Fig. 4

Morphology of GnRH-expressing trigeminal nerve neurons (a) gnrh3:egfp neuron in medaka at 2.5 dpf. EGFP-expressing trigeminal nerves have both rostral and caudal projections (arrowhead). OP: olfactory placode, FB: forebrain, MB: midbrain, TG: trigeminal ganglion. (b) A schematic illustration of the projections of gnrh3:egfp neurons at different developmental stages. Red, GnRH3 neurons in the terminal nerve and the thalamus originating from the olfactory placode, migrating to the brain, and projecting broadly in the brain. Blue, GnRH3 neurons in the thalamus originating from the brain. Because GnRH neurons in the thalamus contain only a few neurons, we did not discuss them in this review (see Takahashi et al., 2015 [9]). Green, GnRH3 neurons in the trigeminal ganglion, Gray, retinal neurons expressing EGFP ectopically. Because these ectopic retinal fiber projections and trigeminal ganglion fibers interfere with the illustration of GnRH3 neuron projections (red), we only indicated these fibers (gray and green) in the right half of the brain. During the late stages of development, red and blue neurons appeared to be intermingled. Both figures were modified from Takahashi et al., 2015 [9]