Changes in the Thyroid Gland of Certain Mammals and Birds Following Hypophysectomy

It is now well established that the anterior pituitary body is actively concerned in the normal development and functional maintenance of the thyroid gland. Hypophyseal extracts have been found to stimulate the thyroid to hyperplasia in a number of species, particularly in the guinea-pig (see Freud et al. 1933). Schockaert(1932)has also shown that the thyroid of the duck is very sensitive to anterior pituitary extracts. This experimentally produced hyperplasia causes symptoms of hyperthyroidism to develop. Siebert and Smith (1930) noticed an increase in the basal metabolic rate in treated guinea-pigs. This increase could not be produced in totally thyroidectomised animals, showing that the pituitary substance affects the metabolic rate through the thyroid. Other symptoms occur such as increased heart rate, exophthalmos, a reduction in the iodine content of the thyroid, an increase in the alcohol-insoluble iodine in the blood, a depletion of liver glycogen and an increase in the acetone bodies of the blood (for references see Anderson and Collip, 1934).

Another line of investigation into this relationship between the pituitary and the thyroid has been the histological examination of the latter after the removal of the pituitary. Hypoplasia of the thyroid has been observed in the rabbit by White (1933), in the rat by Smith (1927), in the dog by Dott (1923) and Houssay et al. (1931b), and in the cat by McPhail (1935). A similar change in the thyroid of birds after partial hypophysectomy was obtained by Mitchell (1929). This atrophy of the thyroid is followed by such symptoms of hypothyroidism as lowering of the basal metabolic rate in rats (Anderson and Collip, 1934); a reduction of the thyroid iodine in dogs (Houssay et al. 1931c), and also a reduction in the blood iodine (Houssay et al. 1931d).

The number of species of mammals on which hypophysectomy can be performed is steadily increasing. The operation has been described by M. Hill and Parkes (1932) for the ferret and by McPhail and Parkes (1933) for the guinea-pig and hedgehog. Hypophysectomy in the domestic fowl (young Brown Leghorns of both sexes) was performed by Mitchell (1929), but he did not succeed in keeping the birds alive for more than two weeks. Very recently R. T. Hill and Parkes (1934) have repeated the operation in adult birds, and they have been able to keep them alive for a much longer period. This paper deals with histological changes in the thyroid of the hypophysectomised ferret, hedgehog, guinea-pig and bird.

The thyroid material has been collected from animals hypophysectomised in this laboratory during the course of the last year.

Two series of ferrets were hypophysectomised. The first consisted of five males and five females and were operated on during the anoestrous period. The males were killed at weekly intervals from 2 to 6 weeks after operation. The females were killed at a similar time except that a 7-week stage was obtained instead of one at 6 weeks. The second series consisted of two males and three females operated on during the breeding season. The first male was killed after 12 days and the second after 18 days. Two females were killed at 6 days and one at 13 days after hypophysectomy. Control material was obtained from six oestrous and seven anoestrous males, and nine oestrous and five anoestrous females.

The thyroids are available from seven completely hypophysectomised adult male guinea-pigs with a body weight ranging from 500 to 800 gm. The times of killing up to 56 days after hypophysectomy are shown in Table IV. Control material consisted of fourteen adult males of a similar range of body weight.

All the guinea-pigs were taken from the inbred stock maintained at the Institute’s Farm Laboratories at Mill Hill.

The thyroids of one male hypophysectomised hedgehog were obtained, together with those from one normal male and three normal females. These animals had been kept under laboratory conditions for a short period and had presumably become acclimatised.

The bird thyroids after hypophysectomy were obtained from three Brown Leghorns (two cocks and one hen), one Rhode Island Red cock, and one Sebright Bantam cock. Control thyroids were obtained from two Sebright Bantam cocks, and two Brown Leghorn cocks.

The thyroids of all the species of animals to be described in this paper can be dissected clean from adhering fat and connective tissue. Fixation overnight in Bouin’s alcoholic picro-formol was followed by dehydration in 70 per cent, alcohol, when the organs were weighed on a torsion balance.

