When cultured in collagen gel-coated dishes, thyroid cells organized into polarized monolayers. The basal poles of the cells were in contact with the collagen gel, whereas the apical surfaces were facing the culture medium. Under these culture conditions, thyroid cells do not concentrate iodide nor respond to acute stimulation by thyroid-stimulating hormone (TSH). To allow the free access of medium components to the basal poles, the gel was detached from the plastic dish and allowed to float in the culture medium. After release of the gel, the iodide concentration and acute response to TSH stimulation were restored. Increased cAMP levels, iodide efflux, and formation of apical pseudopods were observed.When the thyroid cells are cultured on collagen-coated Millipore filters glued to glass rings, the cell layer separates the medium in contact with the apical domain of the plasma membrane (inside the ring) from that bathing the basolateral domain (outside the ring). Iodide present in the basal medium was concentrated in the cells, whereas no transport was observed when iodide was added to the luminal side. Similarly, an acute effect of TSH was observed only when the hormone was added to the basal medium. These results show that the iodide concentration mechanism and the TSH receptoradenylate cyclase complex are present only on the basolateral domain of thyroid cell plasma membranes.In vivo, thyroid epithelial cells are organized into follicles. They concentrate and organify circulating iodide and respond within minutes to acute thyrotropin stimulation by increased cAMP synthesis, iodide effiux, and formation of apical pseudopods (10). As both iodide and thyroid-stimulating hormone (TSH) present in the blood have access to the basal surface of follicular cells, it is likely that the iodide concentration mechanism and the TSH receptor-adenyl cyclase complex are present on the basolateral domain of the plasma membrane of the thyroid epithelial cell. No direct evidence of an asymmetrical distribution of these components between the apical and basolateral domain of the plasma membrane has yet been reported.In vitro, when cultured at high cell density on glass or polystyrene substrates, isolated porcine thyroid cells form polarized monolayers 04,20,26). As in other epithelial systems, the basal surface of the cell layer is in contact with the substrate, 1172 whereas the apical pole of the cells is oriented towards the culture medium. Under these culture conditions, ceils are unable to concentrate iodide and the intracellular cAMP level is not changed upon addition of TSH to the culture medium. This loss of iodide concentration activity and of responsiveness to acute thyrotropin stimulation might be due, at least partly, to the inaccessibility of the basolateral domain of the plasma membrane to molecules present in the culture medium.We used two different experimental procedures to overcome the inaccessibility of the basal surface of the cell layer. (a) Thyroid cells were cultured in petri dishes coated with a th...
