Requirements of follicle structure for thyroid hormone synthesis; cytoskeletons and iodine metabolism in polarized monolayer cells on collagen gel and in double layered, follicle-forming cells.
Thyroid cells take up iodine and synthesize thyroid hormones. Thyroid cell polarity plays an important role in the uptake of iodine. However, we do not know whether polarity itself is enough for thyroid hormone synthesis or whether follicle structure is required for it. Using polarized monolayer porcine thyroid cells, cultured on collagen-coated filters, and double layered, follicle-forming cells, we analyzed the relationships of iodine metabolism and cell polarity (and follicle formation). We demonstrated that follicle structure was required for thyroid hormone synthesis. A quick-freezing and deep-etching method revealed the three-dimensional ultrastructures of cytoskeletons in the thyroid cells. On the collagen gel, the thyroid cells are reorganized into polarized monolayer cells; the basal cell membranes are in contact with the collagen gel and the apical ones face the culture medium. Actin microfilaments predominate under the apical cell membranes and intermediate filaments in the basal cytoplasm. The arrangement of these cytoskeletons determines the polarity of the cells. When the cells are cultured as double layers, follicle structures are reconstructed between the two monolayers. When apical cell membranes are in contact with other apical ones or when the cells are cultured as double layers, the cells are reorganized into follicles; the basal cell membranes are in contact with the collagen gel, and the apical ones face the follicle cavities. Actin microfilaments predominate at the apical cell membranes and intermediate filaments in the basal cytoplasm. Polarized thyroid cells transport iodine from the basal compartments to the apical ones, but cannot organify iodine into thyroid hormones. However, follicle-forming cells, cultured as double layers, take up iodine and organify it into thyroid hormones. Polarity is important for iodine uptake, and follicle structure is required for thyroid hormone synthesis.