Serial sections at 7 μ were cut longitudinally of one thyroid from each pair. In the case of some of the larger ones only one section in ten was mounted. The stain employed was Mayer’s haematoxylin with eosin as a counter-stain.

The base of the skull of every hypophysectomised animal was examined macroscopically for pituitary remains, and sections were cut if necessary. Completely hypophysectomised animals only are considered in this paper.

Changes in the Thyroid Gland of Certain Mammals and Birds 339

The reproductive organs of the ferret are in a state of quiescence during the greater part of the winter (Hammond and Marshall, 1930; Allanson, 1932). The thyroids of normal oestrous and anoestrous male and female ferrets have been collected ; the average body weight and weight of thyroids for the four groups are shown in Table I.

From Table I it is obvious that there is a significant difference between the weight of the thyroids of the oestrous and anoestrous ferret of both sexes, especially in the case of the male. It will be seen that on a body-weight basis the thyroid of the female is slightly smaller than that of the male.

The thyroid of the ferret is long and narrow in shape, some of those in the oestrous ferret attaining a length of 1 · 5−2 cm. In this condition they appear fleshy and well vascularised. In anoestrus they are more pale and appear, macroscopically, to be much less vascularised.

During anoestrus the thyroid of the ferret contains vesicles measuring up to 100µ, in diameter, which are full of colloid in varying stages of absorption. In the majority of vesicles numerous vacuoles can be seen (Pl. I, fig. 1) at the periphery of the colloid mass, in others, vacuolation is absent. The colloid is fairly dense and is well stained by eosin. The vesicular epithelium is cuboidal in shape, the cells having large, round, centrally-placed nuclei. The nucleus contains two or more well-defined masses of chromatin, and a clear, slightly staining nucleoplasm. There is a certain amount of intervesicular connective tissue present which is epithelial in character. Vascularisation is poor.

The histological difference between the thyroid of the oestrous and anoestrous female ferret is very slight in the majority of those examined. The dimensions of the vesicles are similar, although the colloid in the oestrous ferrets may be more vacuolated (Pl. I, fig. 3). Where the absorption of colloid is greater the epithelial cells become more columnar in shape. The thyroids of four of the six oestrous male ferrets (Pl. I, fig. 5) are hyperactive. Colloid may be absent, the epithelial cells being either high cuboidal or columnar, the lumen nearly occluded in some vesicles. The degree of vascularisation in these thyroids is very great, all the vesicles being separated from each other by capillaries.

The average body and thyroid weights of the oestrous and anoestrous hypophysectomised ferret are shown in Table II.

Comparison of Tables I and II shows that hypophysectomy during anoestrus causes a slight decrease in the weight of the thyroid of both sexes. The differences in weight of thyroid between the normal oestrous and the hypophysectomised oestrous ferrets are striking and appear to be significant, in spite of the scanty material. The difference is more marked in the male, although in both sexes the weight of the thyroid is reduced to less than a half. Hypophysectomy reduces the weight of the thyroid of both oestrous and anoestrous ferrets of the same sex to approximately the same weight.

The thyroids of ferrets hypophysectomised during anoestrous have vesicles fairly regular in size, the largest measuring about 100−120μ in diameter. Colloid is present in all the vesicles; it is dense, and very darkly stained in some vesicles. In the majority of vesicles it is non-vacuolated. The epithelium varies from a low cuboidal to a small flattened squamous type. The nucleus is small, it has a greater affinity for stain, and has lost the chromatin granules. Very little cytoplasm surrounds the nucleus and vascularisation is greatly reduced.

Such regression of the vesicular epithelium is noticed in the thyroids of male and female anoestrous ferrets 21 days after hypophysectomy and in all later stages following the operation. At 14 days these changes are not completely developed. Nevertheless, the colloid is becoming less vacuolated, and the vesicular epithelium is composed of low cuboidal-shaped cells that are becoming inactive (Pl. I, fig. 2).

