The development of myelin in the spinal cord of the North American opossum and its possible role in loss of rubrospinal plasticity. A study using myelin basic protein and galactocerebroside immuno-histochemistry.

The aims of this study were to observe the timing and sequence of myelin formation in the opossum's spinal cord by using myelin basic protein (MBP) immuno-histochemistry and to determine whether the onset of myelination, as demonstrated by the presence of MBP or galactocerebroside (GalC)-like immuno-reactivity (LI), correlates temporally with the end of the critical period for rubrospinal plasticity. Rubral axons grow around a lesion of their pathway during early development but they do not do so at later stages of development or in the adult animal. MBP-LI was first observed in the opossum's spinal cord at postnatal day 15 and its development in most tracts followed rostral to caudal gradients. MBP-LI occurred in some tracts before others, however, regardless of level. MBP- and GalC-LI first appeared in the lateral funiculus, the location of rubrospinal axons, around the end of the critical period for rubrospinal plasticity and it was found in the dorsal horn, an area traversed by rerouted axons in the plasticity experiments, shortly thereafter. Since there is a rough temporal correlation between the development of myelin, as demonstrated by the presence of MBP and GalC immuno-reactivity, and the end of the critical period for rubrospinal plasticity, it is possible that myelin proteins which inhibit axonal elongation contribute to loss of that plasticity.