interfollicular connective tissue Clumped, granular, stratified colloid Ectopic thymus Foci of brown pigmentation Foci of lymphocytic infiltration Neoplastic Follicular adenoma Follicular carcinoma C-cell carcinoma

0 0

0 0

5 1

0 1

6 1

1 3

27

28

31

24

34

11

13

3

0

3

1

8

3

0

0

20

7

20

8

20

33

39

28

47

0

0

0

0

0

0

0

0

8

3

4

3

17

4

17

12

0

1

1

0

0

0

0

0

0

0

1

3

10

28

0

0

0

0

0

0

0

0

6

6

0

0

0

0

0

0

17

10

25

11

2 0

0 0

2 0

3 0

0 0

0 0

0 1

0 0

0 1

0 1

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

1

1

7

3

0

0

0

0

0

0

0

0

1

3

0

0

0

0

0

0

0

0

1

1

0 0 Microscopic

* 100 animals per sex per group.

t M = male; F = female.

trend decreasing with increase in age in both males and females (Table 6 ) . At 56 weeks of age most values As the body weight and thyroid weight increased became significant (P < 0.05 or P < 0.001). The relative volumes of the different thyroid comwith age, the follicular epithelial height decreased, with a corresponding increase in the internal follicular di- ponents changed with age (Table 7). The mean relative ameter and the total number of follicles (Table 5). follicular cell volume decreased gradually at weeks 17 The mean number of follicular cells and the mean and 30, but by week 56 there was a major decrease in follicular cell volume showed a statistically significant cell volume that persisted to 108 weeks. The relative Quantitative evaluation

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Fig. 1. Thyroid; 8-week-old Sprague-Dawley rat. Ultimobranchial cyst. Cystic structure is lined by squamous epithelium and contains cellular debris. HE. Fig. 2. Thyroid; 8-week-old Sprague-Dawley rat. Note focal hyperplasia (arrows) of intrafollicular epithelium. HE. Fig. 3. Thyroid; 17-week-old Sprague-Dawley rat. The follicles are lined by columnar epithelium. HE. Fig. 4. Thyroid; 56-week-old Sprague-Dawley rat. Follicles are distended with colloid (C) and lined by flattened epithelium (arrows). HE.

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Changes in Thyroid with Age

283

Fig. 5. Thyroid; 108-week-old Sprague-Dawley rat. Area(s) of C-cell hyperplasia (arrows). Increased numbers of C-cells are present in clusters or scattered in interfollicular space. HE. Fig. 6. Thyroid; 56-week-old Sprague-Dawley rat. Follicular adenoma, a solitary nodule, is well demarcated and compresses the surrounding tissue. HE. Fig. 7. Thyroid; 108-week-old Sprague-Dawley rat. Follicular carcinoma. Note that the tumor cells have invaded the surrounding tissue. HE. Fig. 8. Thyroid; 108-week-old Sprague-Dawley rat. C-cell carcinoma. Tumor cells have invaded the intrafollicular spaces. HE.

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Table 5. Quantitative evaluation of age-related changes in thyroids of Sprague-Dawley rats from 8 to 108 weeks of age. Age in Weeks

8 (group 1)

17 (group 2)

Change

Sex?

Follicular epithelial height (pm) Internal follicular diameter (pm) Total number of follicles

M F

10.02 f 0.439* 10.04 k 0.740

M F

46.18 f 6.55 45.25 f 9.839

M F

1,000.0 f 35.40 965.0 f 60.045

30 (group 3)

56 (group 4)

I08 (group 5)

9.61 k 0.495 8.24 f 1.087

7.95 f 0.450 7.23 & 0.944

5.66 k 0.432 5.67 f 0.882

3.96 f 0.296$ 3.83 f 0.431$

52.22 f 12.79 53.33 f 11.115

53.95 f 12.44 54.10 k 11.114

73.52 f 19.113 73.50 k 15.721

91.78 k 34.312$ 95.77 f 22.328$

997.0 f 48.36 1.062.8 f 86.276

1,227.0 f 31.09 1.129.0 _+ 106.36

1,373.0 f 77.58 1.220.60 f 32.269

1,802.0 k 281.27$ 1,436.0 f 121.40$

* Data are expressed as mean f standard deviation; 100 follicles examined per rat; n = 5 per sex per group.

t M = male; F = female.

