CNS Myelination: What can we learn from phylogeny?

Abstract

The morphology of embryonic and adult spiny dogfish and clearnose skate spinal cords were compared with rat. Myelination occurred in a nearly identical fashion in all three specimens. For example, oligodendrocytes appear first in the ventricle funiculus (vf), coordinate with the development of large (one micron diameter) axons. Since developing dogfish and skate neural cells express the same cell-type-specific antigens, immunofluorescence was used to study their spatial and temporal appearance. In all cases, the order of appearance was: radial glia (GFAP+), neuronal perikaiya (P (non-phosphoiylated) neurofilament), axons (P+ neurofilament) and oligodendrocytes (04+, R-mAb+, 01+, MBP+, P0/PLP+).The morphological and biochemical similarities demonstrate that many features of myelination are conserved in these divergent lineages. Thus, differences between them, which include: lack of tight junctions (chondrichthye myelin), replacement of the P0 isoforms in fish with PLP in mammals, lack of exon 2-containing MBP isoforms, myelin-associated glycoprotein (MAG), myelin oligodendrocyte protein (MOG), CNPase and MBP mRNA translocation (chondrichthye), are all derived features; differences do not alter myelin function. Preliminary evidence suggests that myelinating oligodendrocytes are related to glial cells that ensheath, but do not myelinate, large caliber axons in lamprey spinal cord. MBP-like molecules are detected in lamprey spinal cord by RT-PCR (reverse transcriptase-polymerase chain reaction). Furthermore, polyclonal antibodies to mammalian and shark MBP recognize antigens expressed by lamprey spinal cord glial cells. We conclude that the understanding of human myelination will be enriched through comparative studies of vertebrates that lack (lamprey and hagfish) and display (non-mammalian vertebrates) myelination.