Abstract
Thyroid hormone (3,5,3'-triiodo-l-thyronine or T3) exerts a pleiotropic activity during central nervous system development. Hypothyroidism during the fetal and postnatal life results in an irreversible mental retardation syndrome. At the cellular level, T3 is known to act on neuronal and glial lineages and to control cell proliferation, apoptosis, migration, and differentiation. Oligodendrocyte precursor cells (OPC) found at birth in the optic nerves are self-renewing cells that normally differentiate during the first 3 weeks of rodent postnatal life into postmitotic myelinating oligodendrocytes. In vitro, the addition of T3 to OPC is sufficient to trigger their terminal differentiation. The present analysis of T3 receptor knockout mice reveals that the absence of all T3 receptor results in the persistence of OPC proliferation in adult optic nerves, in a default in myelination, and sometimes in the degeneration of the retinal ganglion neurons. Thus, T3 signaling is necessary in vivo to promote the complete differentiation of OPC.