$ Trends across time: Slope of regression line significantly different from zero (P < 0.001).

lumen volume in male rats showed a progressive minimal increase throughout the 108-week observation period. In females, the lumen volume increased to 56 weeks, and this persisted to 108 weeks. The relative stromal volume increased gradually to 30 weeks of age; the increase was marked in both sexes at week 56 and week 108. Serum thyroid hormone concentrations

The serum T3 and T4 concentrations (Table 8) remained relatively constant in both sexes to week 56 but were markedly reduced at week 108. Discussion Analysis of organ weight is fundamental to many biologic studies and is of considerable importance in toxicologic studies. It is standard practice to present

organ weights as absolute values or as a percentage of body weight, though absolute organ weights are a more reliable parameter to define growth processes than relative organ weights. A significant linear, positive regression was found between body weight and absolute thyroid weight; the allometric coefficient was less than 1. This finding denotes negative allometry or slower organ growth. These findings are in agreement with those of Trieb et al.27 The macroscopic appearance of the thyroid was not a sensitive method for detecting change. The only features that could be detected were increase in size and occasional nodules or masses. Nonneoplastic disease of the thyroid is said to be infrequent and s p ~ r a d i c . We ~ J ~frequently found ultimobranchial cysts that were lined with squamous epithelium. The cysts were located at the center of the

Table 6. Effect of age o n mean number of follicular cells a n d mean follicular cell volume of the thyroid in SpragueDawley rats from 8 to 108 weeks of age. Age in Weeks (x)

Sext

8

17

30

56

108

Mean number of follicular cells (1 O6 cells/ 10 mm3) (Y)§ Mean cell volume bm3) (z)S

M F

2.60 f 0.281* 2.49 f 0.182

2.51 f 0.124 2.37 f 0.222

2.37 f 0.1 10 2.35 f 0.309

2.224 f 0.227 1.74 f 0.36411

1.74 f 0.33 111 1.68 k 0.18311

M F

1,798.05 f 348.79 2,110.07 f 349.12

1,704.14 -+ 131.09 1,883.68 f 139.82

1,708.50 f 169.95 1,741.71 f 306.94

1,138.97 k 544.60# 1,455.23 k 399.42#

1,297.11 f 125.20 1,380.47 f 579.70#

Regression equations

M F

y y

= =

2.67 - 0 . 0 0 9 ~ 2.52 - 0 . 0 0 9 ~

z

=

1,786 - 5 . 9 6 ~

z = 2,020 - 7 . 1 0 ~

* Data are expressed as mean f standard deviation; n = 3 per sex per group.

t M = male; F = female.

$ There are significantly decreasing trends in mean number of follicular cells (lo6 cells/lO mm3) and mean cell volume (pm3)levels with time (P < 0.001 and P < 0.05, respectively). 5 There is no apparent difference between males and females. When analyses of variance were camed out on the data for both sexes combined, there was evidence of a decreased trend in mean number of follicular cells and mean cell volume over time (P < 0.001). 11 Data are significantly different from 8 weeks using Students’ t distribution; P i0.00 1. # Data are significantly different from 8 weeks using Students’ t distribution; P < 0.05.

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Table 7. Effect of age on relative volume of the various thyroid components in Sprague-Dawley rats from 8 to 108 weeks of age.* Age in Weeks

Sex?

