Gliogenesis and myelination in the optic nerve of trisomy 19 mice: A quantitative electron-microscopic study

Trisomy 19 (ts19) of the mouse permits detailed studies on the influence of an extra autosome upon the postnatal development of the central nervous system. To examine gliogenesis and myelinogenesis, the optic nerves of 19 ts 19 pugs aged 1-15 days have been examined by light and electron microscopy and compared to those of litter-mate con-trols. Differentiation of astrocytes and oligodendrocytes, myelinogenesis as well as the opening of the eyes are each delayed by about 2 days. Myelin sheaths are normally structured in ts 19. There is a decrease in the percentage of myelinated fibres. The cross-sectional area of the tsl9 optic nerve is reduced. The fibre density, which decreases with age both in ts 19 and control mice, is higher in tsl9 mice. Both with ts 19 and control animals, the distribution of fibre diameters of myelinated axons overlaps with that of promyelinated and unmyelinated fibres, but myelinated axons cannot be observed belowadiameterof 0.3 µm. and unmyelinated axons are always smaller than lfim. The mean diameterof promyelinated axons is identical in ts 19 and control animals. Myelination is therefore not severely disturbed in the tsl9 optic nerve. As retinal differentiation in ts 19is delayed by 2 days as well, reports on an asynchronous development of neurons and myelin sheaths cannot be confirmed for the visual system.