In the thyroid of the female oestrous ferret 6 days following hypophysectomy regressive changes have occurred in the vesicular epithelium. Shrinkage of the cytoplasm, indicating loss in nuclear activity, occurs in all the cells. Pl. I, fig. 4, shows the condition of the thyroid 13 days after hypophysectomy. The thyroids of the male oestrous ferrets 12 days after hypophysectomy show remarkable atrophic changes. The vesicles have become contracted and have lost their characteristic appearance. The remains of capillaries can be seen running in between the shrunken vesicles. The intervesicular tissue, which is normally of an epithelial nature, has degenerated and appears fibrous. The thyroid of the second oestrous ferret, killed 18 days after the operation, was in a more advanced state of degeneration. The vesicles (Pl. I, fig. 6) are scarcely discernible among the fibrous elements of the intervesicular tissue. The remains of the capillaries are much less obvious in this thyroid.

The thyroid of another mammal undergoing seasonal changes in the reproductive organs, the hedgehog, has been investigated after hypophysectomy. Adler (1920) found definite histological changes between the thyroid of the normal hedgehog in the spring and that during hibernation in the winter. The thyroid may therefore have a seasonal cycle in this species. To what extent this normal cycle in the thyroid is disturbed by housing the animals under laboratory conditions is not known. The changes described therefore are between normal and hypophysectomised hedgehogs kept under these conditions, and irrespective of the time of year.

The hedgehog belongs to that class of animal, including man, where the two lateral lobes are united by a thin median lobe or isthmus passing over the trachea. The glands are large and fleshy, so that dissection can be accurate. The four normal hedgehogs whose body and thyroid weights are recorded in Table III were all killed early in October, having had no previous treatment, and having been in the laboratory for at least 6 months.

The vesicles are fairly small, being on an average from 80 to 100μ in diameter. There is relatively little variation in the size of the vesicles in this species. The vesicles all contain colloid, well stained by eosin and well vacuolated. The epithelial cells appear active, being high cuboidal in shape and possessing large, round nuclei approximately in the centre of the cytoplasm (Pl. II, fig. 1). There is an appreciable amount of intervesicular connective tissue containing numerous blood capillaries.

The thyroids of only one completely hypophysectomised hedgehog are available. The operation was performed on March 7th, 1934, after the animal had been kept in the laboratory for a short time. A period of 12 weeks elapsed before the animal was killed. During that time the body weight had increased from 520 to 670 gm. The weight of the two thyroids, including the isthmus, was 55 mg., which appears to be definitely lower than the thyroids of the control animals.

The average diameter of the vesicles in the thyroid does not differ significantly from the normal. The vesicles are all uniformly atrophic and appear slightly crumpled. The epithelial cells have become greatly reduced, the cytoplasm having almost disappeared. The nuclei are all flat, degenerate and heavily stained. The colloid in the vesicles is dense and non-vacuolated. There is little intervesicular tissue (Pl. II, fig. 2).

The atrophy of the vesicular epithelium, and the accumulation of non-vacuolated colloid in the vesicles, are identical with the description given by Adler (1920) for the normal hedgehog thyroid during hibernation.

The average weight of the thyroids of fourteen adult male guinea-pigs over 500 gm. body weight is 67 mg., varying between 40 and 116 mg. Within these wide limits little correlation is noticed between weight of thyroids and body weight.

The vesicles vary considerably in size, the larger ones measuring from 80 to 100μ in diameter. The epithelium of the large vesicles tends to be flatter, due to the accumulation of colloid. The majority of vesicles are small and contain less colloid, which is usually vacuolated, and the epithelial cells appear more active. The cells are high cuboidal in shape (Pl. II, fig. 3) with the characteristic large, round nuclei with conspicuous chromatin granules. There is a considerable amount of intervesicular connective tissue containing many blood capillaries.

The thyroids of seven hypophysectomised adult male guinea-pigs were obtained, the individual data being shown in Table IV. The weight of the thyroids of six of these animals is known and gives an average of 65 mg., which in view of the great variation in weight of the individual thyroids shows no significant difference from the average normal weight.

