Seasonal changes in morphology of the thyroid gland of a hibernator, Spermophilus richardsoni

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Abstract
Seasonal changes in morphology of the thyroid gland were studied in Richardson''s ground squirrels, S. richardsoni. Animals were sampled at 6 times during the annual cycle, and thyroid tissue was examined at the light microscope and EM levels. Synthesis and resorption of thyroglobulin were assessed on the bases of numbers of apical vesicles, colloid droplets, dense granules and dense bodies, development of the Golgi apparatus and rough endoplasmic reticulum and number and appearance of microvilli and mitochondria. Synthetic and resorptive activities of the thyroid-hormone-producing cells are high during the early prehibernation or preparative phase and decrease to moderate levels before entry into hibernation. Onset of hibernation is accompanied by further reductions in thyroid cell activities which reach minimal levels by midhibernation. Synthetic and resorptive functions increase slightly toward the end of hibernation. The reproductive phase following emergence is marked by a return to high levels of activities. Colloid storage in the follicles increases between early prehibernation and midhibernation and then decreases. Triiodo-L-thyronine (T3) and L-thyroxine (T4) production and secretion are markedly reduced in hibernating ground squirrels. Measurements of the circulating levels of the hormones showed that the titers of T3 and T4 are elevated in hibernating animals. The apparent contradiction between morphological and physiological data is discussed.