Fig. 8. The developmental plan of the lamprey brain. (A,B) Comparison of the developmental plans between lamprey and amniote brains. On the basis of the gene expression patterns and nerve tracts described in this study, expected segments such as P1 and P2, as well as the sulcus limitans, are drawn on the stage 26 lamprey brain as unbroken lines (A). Note that the ventral limit of Pax6 expression corresponds to the sulcus limitans both in the lamprey and the mouse (B). Broken lines in A represent the hypothetical boundaries proposed in other studies but not confirmed in this study. Homologies between the domains are based on a comparison with the model proposed by Puelles and Rubenstein (Puelles and Rubenstein, 1993; B). No data were obtained in the present study to show more subdivisions rostral to the P2/P3 boundary. Boundaries of the rhombomeres are based on the reports of Kuratani et al. (Kuratani et al., 1997) and Horigome et al. (Horigome et al., 1999). The region rostral to the sulcus intraencephalicus anterior (sa) has here been tentatively termed the ‘telencephalon’ (T). Note that three gene expression domains are detected in this telencephalon, possibly corresponding to the dorsal and lateral pallium (DP and LP), and the striatum (S). Also note that LjTTF1(Nkx2.1) expression in the lamprey is restricted to the hypothalamus, and that this gene is not expressed in any region rostral to the optic chiasm. In the amniote brain, the rostral expression domain of TTF1 corresponds to the pallidum (Pa in B) which is believed to be absent in the lamprey brain (Nieuwenhuys and Nicholson, 1998). (C) Polygonal configuration of the ammocoete brain postulated by Bergquist, with the segmental boundaries (broken red lines) and the sulcus limitans (longitudinal unbroken red line) proposed in the present study. Note that some of the boundaries were not defined in Bergquist’s model. This polygonal model is not isomorphic with the pattern shown in B. Brain regions are named according to the postulated model of the larval lamprey brain in this study, and those of Bergquist are shown in parentheses. Redrawn from Bergquist and Källén (Bergquist and Källén, 1953). Abbreviations: a.bulb, olfactory bulb; a.c.th., area caudalis thalami of Bergquist and Källén (Bergquist and Källén, 1953); DP, dorsal pallium; Dth, dorsal thalamus; EP, epiphysis; hab., habenula; hc, habenular commissure; HPT, hypothalamus; ISN, interstitial nucleus; LL, lower lip; M, midbrain; MHB, mid-hindbrain boundary; mlf, medial longitudinal fasciculus; NHP, nasohypophysial plate; nt, notochord; oc, optic chiasm; P1-P3, prosomeres; Pa, pallidum; pc, posterior commissure; pcm, prechordal mesoderm; pp1, pharyngeal pouch 1; r1-4, rhombomeres; S, striatum; sa, anterior intraencephalic sulcus; sl, sulcus limitans; soc, supraoptic commissure; tpoc, postoptic tract; UL, upper lip; VEL, velum; VP, ventral pallium.
