Morphological changes in two subtypes of SH-SY5Y and SK-N-SH under differentiation conditions. The panels show SH-SY5Y-E cells (A-C), SH-SY5Y-A cells (D-F) and SK-N-SH cells (G-I). The cells are nondifferentiated (A, D and G), treated for 5 days with 10 μM RA (B, E and H), and sequentially treated with BDNF in the absence of serum for 5 days in SH-SY5Y-E cells (C) or for 3 days in SH-SY5Y-A (F) and SK-N-SH (I) cells. Arrows indicate S-type cells. Phase contrast microscopy ×100. Scale bar represents 200 μm. Note that RA-treated SK-N-SH cells gradually die in the absence of serum (I).