Neurofilaments Pack Differently in Different Parts of the Same Axon

Quantifying cytological ultrastructure can reveal subtle but consistent differences in the organization of cytological elements, such as the cytoskeletal polymers. From these differences, it is possible to infer the dynamic mechanisms that contribute to those organizational variations which give cells their regional characteristics (Lasek, 1988). Myelinated axons manifest many regional specializations. Moreover, such axons have a repeating stereotyped pattern of interactions with the numerous Schwann cells that are distributed along their lengths. Within this pattern, certain regional specializations occur repeatedly: nodes of Ranvier, internodal regions with compact simple myelin, and Schmidt-Lanterman (S-L) clefts (i.e., internodal regions where the Schwann cell cytoplasm separates the myelin lamellae). We have quantified neurofilament number and density in these different regions of the axon in order to explore the mechanisms that underlie neuronal shape. Twenty-eight day old chickens were perfused by cardiac puncture with 3-4 1 of fixative (4% paraformaldehyde, 2% glutaraldehyde, 0.005% CaCl2, in 0.1M cacodylate buffer, pH7.4). The somatic motor branch of the oculomotor nerve was removed and fixed overnight at 4°C, followed by a buffer rinse and lh in 1% OsO4in 0.1M cacodylate at room temperature.