The effects of intrauterine growth retardation on the structural development of cranial nerves (optic, trochlear) in fetal sheep.

A quantitative morphometric study of the development of myelinated fibres in the optic and trochlear nerves has been made in growth-retarded fetal sheep at 140 days gestation (term = 146 days). Intrauterine growth retardation was induced as a result of the reduction of placental mass, by prior removal of placentation sites in six ewes. In the optic nerve (central nervous system) the mean diameter of myelinated fibres was not significantly reduced but the thickness of the myelin sheath relative to axon diameter was disproportionately reduced. In the trochlear nerve (peripheral nervous system) there was a significant reduction of 23% (P less than 0.01) in the mean diameter of myelinated fibres; however the normal axon:myelin ratio was maintained. The total number of myelinated fibres in the trochlear nerve did not differ between the normal and growth-retarded group, indicating that there was not a greater than normal incidence of cell death during intrauterine growth retardation in the nucleus of the trochlear nerve. The differential effect of intrauterine growth retardation on myelination in the central and peripheral nervous systems suggests that chronic intrauterine deprivation affects oligodendrocyte activity but does not markedly affect the capacity of Schwann cells to produce myelin.