Extraocular Source of Oligodendrocytes Contribute to Retinal Myelination and Optokinetic Responses in Zebrafish

There is no myelination in most mammalian retinas, and if it does happen, it is always accompanied by eye disease. Although lower vertebrates are born with myelin, the precise temporal dynamics of myelination in which oligodendrocytes (OLs) are involved, the origin of OLs, their behaviors in myelination, and their function in retinas have not yet been clearly elaborated. Therefore, we focus on these aspects to study the oligodendrocytes and myelin sheath in the zebrafish retina.

Retinal whole mount, immunohistochemistry, and optic nerve retrograde labeling were performed to monitor the myelination. Taking advantage of whole eye eversion and transplantation techniques, we studied the retinal origin of OLs. By optic nerve transplantation, we can observe single OLs in zebrafish retina. The optokinetic reflex (OKR) behavior test and the lysophosphatidylcholine (LPC)-induced retinal demyelination model were used to test the function of the myelin.

First, we demonstrated that myelination starts at 28 dpf in zebrafish retinas. Second, we directly proved that all the OLs in zebrafish retinas migrated from the optic nerve rather than from a domestic source. Third, we found that compared with adult OLs, younger OLs tend to generate longer but a fewer number of internodes. Finally, we found that the myelin in zebrafish eyes is functionally relevant to the elegant OKR.

Our data suggest that the extraocular source of OLs first appeared at 28 dpf in zebrafish retina and then gradually developed with age, which contribute to optokinetic responses.