Component

Mean relative follicular cell volume Mean relative luminal volume Mean relative stromal volume

M F M&F M F M&F M F M&F

8

17

30

56

108

SED$

46.3% 52.1% 49.2% 33.6% 36.5% 35.1% 14.1% 1 1.4% 12.7%

42.7% 44.6% 43.7% 42.3% 42.3% 42.3% 15.0% 12.5% 13.7%

40.6% 41.1% 40.8% 44.6% 41.5% 43.5% 14.8% 17.4% 16.1%

25.1% 24.3% 24.7%$ 41.4% 51.4% 46.4%11 33.5% 24.3% 28.9%$

22.4% 23.7% 23.0%§ 50.0% 50.3% 50.2%# 27.5% 26.0% 26.8%§

5.71 5.7 1 4.04 4.73 4.73 3.34 5.68 5.68 4.01

* n = 3 per sex per group.

*5 P

t M = male; F = female.

SED = standard error of the difference between the week 8 group and each of the other age groups (residual df = 20). < 0.001. 11 P < 0.05. # P < 0.01.

gland near the hilus. There was a definite decrease in the incidence of these cysts with age. Focal hyperplasia of intrafollicular epithelium was found in a small number of animals up to 1 year of age. Inflammation of the thyroid gland is rare, and this survey did not identify a single example. Foci of brown pigmentation were seen in a few rats aged 56 weeks and older. The pigmentation was seen in one or two of the large irregular follicles in the outer part of the thyroid. Sometimes the pigment was accompanied by cellular debris. The pigment has been shown to be iron and periodic acidSchiff positive. Thymic tissue may be found in the thyroid, but we found it only in rats less than 17 weeks old, suggesting involution in age. Colloid changes in the form of clumps that may appear granular and/or stratified were apparent at 56 weeks of age. Thyroid tumors occur in the rat, the incidence varies with the strain of rat.’0J2J9,25 The data on the incidence of spontaneous thyroid tumors in rats are very contradictory. Some authors have reported the incidence as extremely low and others as very common.18In our study in Sprague-Dawley rats, we have observed a greater incidence of follicular tumors compared to that

reported in other strains, such as Wistar,I4F344,8 Osb~rne-Mendel,~ and a lower incidence of C-cell tumors compared to the incidence reported in Long-Evans,6 O~borne-Mendel,~ and F344.* There are problems in defining the incidence of tumors in laboratory animals, particularly when this is based on a single section, for there are no means of knowing whether the incidence of abnormalities obtained by such a procedure is correct or incorrect, particularly if the lesions are focal. Thompson and Hunt26showed that, when a single section was examined, the incidence of C-cell tumors observed in 177 rats was nine. When the organ was sectioned serially, the incidence was 55. There is marked variation in the literature on the reports of incidence of C-cell tumors in the rat. This might be partially due to the difficulties existing in the differential diagnosis of C-cell hyperplasia, adenoma, and carcinoma. C-cell hyperplasia is an age-related lesion. Hyperplasia denotes an increase usually of interfollicular C-cells and sometimes within and filling the follicles. These cells have cytological features not dissimilar to the normal C-cells. Care must be taken to prevent overinterpretation of hyperplasia. The term “C-cell adenomas”

Table 8. Serum T3 and T4 concentrations of Sprague-Dawley rats from 8 to 108 weeks of age. Hormone

Age in Weeks

Sex?

T 3 ngper 100/ml M F T 4 p g p e r 100/ml M F

8

91.60 90.80 5.94 4.59

30

17

f 13.970* 74.40 f 12.420 f 16.572 90.10 f 11.338 5.69 f 0.597 f 0.679 4.55 f 0.916 f 0.717

71.40 103.70 5.57 4.38

f 12.695 f 13.516 -t 0.701 k 0.892

56

81.10 108.20 5.44 3.98

* Data are expressed as mean f standard deviation.

t M = male; F = female; n = 10 per sex per group.

