Structural and functional polarity of inside-out follicles prepared from pig thyroid gland

Within 24 h of suspension culture, follicle segments isolated from pig thyroid gland form closed follicular spheres. Their wall consists of a single layer of epithelial cells and is impermeable to various tracers. The structural polarity of all follicle cells is reversed: The apical plasma membrane faces the culture medium and the basal plasmalemma is directed towards the central cavity. The reversed polarity of follicles in suspension culture is retained for several days. Newly synthetized thyroglobulin does not accumulate in the central cavity but is secreted into the culture medium. After stimulation with thyrotropin thyroglobulin is internalized and transferred to lysosomes. The findings show that the reversed bidirectional transport of thyroglobulin coincides with the structural inside-out polarity of reconstituted follicles. After uptake of thyroglobulin, thyroxine and triiodothyronine are released by follicle cells and accumulate in the culture medium. The amount of thyroid hormones released depends on the concentration of thyroglobulin available for endocytosis and on the stimulation of inside-out follicles by thyrotropin.