Light and electron microscopy observations in the thyroid gland of the pheasant (Phasianus colchicus L.).

In the summer, the pheasant thyroid presents as a relatively quiescent organ with reduced secretory activity; the follicular epithelium is very low and mostly of the pavement type, while the follicles are unwontedly large and contain an increased amount of colloid. The follicular cell cytoplasm contains numerous characteristic spherical vacuolar structures. In the winter, however, the thyroid displays remarkable reconstruction and raised activity of the follicular cells, which are much higher and columnar; the small follicles contain less colloid. Peculiar rod-like or fusiform structures, some measuring up to 6,5 microgram, were observed in the cytoplasm of the follicular cells of both hen and cock pheasants in November and March. At the beginning of the winter (November), specific dark cells with pycnotic nuclei, evidently in process of degeneration, appeared in the follicular epithelium. In March, specific granulated cells (probably parafollicular cells) were found sporadically in the immediate vicinity of the wall of subcapsular follicles in cock pheasant.