DEVELOPMENT OF NODES OF RANVIER IN FELINE NERVES - AN ULTRASTRUCTURALPRESENTATION

The ultrastructure of developing nodes of Ranvier and adjacent paranod es of future large myelinated fibers in feline lumbar spinal roots is described. The development starts before birth concurrent with myelina tion and is finished at the end of the first postnatal month when the nodal regions of future large fibers, now 4-5 mu m of diameter, for th e first time appear like miniatures of those of their 4 times thicker and fully mature counterparts. At this stage the fibers also begin to show mature functional properties. The latent maturation process is de noted ''nodalization'' and includes two major events: (1) the formatio n of a narrow node gap bordered by compact myelin segments and filled with Schwann cell microvilli that interconnect an undercoated nodal ax olemma with rapidly increasing accumulations of mitochondria lodging i n the longitudinal cords of Schwann cell cytoplasm that is distributed outside a more and more crenated paranodal myelin sheath; (2) the set ting of a fixed number of nodes along the axons; an event that include s segmental axonal and myelin sheath degeneration and is concluded by the elimination of supernumerary Schwann cells. (C) 1996 Wiley-Liss, I nc.