Fig. (4) PNCA assay of embryos treated with DMSO, 13-cis-RA (RA) and a combined folic acid and 13-cis-RA (FA-RA). In DMSO-treated embryos, numerous PCNA positive cells (brown staining) were found in epithelium and mesenchyme of midfacial processes (A, D, G, J, M). The 13-cis-RA-treated embryos showed lower numbers of PCNA positive cells found in epithelium and mesenchyme of LNP, MNP and MxP (B, E, H, K, N), compared to those of the DMSO-treated embryos. The FA-RA-treated embryos showed an increased numbers of PCNA positive cells in epithelium and mesenchyme of LNP, MNP and MxP (C, F, I, L, O), compared to those of the RA-treated embryos. G-O showed a higher magnification of square area in A-F. Scale bar= 100 µm for A-F and 20 µm for G-O.