Thirteen and 21 days following hypophysectomy, the thyroids show definite regressive changes, particularly in the vesicular epithelium. The cytoplasm surrounding the nuclei becomes contracted, indicating nuclear atrophy, while the nuclei themselves become heavily stained and lose their characteristic chromatin granules. The epithelium becomes low cuboidal in shape.

After this time (28−56 days) all the thyroids examined show intense atrophy of the epithelium, with large accumulations of colloid that distend the vesicles. In very few vesicles is the colloid vacuolated. The vesicles, so distended, measure approximately 160μ in diameter. The epithelial cells are completely atrophic. The intervesicular tissue is much reduced and the gland as a whole is poorly vascularised (PL II, fig-4).

The position of the thyroids differs considerably in the bird from the mammal. The thyroid apparatus consists of two quite separate glands situated on either side at the posterior end of the trachea, each lying on the ventral side of the carotid where it touches the jugular vein, near the origin of the vertebral artery. They are widely separated from one another. Each gland is small and oval.

One thyroid was dissected from each of two Brown Leghorn cocks (COH 19 and TH 2), and these weighed 95 and 22 mg. respectively, and one from each of two Sebright Bantam cocks (Sebright 12 and Sebright 13) weighed 72 and 27 mg. respectively.

The thyroids of COH 19 and Sebright 12 appear to be hypoactive as judged by the large accumulations of colloid in the vesicles, and the low cuboidal epithelium. The vesicles are large, some in Sebright 12 measuring up to 150µ in diameter while some in COH 19 are much larger. There is great variation in the size of the vesicles in these thyroids. The thyroids of TH 2 and Sebright 13 are more active. The vesicles are more regular in size, the epithelium of a high cuboidal type, and contain well-vacuolated colloid, particularly in TH2 (Pl. II, fig. 5). Vascularisation of these thyroids is slight.

Table V gives the weight of one or both thyroids, together with the time at which the birds were killed following the removal of the pituitary.

The left thyroid was removed from HF36, 121 days following hypophysectomy. It is not possible to make any quantitative deductions from the above table apart from a suggestion that the weight of the Sebright thyroids is below normal.

In most of these thyroids there is great variation in size of the vesicles due to their being distended with large accumulations of colloid. The vesicles of the left thyroid of HF36 are full of non-vacuolated colloid. The epithelium is not very atrophic, the cells being of a low cuboidal type. The nuclei are small and are surrounded by a fair amount of cytoplasm.

The vesicles in the thyroid of Sebright 7 appear to be much smaller than those of the normal Sebrights. They measure about 40 − 60 μ in diameter. Regressive changes have occurred in the vesicular epithelium. The cells are of a low, flattened type, with little or no cytoplasm surrounding the nuclei. The colloid is fairly dense and slightly vacuolated. The degree of vascularity is very slight.

Atrophic changes have occurred also in the vesicular epithelium of the hypophysectomised Brown Leghorns, especially in HF 65. This bird was killed 51 days after the operation. The condition of the epithelium is similar to that described for the Sebright. All the vesicles are distended with non-vacuolated colloid. The regression of the epithelium of HF 70 killed after 20 days is as complete as that in HF 71 (Pl. II, fig. 6), which was killed 50 days after the operation. The histological changes are similar to those described by Mitchell (1929) for young Brown Leghorns after partial hypophysectomy.

These birds all showed the plumage changes characteristic of hypothyroidism (R. T. Hill and Parkes, 1935).

There is a considerable decrease in the weight of the thyroid during anoestrus. From the figures given in Table I it can be seen that the weight of the thyroid is approximately halved during this period. This suggests that the amount of thyrotropic hormone secreted by the pituitary is less during anoestrus. The height of the vesicular epithelium and the vascularisation of the thyroids of the males are greater during oestrus, indicating a higher level of activity. In the female the vesicular epithelium remains more or less constant throughout oestrus and anoestrus. Since, as the results show, removal of the pituitary produces severe atrophic changes in the thyroid, the amount of thyrotropic hormone secreted by the pituitary is apparently sufficient to keep the thyroid active throughout the year, irrespective of the breeding season.

