FIGURE 1. Transwell bicameral culture chamber system. Porcine thyroid cells form continuous monolayer epithelium on microporous filter, which constitutes bottom of Transwell insert. Monolayer is continuous and thus separates chamber into lower (basal) and upper (apical) compartment. Insert that holds filter is placed at fixed distance of 1 mm from base of basal well. Filter area, 0.33 cm²; filter height, 10 µm; filter pore size, 0.4 µm.