Fine structural aspects of follicle-like cavity formation from dispersed porcine thyroid cells cultured in a collagen substrate.

The mechanism of follicle-like cavity formation from a monolayer of porcine thyroid cells grown in a collagen sandwich was studied. When porcine thyroid cells cultured in a monolayer on the type I collagen gel are overlaid with a second layer of collagen, changes occur in the polarity and arrangement of the cells. After 1 day of culture in the collagen sandwich, some cells in the monolayer proliferate, migrate and form two cell layers. The cell surfaces in contact with the upper or lower collagen gels always show basal characteristics. The primitive follicle lumen sealed by the zonula occludens first appears between two adjacent cells in a monolayer, or between the cells of the upper layer and those of the lower layer. Within 2 days of incubation, the cultured thyroid cells form rather flattened follicle-like cavities, even in the absence of TSH. Proliferation of the cells surrounding the lumen and luminal fusion would contribute to the development of such cavities. Bundles of numerous filaments are located along the apical and basal plasma membranes, and microtubules are markedly developed in the cytoplasm. The formation of the follicle-like cavities is inhibited by colchicine, suggesting the involvement of microtubules in the lumen-forming process. Immunohistochemistry shows that thyroid cells cultured in the collagen gel preserve the ability for thyroglobulin synthesis and basal lamina formation.