Volume 17, Number 22,
Issue of November 15, 1997
pp. 8792-8803
Preferential Termination of Corticorubral Axons on Spine-Like
Dendritic Protrusions in Developing Cat
Received June 16, 1997; revised Aug. 25, 1997; accepted Aug. 28, 1997.
Yasuhiko Saito,
Wen-Jie Song, and
Fujio Murakami
Department of Biophysical Engineering, Faculty of Engineering
Science, Osaka University, Toyonaka, Osaka 560, Japan
The formation of synaptic contacts is a crucial event during neural
development and is thought to be achieved by complex interactions between incoming axons and the neurons in the target. We have focused
on spine-like dendritic protrusions (SLDPs), which are transient
pleomorphic protrusive structures seen in developing brains. Although
the functional significance of SLDPs remains unknown, accumulating
in vitro evidence suggests that the SLDP plays an
important role in synaptogenetic interactions with axons. As a test of
this idea, the present study was performed to examine whether the SLDPs
are the preferential sites of synapse formation in
vivo.
The ultrastructure of biocytin-labeled corticorubral (CR) terminals was
examined in serial thin sections during the period of synaptogenesis in
newborn cats. We found that a major proportion (86%) of the CR
synapses was formed on SLDPs. The presynaptic terminals were often
invaginated by fine processes extending from the tips of SLDPs.
Synaptic structures presumably of cortical origin were also found on
SLDPs of HRP-labeled rubrospinal cells, suggesting that SLDPs
postsynaptic to labeled CR terminals originate at least in part from
rubrospinal cells. Taken together, these results indicate that SLDPs
may represent preferred sites of synapse formation and support the
notion that SLDPs play a role in synaptogenic interactions during brain
development.
Key words:
cat;
dendritic spine;
dendritic filopodia;
synaptogenesis;
corticorubral;
rubrospinal;
electron
microscopy;
biocytin