Journal of Neuroscience, Vol 10, 975-984, Copyright © 1990 by Society for Neuroscience
Adaptations of synaptic form in an aberrant projection to the avian cochlear nucleus
TN Parks, DA Taylor and H Jackson
Department of Anatomy, University of Utah School of Medicine, Salt Lake City 84132.
Surgical removal of the otocyst in chick embryos induces axons from the
contralateral cochlear nucleus (nucleus magnocellularis, NM) to form, in
addition to their normal endings in nucleus laminaris (NL), anomalous and
persistent functional contacts in the ipsilateral NM (Jackson and Parks,
1988). We have examined how interaction between the abnormal synaptic
partners during development influences the form of the axon terminal and
its relation to the target neuron. In the light microscope, aberrant axon
terminals labeled in vitro with HRP appear to form boutons quite unlike the
large calycine endbulbs made by the normal cochlear nerve (CN) endings in
NM. In the electron microscope, however, the anomalous endings appear
embedded in the NM cells, something never seen normally in NM or NL.
Morphometric analyses were performed on electron micrographs from NM and NL
in animals aged embryonic day (E) 19 to posthatching day (P) 2 from which
the right otocyst had been removed on E3 and in normal control animals.
Aberrant endings appose 18% of the circumference of operated NM cells,
versus 45% for CN axons in the normal NM at this age. The mean length of
membrane apposition for the anomalous NM-to-NM endings was 215% greater
than for normal NM-to-NL endings but 54% smaller than that in normal CN
endings. These results support the idea that developmental interactions
between synaptic partners can influence the form of the contact between the
2 neurons. The results also demonstrate, however, that formation of
persistent and functional synapses with NM neurons throughout development
is not sufficient to induce any axon to assume the calycine form of a
cochlear nerve endbulb.(ABSTRACT TRUNCATED AT 250 WORDS)