Mann (1916) was unable to detect any appreciable difference histologically between the thyroid of the spermophile or gopher (Spermophilus tridecimlineatus) in spring when breeding, and in winter when the animal becomes torpid. His results were obtained on animals living under laboratory conditions at varying temperatures. On the other hand, Adler (1920) found in the hedgehog great histological differences, already referred to, between the thyroid in spring and in winter, when hibernation occurs, although in this species the degree of torpor is slight and very variable. There is, therefore, a species difference in the degree of activity of the thyroid during anoestrus.

Loss in weight of the thyroid, following the removal of the pituitary, does not occur in all species. In the oestrous ferret, both male and female, there is a significant loss in weight, while during anoestrus there is only a slight drop. In the guinea-pig there is no loss in weight. Houssay et al. (1931a) found that in the dog there was a slight initial rise accompanied by congestion for the first few days and, following that, the weight became very irregular. Smith (1927) recorded a decrease in the size of the thyroid of the rat, possibly due to the high activity of the normal gland. Mitchell (1929) found, in spite of the great variation in the weight of the thyroids of the normal bird, that there was a significant decrease in the weight of those of the partially hypophysectomised bird.

In spite of the species variation as regards change in weight of the thyroid following hypophysectomy, the regressive changes of structure that have been described occur in most animals. The changes occurring in the thyroid of the guinea-pig, hedgehog and the bird are similar to those described by Smith (1927) for the rat, Houssay et al. (1931b) and Dott (1923) for the dog, and White (1933) for the rabbit. The distension of the vesicles by colloid, which is characteristic of these species, does not occur in the ferret.

The changes that occur in the ferret, especially those in the thyroid of the oestrous male, are somewhat different. In the ferret there is less distension of the vesicles by the colloid which accumulates in consequence of the atrophy of the vesicular epithelium, while there is a greater proportion of intervesicular tissue than there is in the guinea-pig, hedgehog and the fowl. In the male ferrets hypophysectomised during the breeding season the appearance of the vesicles is nearly lost. The thyroids of both animals operated at this time are similar, which fact tends to exclude the possibility of any pathological condition. The thyroids of normal male ferrets during oestrus were among the most active obtained (see Pl. I, fig. 5), especially in their great vascularity. Remains of similar intense vascularisation can be seen in those hypophysectomised, and it would appear that the degree of atrophic change is in some way dependent on the state of activity before the operation.

Dott (1923), confirmed by Houssay (19316), found that in dogs, following hypophysectomy, the initial change in the thyroid was hyperplasia, accompanied by an increase in height of the epithelium of the vesicles and intense vascularity. This, however, lasted only for a few days, before involution set in at about 7 days after the operation. From 14 to 21 days very marked atrophic changes had occurred. Although the earliest stage obtained following hypophysectomy was only 6 days (oestrous female ferret) no such hyperplasia, preceding the regressive changes, was noticed.

Quantitative observations are only available on two species, viz. the ferret and the guinea-pig. In the anoestrous ferret the atrophy of the thyroid is well advanced at 14 days following hypophysectomy, while two oestrous ferrets killed after 6 days showed changes already visible. Another one killed after 13 days showed only a slightly greater atrophy. In the oestrous male ferret the changes described above occurred after 12 days. The guinea-pig is similar to the ferret, the changes being well marked after 13 days.

The time given by other authors for different species is in close agreement. Dott found that involution of the thyroid of the dog begins at 7 days, is continued up to 14 days, and is at a maximum at about 3 weeks, while Smith found that the changes in the rat were pronounced after 15 days and were at a maximum at about 30 days. In the thyroid of the bird Mitchell noticed that the changes were in progress on the third day after the operation.

My best thanks are due to Dr R. T. Hill for the thyroids of the birds and the hedgehog, to Dr M. K. McPhail for the guinea-pig thyroids and to Dr A. S. Parkes for assistance and helpful criticism throughout the work.