More than 130 rats inclu d ing normal controls was used for studying the histology of the thyroid gland after partial thyroidectomy, with special reference to its regeneration. Partial thyroidectomy was performed by removal of one-third of the left lobe at 50 days of age (first group), and in the same way at 170 days and 300 days of age (second group) and by removal of the left lobe and isthmus at 50 days (third group). Long-term observation was tried only in the first group.
1. Partial re m oval induces regeneration of follicle cells and follicles in the remaining part. Regeneration of the follicle cells occurs by mitotic division, and not by amitotic division. The division occurs most frequently at three to 4 days after operation, ranking in frequency first in an injured area of the operated lobe, next in order in an non-injured area of the operated lobe and in the non-operated lobe. The frequency of mitotic division depends upon the age of animals, and division is more frequent in younger anima/s.
2. Regeneration of follicles occurs less frequently than might be expected, and chiefly occurs from,1. surviving cell cords and conglomerates originating from destroyed follicles in an injured area, and 2. from buds of follicles in other areas and continues to approximately 7 days after operation. No other modes of -regeneration of follicles by fusion and division of follicles, by re-arrangement of follicle cells broken up and by forming As c h of f's proliferative Regeneration of Thyroid Gland 227buds and S a n d e r so n's bolster-like swellings in the follicle walls are found.
3. The problem of regeneration of follicles from parafollicular cells remains undetermined. Cysts and their extending cell cords of ultimobranchial origin indicated no sign of production of follicles.
4. Partial removal soon induces transient increased activ i t y in the remaining part of the gland, indicated by various signs such as irregular change of follicles, swelling and height-increase of follicle cells with the formation of colloid droplets, feeble staining, dilution and reduction of colloid, and marked engorgement of capillaries.
5. This histological picture depends in intensity i n direct proportion to the extent of removal of the gland, and is more conspicuous by removal of the left lobe and isthmus than by removal of one-third of the left lobe. On the other hand, radioiodine is contained in inverse proportion to the extent of removal and the turnover and excretion of iodine may occur more rapidly by a greater extent of removal.
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