Fig. 5. Comparative expression of regulatory genes in the lamprey brain. Whole-mount in situ hybridization of stage 24 lamprey embryos. (A) Expression of LjPax6. (B) Expression of LjDlx1/6. Transcripts are seen in the anterior forebrain and mandibular arch (MA). (C) Expression of LjTTF1. Transcripts are restricted to the hypothalamus (HPT) and the endostyle (ES). (D) Expression of LjPax2/5/8 at the mid-hindbrain boundary (MHB) and mandibular arch (MA). (E) Expression of LjOtxA is seen in the CNS rostral to the mid-hindbrain boundary (MHB) and in the olfactory placode (OP). (F) Immunohistochemical staining of nerve fibers. Several nerve tracts are observed including the habenular commissure (hc), the medial longitudinal fascicle (mlf), the posterior commissure (pc), the posterior optic tract (pot), the supraoptic tract (sot) and the ventral longitudinal fascicle (vlf). Abbreviations: EP, epiphysis; ES, endostyle; FMB, fore-midbrain boundary; hc, habenular commissure; HPT, hypothalamus; ISN, interstitial nucleus; MA, mandibular arch; MB, midbrain; mlf, medial longitudinal fascicle; MHB, mid-hindbrain boundary; NT, notochord; OE, olfactory epithelium; OP, olfactory placode; OPT, optic stalk; pc, posterior commissure; pot, posterior optic tract; PP, pharyngeal pouch; r4, rhombomere 4; sot, supraoptic tract; vlf, ventral longitudinal fascicle.