FIGURE 3.

Hypoxia delayed the onset of myelination and led to thinner myelin sheaths. (A) Myelination of the Mauthner axon, the most posterior myelin sheath in the spinal cord, at 80 hpf. Myelination can be seen between somites 18 and 20 in normoxic animals but between somites 11 and 13 in hypoxic animals. Scale bars: 10 μm. (B) The most posterior myelin sheath at 70 hpf is somite 2.875 ± 0.3981 in normoxic animals (n = 8) and somite 0.0 ± 0.0 in hypoxic animals (n = 8); at 75 hpf is somite 12.00 ± 0.6814 in normoxic animals (n = 8) and somite 4.857 ± 0.5084 in hypoxic animals (n = 7); and at 80 hpf is somite 19.27 ± 0.7519 in normoxic animals (n = 8) and somite 12.13 ± 0.7892 in hypoxic animals (n = 7). Two-way ANOVA, P < 0.0001. (C–E) A large number of unmyelinated axons (shaded in blue) were observed in the spinal cord axons of the hypoxic and CoCl₂ groups as compared to the myelinated axons (shaded in purple) observed in the normoxic group. Scale bars: 200 nm. (F) The hypoxic group had a higher G-ratio in the spinal cord axon than did the normoxic group, indicating delayed-onset myelination under hypoxia. One-way ANOVA, P < 0.05. (G) qRT-PCR analysis indicated that myelin basic protein (mbp) mRNA transcription was significantly decreased under hypoxia. (H) The expression of the Mbp protein was analyzed with western blots. The results showed that hypoxia blocked Mbp protein translation. Larvae at 3 and 4 dpf (30 per day) were pooled for western blot experiments. ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001; ^∗∗∗∗P < 0.0001.