Figure 2

Expression of the retinoic acid receptor (rar) gene rar1 during development of the European river lamprey, Lampetra fluviatilis, of the sea lamprey, Petromyzon marinus, and of the Japanese lamprey, Lethenteron japonicum. The expression of L. fluviatilis rar1 was characterized by in situ hybridization in embryos at stages (st) 19 (A) and (E), 23 (B) and (F), 25 (C) and (G), as well as 26 (D) and (H). Higher magnifications of the anterior regions of embryos at stages 19, 23, 25, and 26 are shown in (E), (F), (G), and (H), respectively. Cross sections of the embryo at stage 25 are midway through the hindbrain (G’) and through the anterior spinal cord (G”). The expression of P. marinus rar1 was characterized by in situ hybridization in embryos at stages 19 (I) and (M), 23 (J) and (N), 25 (K) and (O), as well as 26 (L) and (P). Higher magnifications of the anterior regions of embryos at stages 19, 23, 25, and 26 are shown in (M), (N), (O), and (P), respectively. The cross section of the embryo at stage 23 is through the anterior spinal cord (N’) and those of the embryo at stage 25 are through the mouth region (O’), otic vesicle (O”), and posterior pharynx (O'''). The expression of L. japonicum rar1 was analyzed by in situ hybridization in embryos at stages 24 (Q) as well as 25 (R) and (S). A higher magnification of the anterior region of the embryos at stage 25 is shown in (S). Cross sections of the embryo at stage 25 are through the hindbrain (S’), mouth region (S”), and anterior spinal cord (S'''). The exact anteroposterior levels of the sections shown in (S’), (S”), and (S”’) are indicated by dotted lines in (S). (S'''') shows a magnification of the anterior spinal cord in (S”’). All embryos are oriented with anterior to the left. White arrowheads indicate hindbrain expression and black arrowheads expression in the spinal cord. Arrows in (A) and (E) as well as in (G’) and (O”’) show pharyngeal expression, while arrows in (B), (F), (J), (N), (Q), as well as in (O’) and (S”) highlight mandibular expression. Scale bars for L. fluviatilis: 100 μm for (A), (B), (C), and (D); 50 μm for (E), (F), (G), (H), (G’), and (G”). Scale bars for P. marinus: 500 μm for (I), (J), (K), and (L); 100 μm for (M), (N), (O), (P), (N’), (O’), (O”), and (O'''). Scale bars for L. japonicum: 500 μm for (Q); 1,000 μm for (R); 100 μm for (S); 50 μm for (S’), (S”), (S”’), and (S''''). Abbreviations: hb, hindbrain; llp, lower lip; n, notochord; ov, otic vesicle; ph, pharynx; sc, spinal cord; tb, tail bud; ulp, upper lip; 1 to 8, pharyngeal pouches 1 to 8.