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Age related changed of TSH-responsiveness and the iodinating capacity of thyroid follicles in mice 3, 10 and 19-20 months of age were investigated by light and electron microscopy partly using 125I radioautography. In 3-month-old mice almost all the thyroid follicles respond to exogenous TSH stimulation and many reabsorbed colloid droplets appear in most follicle epithelial cells. In aged mice extremely large follicles appear in the thyroid. Though some of them react to TSH, others do not show any responsibility to repeated injections of TSH. In 10-month-old mice, TSH-unresponsive follicles occupy about 4.5% of the total number of follicles, and in 19-20-month-old animals about 20%. The follicles of this type, named "cold" follicles, are very large in size and fail to concentrate radioiodine even after repeated injections of TSH. The large lumina of these follicles are surrounded by strongly attenuated epithelial cells. The apical surfaces of these atrophied cells are almost flat. The cytoplasm is occupied by large mitochondria, large secondary lysosomes, rough endoplasmic reticulum with dilated cisternae, and small Golgi apparatus. Some cells in normal follicles are degenerated without losing their cuboidal or columnar shape, showing swelling of the cisternae of rough endoplasmic reticulum and accumulation of secondary lysosomes. The present study indicates that the aging of the thyroid may be manifested by two changes: one is the appearance of "cold" follicles surrounded by attenuated and atrophied cells, and the other is degeneration in normal-shaped, TSH-reactive follicles.

Fine structural aspects on the cold follicles in the aged mouse thyroid.