Relative mRNA levels of receptors associated with two-stage differentiation. RT-PCR analysis was used to examine the expression of human transforming growth factor-β receptor 1 (HTGFBR1), human erythropoietin receptor (HEPOR), protein tyrosine phosphatase receptor-type N (PTPRN) for stage 1, human transforming growth factor beta receptor 2 (HTGFBR2), human transforming growth factor beta receptor 3 (HTGFBR3), multiple epidermal growth factor-like-domains 9 (MEGF9), prostaglandin F2 alpha receptor (PTGFR), EGF-like repeat and discoidin I-like domain-containing protein 3 (EDIL3), and prostaglandin E receptors 1, 2, and 4 (PTGER1, 2, and 4) for stage 2. GAPDH was used as the reference gene. Data from three independent experiments (n = 3) were statistically analyzed by Tukey's multiple comparisons. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.