Major sex differences in the development of myelination are prominent in song system nucleus HVC but not lMAN

Abstract Myelination of axonal processes is an important feature for setting up the connectivity in the neuronal network. Processes of myelination are also indicatives for changes in neuronal plasticity. The primary objective of this paper was to determine the time course of myelination—expressed in the number of myelinated profiles and the overall coverage of myelinated profiles per unit area of tissue—in two well-known song nuclei (HVC and lMAN) that have been implicated in song recognition in both male and female birds. While large sex differences have been described for many aspects of morphology in HVC, lMAN is much less dimorphic. Therefore, we wanted to know: 1) whether these differences in the extent of sexual dimorphism are also reflected in the extent of myelination; and 2) whether spatial and temporal features of myelin appearance differ in these two song nuclei in males. We analysed toluidine blue-stained Epon-embedded semithin sections with high resolution light microscopy using a computer-based morphometric system. Myelination in HVC increases substantially in males at 60 days of age, whereas, in lMAN, myelination starts early in development with an additional increase between 60 days and adulthood. In contrast to female HVC, where no changes in myelination can be observed throughout development, myelination in female lMAN increases. This increase, however, is of a different dynamic and of lesser extent in comparison to males. Henceforth, myelination in males lasts until adulthood, indicating long lasting plastic changes in both song nuclei, HVC and lMAN, whereas in females myelination is only prominent in lMAN.