In the course of an investigation into the effects of varying diets on the structure of the tissues and organs in tame rats, my attention has incidentally been directed to the varying histological appearances of the thyroid gland in wild rats. The examination of the thyroid gland in a series of forty wild rats, all apparently healthy, has shown wide variations in the histological appearances of this gland. The source from which the animals were obtained was noted in all instances, the rats being caught in such diverse places as a sewer, a hotel basement, outhouses of an asylum, railway station, a canal bank, and a bird shop. The weight of the rats ranged from 60 to 370 grammes, twenty of the number weighing 200 grammes and upwards. As it was observed that the histological appearances of the gland in quite young animals were not strikingly different from those of the fully grown subjects in the same series, it is assumed that the differences in the histological appearances to be described are not directly associated with the age of the animal, and accordingly no further differentiation will be made between the condition of the gland in young and adult subjects.

1 Ch-il mers Watson, Quarterly Journal of Experimental Physiology, vol. ii., 1909.

Thyroid Gland of Wild Rat.

Fig. 19A - Thyroid Gland of Wild Rat. (x 50).

Thyroid Gland of Wild Rat.

Fig. 20. - Thyroid Gland of Wild Rat ( x 250.) From same specimen as that shown in Fig. 19A.

[Between pages 600-601].

Thyroid Ghind of Wild Rat.

Fig. 21. - Thyroid Ghind of Wild Rat. ( x 50).

Thyroid Gland of Wild Rat.

Fig. 22. - Thyroid Gland of Wild Rat. (x 250.) From same specimen as that shown in Fig. 21.

Thyroid Gland of Wild Rut.

Fig. 23. - Thyroid Gland of Wild Rut. (x 50).

Thyroid Gland of Wild Rat.

Fig. 24. - Thyroid Gland of Wild Rat. (x 250.) From same specimen as that shown in Fig. 23.

Thyroid Gland of Wild Rat.

Fig. 25. - Thyroid Gland of Wild Rat. ( x 50).

Thyroid Gland of Wild Rat.

Fig. 26. - Thyroid Gland of Wild Rat. (x 250.) From same specimen as that shown in Fig. 25.

Histological Appearances

The microscopic appearances of the gland vary considerably. In many glands the vesicles are large (Fig. 19a) and full of colloid, which takes a very delicate pink stain with the hematoxylin and eosin stain; a few of the vesicles may be empty or may contain a clear watery secretion. The secreting cells lining the vesicles are flat in character, and consist of a darkly stained nucleus with a very small amount of protoplasm in the body of the cell, the nucleus occupying the entire breadth of the cell. There is a relatively small amount of intervesicular glandular tissue (Fig. 20). Blood-vessels are inconspicuous. Fig. 21 illustrates another type of gland which differs from the first chiefly in presenting a more cellular appearance and containing less colloid; as in the former instance, the colloid may be stained a delicate pink, or may have no affinity for the hematoxylin and eosin stain. Blood-vessels are more prominent. On high-power examination the secreting cells are of a low columnar or cubical shape, the nucleus being of moderate size and circular shape, and occupying the entire breadth of the cell (Fig. 22). There is a moderate amount of intervesicular glandular tissue, the nuclei in these cells being considerably larger than those of the cells lining the vesicles. There are also glands intermediate in structure between those represented in Figs. 19a and 20, and in Figs. 21 and 22, respectively. The appearances represented in these figures were present in thirty-one of the series of forty animals; in twenty-six out of the thirty-one the type of gland was similar to that shown in Fig. 19a.

A third type of gland is depicted in Figs. 19a and 21, and is entirely different from 23 and 24. The vesicles are small, and contain little or no secretion. Their contents take the form of a perfectly clear unstained or of a faintly stained granular material. The secreting cells are very large and prominent; the nucleus is large and situated at the basal part of the cell, there being a wide free margin of faintly granular protoplasm. In some vesicles the lumen is not sharply defined, the granular secretion in the vesicles being apparently continuous with the similarly stained protoplasm of the outer part of the secreting cell. The nuclei of the secreting cells are unequally stained, some of these presenting a pale, granular appearance. The gland appears more vascular than either of the foregoing, capillaries and large vessels being alike prominent. This type of gland was present in five of the series.

Yet another type of gland is illustrated in Figs 25 and 26. The appearance here is different from all the foregoing, and at first sight is not readily recognisable as thyroid tissue. The normal arrangement of lumen and secreting cells is disturbed. The secreting cells are swollen and distorted, and many are detached and lie free in the lumen. There is no colloid. On high-power examination it is also seen that the majority of the secreting cells are swollen, distorted, and detached; others are smaller in size and are represented by a very small deeply stained nucleus with a small amount of granular protoplasm around. There are few blood-vessels. This type of gland was present in four of the series. Those results therefore show that the minute Structure of the thyroid gland in apparently healthy wild rats differs widely in different animals. The differences must, it seems to me, be explained in one or other of the following ways. Either the appearances represent different stages of activity of the gland comparable to those observed in other secreting glands, these variations being dependent on the stage of secretion; or they represent modifications in structure and function of the gland which have been induced by dietetic or other factors in the animal's environment. The former explanation seems not to hold good, since a careful investigation into the nature of the stomach contents in the different animals, along with a consideration of the microscopic appearances of the thyroid in each case, has yielded evidence against it. For instance, the appearances of the gland shown in Fig. 19a were observed both in animals in which the stomach was full and also when it was empty. And again, the different appearances represented in Figs. 23 and 25 were alike observed in animals in which the stomach was about one-third full, while in other animals with an equal amount of stomach contents the condition of the thyroid gland was entirely different from both. The second explanation seems, therefore, to be the more probable, the facts observed indicating that modifications in structure and function of the thyroid gland occur in association with different environmental conditions. In view of the fact recently demonstrated by the writer, that certain diets modify the structural appearances of the thyroid gland in tame rats, it is reasonable to conclude that diet has been the chief factor in inducing the modifications of structure described in the present paper. In this connection it is of interest to note that it was observed that the thyroid gland of all the animals from any one source invariably presented the same general characters.