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

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Abstract
Age related changes of TSH[thyrotropin]-responsiveness and the iodinating capacity of thyroid follicles in mice 3, 10 and 19-20 mo. of age were investigated by light microscopy and EM partly using 125I-radioautography. In 3 mo.-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-mo.-old mice, TSH-unresponsive follicles occupy .apprx. 4.5% of the total number of follicles, and in 19-20 to mo.-old animals .apprx. 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 aging of the thyroid may be manifested by 2 changes: one is the appearance of cold follicles surrounded by attentuated and atrophied cells, and the other is degeneration in normal-shaped, TSH-reactive follicles.

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