ABSTRACT: Myelin is one of the most important features of the vertebrate nervous system, formed by glial cells. In the peripheral nervous system, Schwann cells produce myelin. While much is known about the molecules and processes involved in mammalian myelination, little is known about it in other vertebrate groups. In this study, we examined myelin development in the peripheral nerves of the turtle Trachemys scripta using qPCR, RNA sequencing, immunofluorescence, and TEM. Our findings indicate that in T. scripta, myelination begins during the late stages of embryonic development and continues beyond hatching. Expression profiles reveal both conservation and divergence of core myelination components between mammals and amphibians; however, direct gene-level comparisons across species require further investigation given our small sample size. The upregulation of orthologous myelin genes in the turtle PNS supports the idea that these components are conserved, although their timing, regulation, or network structure may differ across tetrapods. We based our inference of conservation on orthology and the coordinated expression of key myelin genes (e.g., ErbB2, Mag, Mpz, Mbp, Pmp22) in T. scripta. TEM analysis of turtle sciatic nerves shows myelin rings in a few axons starting at stage 21, increasing significantly by stage 24, and becoming prominent in most axons of the adult nerve. Furthermore, antibodies against MPZ, DRP2, and Kv1.1 indicate that T. scripta adult peripheral nerves at various axial levels contain myelin segments with structures consistent with appositions and Schmidt-Lanterman incisures. Overall, this is the first study of PNS myelin development in a reptile, demonstrating that myelination is a highly conserved process in vertebrates.
