A quantitative study of morphological changes accompanying the initiation and progress of myelin production in the dorsal funiculus of the rat spinal cord

Abstract

The dorsal funiculus in cervical spinal cords of rats from 3 to 120 days postnatal was studied in order to document and quantitate glial cell development and axonal growth as related to the initiation and progress of central myelination. Within the dorsal funiculus are three major and distinct tracts, each having distinct developmental trends and adult characteristics in terms of fiber sizes and amount of myelin. These tracts are the cuneate and gracile fasciculi and the cortico‐spinal tracts. Glial cell counts and cross‐sectional surface area determinations of each tract at increasing ages show that the initial rate of glial population increase is similar. However, each tract is unique in terms of the age at which a maximum population density is reached and the rate at which the expected population dilution takes place. An electron‐microscopic examination indicates that oligodendrocytes constitute over 85% of the total glial population throughout the development period surveyed. As such, these cells are primarily responsible for the population density changes. The diameters of unmyelixgnated fibers, promyelin fibers and some myelinated fibers in these tracts were measured at 5, 10, 15, 20 and 120 days postnatal. This was done both for the purpose of relating glial population density changes with the initiation and decline of active myelination, and for determining whether or not a critical diameter for myelination exists in the CNS as was found in peripheral nerves (Matthews, '68). For each tract there is a characteristic sequence of events involving not only myelination, but also changes in diameter distribution just prior to the appearance of myelin and during the period of active myelin formation. These events coincide with the concentration and dilution of the glial population, but it is also evident that there is no critical and constant diameter in the CNS above which all axons are myelinated and below which all are unmyelinated. Myelin appears first on larger axons, but as the animal matures, it is found on progressively smaller axons until between 20 and 120 days, axons 0.2–0.4 μ in diameter acquire myelin. Thus, myelination begins with axons destined to be large and then extends down to those which enlarge very little prior to acquiring myelin and remain very small even in adult animals. Finally, from the determination, in adult rats, of the number of axons and oligodendrocytes in a defined volume of each tract and an estimation of internode length, the ratio of internodes to oligodendrocytes was calculated. The specific values obtained could vary by as much as ±50% and are only meant to serve as indicators of a trend. However, it is suggested that the number of internodes per oligodendrocyte may be inversely proportional to the length of the internode.