Fine structural relationships between neurites and radial glial processes in developing mouse spinal cord

Summary

Neurites have been observed in various types of fine structural associations with developing radial glial processes in the lateral marginal zones of embryonic mouse spinal cords. The least complex of the neurite-glial relationships involves a simple apposition of dendrites and dendritic growth cones with radial glial processes. We interpret this relationship to mean that dendrites, growing from the somata of the lateral motor nucleus, may preferentially move along the interface between radial glial processes and the adjacent axons of the marginal zone. A somewhat more specialized neurite-glial association is the occurrence ofpuncta adhaerentia between neurites and radial glial processes. Finally, the most elaborate morphological relationship observed is that of synapse-like contacts between axons and radial glial processes. These ‘axoglial synapses’ begin to appear at E11 and E12, are most numerous in E13 and E14 spinal cords, but they can not be found after embryonic day 15. Based on these observations, we make the following speculation: the formation of synaptic junctions occurs in a hierarchy of steps with a process of trial and error primarily involved in the initial stages of synapto-genesis, while more specific requirements are involved in the later steps of synaptic differentiation. Thus, the formation of ‘axoglial synapses’ may reflect errors in development while their disappearance may result when more specific mechanisms of synaptic differentiation become operational.