Ultrastructural pathology of the thyroid gland

Abstract

Morbid changes which occur in the human thyroid gland can be understood only from the standpoint of normal histology and physiology of the thyroid. Conventional transmission electron microscopy (TEM), freeze-etching and scanning electron microscopy (SEM) have revealed that the ultrastructural pattern of the normal human thyroid gland is quite similar to normal patterns already described in other mammals (mouse, rat, rabbit, guinea pigs, dogs), resembling especially the thyroid glands of guinea pigs and dogs (1–4). The functional unit is the follicle (Fig. 1). The human thyroid gland is composed of follicles separated by capillaries, lymph vessels, large areas of connective tissue and interfollicular cells (mast cells, fibroblast). Thyroid follicles are heterogenous; some are large, composed of flat cuboidal cells and filled with a homogenous dense colloid; others are small and outlined by columnar cells. No vacuoles can be seen at the periphery of the colloid (Fig. 2). Light microscopic autoradiography further delineates the heterogeneity of the thyroid gland. Radioiodine (131I or 125I) is incorporated differently; fewer grains are seen in the larger follicles compared with smaller, in which a more isotopic incorporation can be seen as mottled grains within the follicular colloid.