The Journal of Cell Biology, Vol 92, 765-776, Copyright © 1982 by The Rockefeller University Press
High voltage electron microscopy studies of axoplasmic transport in neurons: a possible regulatory role for divalent cations
ME Stearns
Light and high voltage electron microscopy (HVEM) procedures have been
employed to examine the processes regulating saltatory motion in neurons.
Light microscope studies demonstrate that organelle transport occurs by
rapid bidirectional saltations along linear pathways in cultured
neuroblastoma cells. HVEM stereo images of axons reveal that microtubules
(Mts) and organelles are suspended in a continuous latticework of fine
microtrabecular filaments and that the Mts and lattice constitute a basic
cytoskeletal structure mediating the motion of particles along axons. We
propose that particle transport depends on dynamic properties of nonstatic
microtrabecular lattice components. EXperiments were initiated to determine
the effects of changes in divalent cation concentrations (Ca2+ and Mg2+)
on: (a)the continuation of transport and (b) the corresponding structural
properties of the microtrabecular lattice. We discovered that transport
continues or is stimulated to a limited extent in cells exposed to small
amounts of exogenously supplied Ca2+ and Mg2+ ions (less than 0.1 mM).
Exposure of neurons to increased dosages of Ca2+ and Mg2+ (0.2-1.0 mM)
stimulates transport for 2-4 min at 37 degrees C, but after a 5- to 20-min
exposure the saltatory movements of organelles are observed gradually to
become shorter in duration and rate particle motion ceases to occur. HVEM
observations demonstrated that Ca2+ - and with the cessation of motion.
Ca2+-containing solutions produced contractions of the microtrabecular
filaments, whereas Mg2+-containing solutions had the opposing effect of
stimulating an elongation and assembly (expansion) of microtrabeculae. On
the basis of these observations we hypothesize that cycles of
Ca2+/Mg2+-coupled contractions and expansions of the microtrabecular
lattice probably regulate organelle motion in nerve cells.
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