High levels of serum unconjugated bilirubin (UCB) in newborns are associated 
with axonal damage and glial reactivity that may contribute to subsequent 
neurologic injury and encephalopathy (kernicterus). Impairments in myelination 
and white matter damage were observed at autopsy in kernicteric infants. We have 
recently reported that UCB reduces oligodendrocyte progenitor cell (OPC) 
survival in a pure OPC in vitro proliferative culture. Here, we hypothesized 
that neonatal hyperbilirubinemia may also impair oligodendrocyte (OL) maturation 
and myelination. We used an experimental model of hyperbilirubinemia that has 
been shown to mimic the pathophysiological conditions leading to brain 
dysfunction by unbound (free) UCB. Using primary cultures of OL, we demonstrated 
that UCB delays cell differentiation by increasing the OPC number and reducing 
the number of mature OL. This finding was combined with a downregulation of 
Olig1 mRNA levels and upregulation of Olig2 mRNA levels. Addition of UCB, prior 
to or during differentiation, impaired OL morphological maturation, extension of 
processes and cell diameter. Both conditions reduced active guanosine 
triphosphate (GTP)-bound Rac1 fraction. In myelinating co-cultures of dorsal 
root ganglia neurons and OL, UCB treatment prior to the onset of myelination 
decreased oligodendroglial differentiation and the number of myelinating OL, 
also observed when UCB was added after the onset of myelination. In both 
circumstances, UCB decreased the number of myelin internodes per OL, as well as 
the myelin internode length. Our studies demonstrate that increased 
concentrations of UCB compromise myelinogenesis, thereby elucidating a potential 
deleterious consequence of elevated UCB.