Abstract
The localization of vimentin, an intermediate filament protein, in the dog thyroid cells was examined by immunohistochemistry with a monoclonal antibody against vimentin and polyclonal anti-19S-thyroglobulin and anti-calcitonin antibodies. In the dog thyroid gland, C-cells are usually distributed as large cell groups in an interfollicular location. In addition, C-cell complexes that are special C-cell clusters associated with other epithelial elements and cysts are frequently located around the parathyroid gland IV. The C-cell complexes are remnants of embryonic ultimobranchial bodies and retain fetal characteristics for long periods. Follicular cells in various stages of differentiation are observed in the C-cell complexes and large C-cell groups of postnatal dogs: (a) cell masses not yet organized into follicles, (b) primitive and small follicles storing colloid, and (c) small follicles without colloid storage. The immature follicular cells in the C-cell complexes and large C-cell groups express immunoreactivity for vimentin in addition to 19S-thyroglobulin. When the follicular cells stored considerable amounts of colloid in the follicular lumina, vimentin immunoreactivity disappeared from them. Typical thyroid follicles showed no immunoreactivity for vimentin. In dog fetuses in which follicular cells were arranged in cell clusters or were vigorously forming follicles, no immunoreactivity for vimentin was observed in the thyroid parenchyma. These results indicate that vimentin is specifically expressed in the immature follicular cells derived from the ultimobranchial anlage. The vimentin filaments may participate in increased cellular activities in the cells, i.e., thyroglobulin synthesis and folliculogenesis.