Sprouting and nerve retraction in frog neuromuscular junction during ontogenesis and environmental changes

Based on recent evidence for a physiological remodeling of neuromuscular contacts (Wernig et al.), a morphometric study was performed on axon- and cholinesterase-stained cutaneous pectoris muscle of frog. The aim of this investigation was to separate changes due to aging, growth, and environmental conditions. Within a single muscle, fiber diameters, synaptic lengths, number of intraterminal branches, and lengths of abandoned gutters differ considerably (with coefficients of variation from 40 to 56%). On the other hand, these parameters are correlated and correlations hold when muscle fibers grow during ontogenesis: large muscle fibers bear larger and more complex junctions than small fibers. Obviously there exist growth regulating interactions between muscle fiber and the presynaptic nerve. To dissociate between age- and growth-related changes muscle fibers of equal diameters in frogs of different age are compared. With increase in age there is an additional increase in abandoned gutters, synaptic length, and complexity independent of muscle fiber growth. Possibly, abandoned gutters accumulate with time and synaptic length increases with age as the net outcome of continual synapse remodeling. When freshly caught frogs (October) were compared with frogs kept under laboratory conditions for a period of 16 weeks (which in addition included a change in season) the number of sprouts in a junction increased by about 2, the average length of presynaptic nerve terminals with small circumscribed contacts increased by 30-150 microns, and abandoned gutters tended to be shorter on fibers with large junctions. The hypothesis is discussed that remodeling is "inherent" to nerve terminals whereby sprouting is counterbalanced and reversed by nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)