Development of the Early Axon Scaffold in the Rostral Brain of the Small Spotted Cat Shark (Scyliorhinus canicula) Embryo

Figure 1

Detailed formation of the MLF in the embryonic S. canicula brain. Lateral view of whole mount embryonic brain. Scale bars, 100 μm. (a)–(c) Overview of embryos. (a) Stage 18. The first neurones appeared at the ventral DMB (arrow). These neurones formed the nMLF. Box indicates high magnification in D. (b) Stage 19. The number of MLF neurones has increased. Box indicates high magnification in E. (c) Stage 22. The MLF was still the prominent tract, with scattered neurones located rostrally projecting axons into the MLF (unfilled arrow). Orange spot indicated where DiO was injected. (d) Stage 18. The first MLF neurones appeared (arrow). (e) Stage 19. The MLF neurones were projecting their first axons caudally. (f) Stage 20. The MLF axons have projected further caudally and the first rostrally located; scattered MLF neurones appeared (unfilled arrow). (g) Stage 21. The MLF axons have formed a tight bundle. The scattered MLF neurones were projecting into the MLF tract (unfilled arrow). (h) Stage 23. The MLF has increased in number of axons and neurones. Scattered MLF neurones projected into the MLF axon tract (unfilled arrow). A tight population of MLF neurones was present ventrally (arrow). (i) Stage 25. The MLF was well established and the VC was also present. (j) Stage 23, double labelling with Tuj1 and HuC/D. Caudal to the scattered MLF neurones, the MLF nucleus appeared to be arranged into a further two populations of MLF neurones: one located dorsally (unfilled arrowhead) and the other as a tight cluster of centrally located neurones along the ventral midline (arrow). There were also axons projecting ventrally to the MLF (arrowhead) that appeared to be a separate axon tract. (k) HuC/D labelling only of (j). (l) Stage 22. The MLF labelled with DiO. DiO was injected into the rhombencephalon close to the ventral midline to label MLF axons. Mes: mesencephalon; MLF: medial longitudinal fascicle; nMLF: nucleus of the medial longitudinal fascicle; op: olfactory pit; os: optic stalk; pros: prosencephalon; rh: rhombencephalon; tel: telencephalon; VC: ventral commissure.