FIGURE 7.

Lamprey giant reticulospinal (RS) neurons have different regeneration capacities. (A) Diagram of the lamprey midbrain and hindbrain showing the 30 giant RS neurons. RS neurons are further designated as mesencephalic (M), isthmic (I), bulbar (B), and Mauthner (Mth) cells. Upon spinal transection, all the giant RS neurons are axotomized, after which a reproducible subset regenerates their axons with high probability (“good regenerators”) while the remaining undergo delayed apoptosis (“poor regenerators”). While most giant RS neurons reside in the midbrain and hindbrain, M1 and M2 are located in the caudal diencephalon (Pombal and Puelles, 1999); moreover, I3 and I4 reside outside of the isthmic region in rhombomeres 2 and 3 of the hindbrain (Murakami et al., 2004). (B) Image of a toluidine blue stained uninjured, control lamprey brain showing all 30 giant RS neurons, which are darkly Nissl stained, indicating healthy cells. (C) After 11 weeks post-transection (Trans), the “poor regenerators” (red arrows) swell and lose their Nissl staining due to injury-induced cell death, while the “good regenerators” (black arrows) remain healthy. Scale bars = 500 mm. Adapted from Fogerson et al., 2016, and used with permission by Elsevier.