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f 9.146 f 18.719 f 0.749 f 1.058

108

32.90 48.10 3.04 2.77

f 10.878 f 8.875

f 0.604 f 0.768

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Rao-Rupanagudi, Heywood, and Gopinath

must be restricted to well-circumscribed nodules of C-cells showing varying degrees of compression in the surrounding parenchyma. This is the convention adopted by the National Toxicology Program (Research Triangle Park, North Carolina, USA) for carcinogenicity evaluation. The tumor consisted of uniform cells with abundant pale-staining cytoplasm and was not encapsulated. The incidence of these tumors varies considerably with age and strain of rat and how diligently the pathologist searches for them. In C-cell carcinomas, the cytological features often do not differ from those of the benign tumor, but there are irregular growths showing evidence of local invasion, abnormal vascularity, hemorrhage, and other features of malignancy. A proportion of these tumors are known to metastasize to the regional deep cervical lymph nodes. A higher incidence of C-cell carcinomas is to be expected in Wistar rats.3 The follicular epithelial cells, the internal follicular lumens in which the colloid is stored, and the interfollicular stroma, all change with increased age. As the thyroid weight increases with increasing age, the number of thyroid follicular epithelial cells per unit weight decreases progressively from week 17. The height of the follicular epithelium and mean cell volume decrease significantly from 17 weeks of age, with a resultant increase in the internal follicular luminal diameter and associated with accumulation of colloid. There is a significant increase in the total number of follicles with age, and this persists to 108 weeks. There is an increase in the growth of connective tissue and C-cells at 56 weeks. This increase in the growth of stroma with age can be seen in several organs of the body; it may be due to the replacement of lost functional structures with connective tissues. Serum T3 concentrations were shown to be constant in both sexes until 56 weeks of age; at 108 weeks, the values were markedly reduced. The serum T4 values remained constant in both sexes to week 56; a marked reduction of about 50% occurred at week 108. The serum T3 concentrations were decreased by about 60% at 108 weeks compared to 56-week values; this marked decrease in the T3 concentrations may be due to the decreased S'deiodinase activity in the liver and kidneys that is responsible for conversion of T4 to T3. The conversion of T4 to T3 and the functional removal rate of T4 were found to decrease with age. Age-related reduced S'deiodinase activity presumably results in a fall in serum T3 levels.28Latham and Tseng16 reported that in Wistar rats the serum concentrations of T3 increased by about 40% in male and female rats from 3 months to 15 months, while serum T4 concentrations increased in males by 22% but decreased slightly in females. These differences in serum concentrations of thyroid hormones between Wistar rats and

Vet Pathol 29(4), 1992

Sprague-Dawley rats indicated strain difference with respect to the thyroid hormone concentrations. The decrease in thyroid hormone concentrations with age could be due to reduced pituitary function, though no variation in serum thyroid stimulating hormone with age has been found in Sprague-Dawley rats at 12 or 24 month^.^ In adult animals of several species, thyroid activity appears to decrease with increasing age. The response, as in human beings, may be homeostatic.22The data of this survey suggest that the functionality of the thyroid reduces as the age of Sprague-Dawley rats increases. Thus, the age of the animal should be taken into consideration when investigating toxicity-induced structural and functional alterations in the thyroid. References 1 Abercrombie M: Estimation of nuclear population from microtome section. Anat Rec 94:239-240, 1946 2 Anderson MP, Capen C: The endocrine system. In: Pathology of Laboratory Animals, ed. Benirsche K, Gamer FM, and Jones TC, pp. 443-450. Springer Verlag, New York, NY 1978 3 Boorman GA, Hollander C F Medullary carcinomas of the thyroid. Am J Pathol83:237-240, 1976 4 Burek JD: Pathology of Aging Rats, pp. 29-38. CRC Press, Boca Raton, FL, 1978 5 Chalkley H W Methods for the quantitative morphologic analysis of tissues. J Natl Cancer Inst 4:47-5 1, 1943 6 De Lellis RA, Nunnemacher G, Bitman WR, Gragel RF, Tashjian AH Jr, Blount M, Wolfe JH: C cell hyperplasia and medullary thyroid carcinoma in rat: an immunohistochemical and ultrastructural analysis. Lab Invest 40: 140-154, 1979 7 Emanuele NV, Baker G, McDonald D, Kirsteins L, Lawrence AM: The impact of aging on luteinising hormone (LH) and thyroid-stimulating hormone (TSH) in the rat brain. Brain Res 20:179-183, 1985 8 Goodman DG, Ward JM, Squire RA, Chu KC, Linhart MS: Neoplastic and non-neoplastic lesions in aging Osborne-Mendel rats. Toxicol Appl Pharmacol48:237-248, 1979 9 Goodman DG, Ward JM, Squire RA, Paxton MB, Reichardt WD, Chu KC, Linhart MS: Neoplastic and nonneoplastic lesions in aging Osbome-Mendel rats. Toxicol Appl Pharmacol55:433-447, 1980 10 Greaves P, Faccini JM: Rat histopathology, pp. 1872 10. Elsevier Press, Amsterdam, The Netherlands, 1984 11 Haensley WE, Getty R: Age changes in the weight of thyroid gland of swine from birth to eight years. Exp Gerontol5203-206, 1970 12 Haxman JD: Patterns of tumor incidence in two year cancer bioassay feeding studies in Fischer 344 rats. Fundam Appl Toxicol 3: 1-9, 1983 13 Jacobs B: Variations in thyroid morphology in mice. ProcSocBiol97:115-118, 1958 14 Kroes R, Garbis-Berkvens JM, de Vnes T, Van Nesselrooy JHJ: Histopathological profile of a Wistar rat stock

