The dorsal funiculus in cervical spinal cords of rats from 3 to 120 days postnatal was studied in order to document and quantitate d i a l 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 d i a l 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 unmyelinated 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 CL 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 C 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.Characteristic changes jn the mitotic activity of glial cells prior to, during and after the initiation of myelin production have been studied by such workers as, Allen l T h i s T Z a r c h supported by NIII grants DHEW 5 T T z "~~"~~~~~~~~~~~~0...
Mid-thoracic dorsal and ventral roots from adult rats, cats and cows were prepared for electron microscopy using standard techniques. Axon diameters were measured on photographs of known magnification. Minimal diameters of the unmyelinated fibers in the spinal cords of these animals were measured also. With the exception of a few overlapping diameters unmyelinated fibers were smaller than the smallest myelinated fibers in all the material examined. In the dorsal roots of the cow, the unmyelinated fibers, as a group, were larger than those of the rat and cat, and very few were enclosed by any one Schwann cell. The largest unmyelinated fibers were singly enclosed. In addition, the dorsal root of the cow contains many more small myelinated fibers than those of the other two species. In the ventral roots, a progressive increase in the size and relative numbers of myelinated preganglionic fibers from rat through cat to cow was noted. These facts indicated that myelination is directly dependent on fiber diameter irrespective of age, species or function although less precisely than was indicated by the earlier studies of Duncan ('34). Although the critical diameter for myelination in the peripheral nervous system is about one micron, in the central nervous system it is 0.3 P or less.
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