The Journal of Neuroscience, August 1, 1998, 18(15):5652-5662
Neurite Growth Patterns Leading to Functional Synapses in an
Identified Embryonic Neuron
David
Reese and
Pierre
Drapeau
Departments of Biology and of Neurology and Neurosurgery, McGill
University and Center for Research in Neuroscience, Montreal General
Hospital Research Institute, Montreal, Quebec, Canada, H3G 1A4
We explored the relationship between neurite outgrowth and the
onset of synaptic activity in the central neuropil of the leech embryo
in vivo. To follow changes in early morphology and the onset of synaptic activity in the same identified neuron, we obtained whole-cell patch-clamp recordings and fluorescent dye fills from dorsal
pressure-sensitive (P) cells, the first neurons that could be reliably
identified in the early embryo. We followed the development of the P
cell from the first extension of neurites to the elaboration of an
adult-like arbor. After the growth of primary neurites, we observed a
profuse outgrowth of transient neurites within the neuropil. Retraction
of the transient neurites left the primary branches studded with spurs.
After a dormant period, stable secondary branches grew apparently from
the spurs and became tipped with terminals. At this time, neurites of
the Retzius (R) cell, a known presynaptic partner in the adult, were
observed to apparently contact the terminals. Although
voltage-dependent currents were seen in the P cell at the earliest
stage, spontaneous synaptic activity was only observed when terminals
had formed. Spontaneous release was observed before evoked release
could be detected from the R cell. Our results suggest that transient
neurites are formed during an exploratory phase of development, whereas
the more precisely timed outgrowth of stable neurites from the spurs
signals functional differentiation during synaptogenesis. Because spurs
have also been observed in neurons of the mammalian brain, they may
constitute a primordial synaptic organizer.
Key words:
synaptogenesis; embryo; development; synaptic current; neurites; confocal microscopy; patch clamp
Copyright © 1998 Society for Neuroscience 0270-6474/98/18155652-11$05.00/0