Paracrystalloids from mitochondria in thyroid follicular cells. Normal occurrence and experimental induction.

The ultrastructure of the follicular cells of the thyroid gland of bats caught in early June, when the euthyroid state is being restored following the phase of great thyroid activity that occurs at arousal, was studied. Several ultrastructural features were found which had not been observed in animals caught at other periods of the annual life cycle. The most striking ultrastructural feature was the presence in many cells of prominent, large membrane-enclosed paracrystalloid bodies. The paracrystalloids exceeded 6.0 mum in length and were approximately 1.0 mum. in width. Examination of transition forms showed that paracrystalloids were derived from mitochondria and were formed by the reorganization of the cristae into parallel closely apposed membranes. The membranes run longitudinally in loops from one end of the mitochondrion to the other. Similar mitochondrial alterations were observed in rats by duplicating the thyroid conditions of the June bat. Rats were given thiouracil for 10 weeks followed by L-triiodothyronine injection. These animals all showed similar mitochondrial-paracrystalline alterations which were not seen in hypophysectomized animals or animals given L-triiodothyronine alone. Mitochondria from thyroids of rats given thiouracil alone showed an increase in size and number and changes in shape and internal structure. The results of the bat and rat studies suggest that rapid shutdown of active follicular cells appears to promote thyroid paracrystalloid formation from mitochondria.