Expression of rtn4a and rtn4b genes during zebrafish embryogenesis. The developmental expression pattern of rtn4a and rtn4b was examined by whole-mount in situ hybridization using gene-specific probes. (A) At 18 hours postfertilization (hpf), rtn4a is expressed in the anterior part of the embryo. Between 1 day postfertilization (dpf) (B) and (C) and 2 dpf (G) and (H), we detected increased transcription of rtn4a in the somites (s) and the eye anlage (e), as well as in the presumptive optic tectum (t). (D) At 18 hpf, rtn4b transcripts appeared along the trunk of the embryo. (E) and (F) At 1 dpf, rtn4b expression is evident in the posterior spinal cord (sc), the developing forebrain (fb), eye anlagen (e) and midbrain and hindbrain (hb), including the otic vesicle (ov). (G) and (H) At 2 dpf, rtn4a mRNAs are produced in retinal ganglion cells (RGCs) (arrow), the olfactory organ (o) and forebrain (fb), as well as in somites (s). (I) and (J) At 2 dpf, rtn4b is transcribed in the forebrain (fb), including the olfactory organ (o) and RGCs (arrow), and in the midbrain and hindbrain. At this stage, the spinal cord was no longer labeled, but the notochord (n) began to express rtn4b. Lateral views are shown, except in (C) and (F) (dorsal views of 1-dpf embryos) and (H) and (J) (ventral views of 2-dpf embryos).