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 pro les and the overall coverage of myelinated pro les 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 re ected 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.

Sex differences in myelination of the zebra finch vocal control system emerge relatively late in development

“…Overall, myelin immunoreactivity distribution (MBP‐ir) was identical to the distribution of myelin described in previous studies (Champoux et al., 2021; Herrmann & Bischof, 1986; Karten et al., 2013; Von Bohlen und Halbach & Nixdorf‐Bergweiler, 2004) and was broadly distributed throughout the brain in males and females (Figures 2 and 3).…”

“…In fact, there was no significant effect of sex or sex–age interaction. This reduced sex difference in LMAN's myelination has been previously found (Von Bohlen und Halbach & Nixdorf‐Bergweiler, 2004). The volume of LMAN was discriminable using MBP‐ir in males and females by 20–25 dph, and the volume of MBP‐ir increased by 35–40 dph, then slightly decreased by 65–70 dph where it reached adult size.…”

“…This differs from the large sex differences found in the myelination of HVC and RA. Smaller sex differences in LMAN myelination compared to other areas of the vocal control system were previously observed (Von Bohlen und Halbach & Nixdorf‐Bergweiler, 2004). There was a significant age effect (regardless of sex), with the highest levels of MBP‐ir in LMAN by 35–40 dph when adult levels of myelination were reached.…”

“…Asynchrony in the myelination of different brain regions has also been found in other species, such as songbirds. For instance, in zebra finches, the myelination of visual areas happens earlier than the myelination of areas related to song production (Herrmann & Bischof, 1986), and within the vocal control system, some areas myelinate earlier than others (Champoux et al., 2021; Herrmann & Bischof, 1986; Von Bohlen und Halbach & Nixdorf‐Bergweiler, 2004). This asynchrony in myelination rate seems to have behavioral outcomes.…”

“…Although prior research has demonstrated the existence of sex differences in myelination of the songbird vocal control system, this study relied on histological staining, such as toluidine blue/methylene (Von Bohlen und Halbach & Nixdorf‐Bergweiler, 2004) and Gallyas protocol for myelin impregnation (Champoux et al., 2021; Herrmann & Bischof, 1986; Karten et al., 2013). Moreover, it remains unclear how development of myelination in this circuit corresponds to transitions through different stages of male‐specific song learning.…”