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including a survey of the literature. J Gerontol 36:259279, 1981 Lansing W, Wolfe JM: Structural changes associated with advancing age in the thyroid gland of the female rat with particular reference to alteration in the connective tissues. Anat Rev 88:3 11-325, 1944 Latham KR, Tseng Y L Nuclear thyroid hormone binding activities and serum iodothyronine in aging rats. Age 2~48-54, 1985 Nakamura H, Yokota T, Imura H: Metabolism, receptors and action of thyroid hormone in the liver and brain during aging. In: Liver and Aging, ed. K Kitani, pp. 18319 1. Elsevier Science Publishers B.V. (Biomedical Division), Amsterdam, 1986 Napalkov NP: Pathology of Tumors in Laboratory Animals, Volume I: Tumors of the Rat, ed. Turusov VS, pt. 2, pp. 239-272. IARC ScientificPublications No. 6, Lyon, 1976 Nunziata A, Storino AA: Spontaneous neoplastic pathology in control rats. Vet Hum Toxicol 24:243-247, 1982 Pasternack B, Gianutsos R: Application of the exponential and power functions to the study of allometric growth with particular reference to doryline ants. Amer Nat 103:225-234, 1969

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21 Philip JR, Crooks J, MacGregor AG, McIntosh JAR: The growth curve of rat thyroid under a goitrogenic stimulus. Br J Cancer 23:515-523, 1969 22 Pipes GW, Bauman TR, Brooks JR: Effect of season, sex and breed on the thyroxine secretion rate of beef cattle and a comparison with dairy cattle. J Anim Sci 22: 476, 1963 23 Pittman JA: The thyroid and aging. J Am Geriatr SOC 10~10-14, 1962 24 Russfield AB: Pathology of the endocrine glands, ovary and testes of rats and mice. In: Pathology of Laboratory Rats and Mice, ed. Cotchin E and Roe FJC, pp. 391467. Blackwell ScientificPublications, Oxford, UK, 1967 25 Sher SP: Tumors in control hamsters, rats and mice. CRC Crit Rev Toxicol 10:49-79, 1982 26 Thompson SW, Hunt RD: Spontaneous tumors in the Sprague-Dawley rats. Ann NY Acad Sci 108:832-845, 1963 27 Trieb G, Pappritz G, Lutzen L: Allometric analysis for organ weight. Toxicol Appl Pharmacol3553 1-542, 1976 28 Visser TJ, Van Der Does-Tobe I, Docter K, Hennemann G: Conversion of thyroxine into tri-iodothyronine by rat liver homogenate. Biochem J 150:489-493, 1975

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Age-related changes in thyroid structure and function in Sprague-Dawley rats.

Investigation of thyroid glands from 500 male and 500 female Sprague-Dawley rats, at time points of 8, 17, 30, 56, and 108 weeks of toxicity studies c...
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