Abstract
The thyroid gland of silver eels, specimens of the Anguilla japonica before breeding migration was examined with the electron microscope. The apical surface of the follicle cell is irregular in outline and microvilli are scarce. Though the rough-surfaced endoplasmic reticulum is well developed throughout, especially at the basal part of the cytoplasm, the cisterna of this structure is not so largely vacuolated but flattend, and the cytoplasm looks relatively compact as compared with that of the higher vertebrate. There are three kinds of vesicles or granules in the apical cytoplasm of the follicle cell; (1) less-dense small vesicles, (2) large less-dense or moderately dense droplet and (3) large or small dark granules. Small less-dense vesicles and some of the dark granules are considered to be derived from the Golgi region. A central flagellum is observed in a few follicle cells. The fine structure of the pericapillary region is similar to that of the higher vertebrate, though the pericapillary space with connective tissue cells and fibrils is relatively thicker and the endothelial pores are not observed. Numerous pinocytotic vesicles are found in the endothelial cell.
