Induction of thyroid differentiation markers in monolayer culture of SPTL cells. (A) SPTL cells in monolayer culture were collected at days 0, 1, 3, and 6 after serum concentration was changed to 2%. Day 0 is equivalent to 10% culture condition. The expression levels of thyroid primary culture cells (Thy) are shown as comparison. Relative expression is based on the expression level of SPTL day 0 cultured cells as 1. Mean ± SD from three experiments, determined in triplicate are shown. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; NS, not significant for day 0 vs. days 1, 3, and 6 of SPTL cultures by one-way analysis of variance. ****p < 0.0001 for day 0 SPTL cells versus Thy (thyroid primary culture cells) by unpaired t-test. (B) Immunocytochemistry for NKX2-1 using SPTL cells collected at days 0 and 6 under 2% serum conditions. Experiments were carried out more than three times, and each time, similar results were obtained. Representative results are shown.