Seasonal changes in the ultrastructure of the thyrotroph in the wookchuck (Marmota monax) adenohypophysis.

Throtrophs of the woodchuck anterior pituitary are described through the four seasons of the year. Evidence of an increase in number and activity in the spring is presented. Activity of the thyrotroph is seen to continue through the summer and decline in the autumn to quiescence during the hibernatory period of winter. The significance of these findings is discussed in relation to thyoid follicular cell morphology, serum T3 and T4 levels and the physiological activities of the animal. High levels of thyroid hormones in the serum may be consistent with the appearance of great activity seen in the thyrotroph and follicular cell when taken in context with breeding season, ambient temperature and lack of food.