Macrophage infiltration into thyroid follicles: an immunohistochemical study using donated elderly cadavers

To describe and discuss the morphology of the aged thyroid gland, with particular reference to the contribution of macrophages.With the aid of immunohistochemistry, we examined 1) macrophage accumulation, 2) infiltration of lymphocytes, and 3) the size and density of follicles in the unilateral lobe of the thyroid gland obtained from elderly donated cadavers (mean age, 84 years) without macroscopic malignancy. Each almost entire unilateral lobe of the thyroid showed 2554-9910 follicles per section, and each of the follicles ranged in area from 0.014-0.072 mm². We often found evidence suggesting absorption and fusion of follicles to provide a larger colloidal lumen, containing small follicles and/or epithelial fragments. In addition to dendritic perifollicular macrophages, large and round macrophages often formed clusters in the colloid. Colloidal lumina with weak macrophage immunoreactivity were intermingled with those showing strong reactivity. Notably, a greater number of macrophage foci in the colloid was usually associated with a lower density of perifollicular macrophages. Likewise, perifollicular macrophages were not always associated with lymphocyte infiltration. In the elderly, the initial appearance of colloidal macrophages does not appear to be associated with perifollicular infiltration of mononuclear cells. Macrophage invasion into a follicle might depend on the functional state of each follicle. After destruction of a follicle, a macrophage cluster appears to remain in the perifollicular tissue, and perhaps lymphocyte infiltration occurs secondarily. This course is likely to represent the process of degeneration of the thyroid gland structure with age.