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Abstract

Here we report on ultrastructural features of brain synapses in the fly Drosophila
melanogaster and outline a perspective for the study of their functional significance.
Images taken with the aid of focused ion beam-scanning electron microscopy (EM) at
20 nm intervals across olfactory glomerulus DA2 revealed that some synaptic boutons
are penetrated by protrusions emanating from other neurons. Similar structures in
the brain of mammals are known as synaptic spinules. A survey with transmission
EM (TEM) disclosed that these structures are frequent throughout the antennal lobe.
Detailed neuronal tracings revealed that spinules are formed by all three major types
of neurons innervating glomerulus DA2 but the olfactory sensory neurons (OSNs)
receive significantly more spinules than other olfactory neurons. Double-membrane
vesicles (DMVs) that appear to represent material that has pinched-off from spinules
are also most abundant in presynaptic boutons of OSNs. Inside the host neuron, a
close association was observed between spinules, the endoplasmic reticulum (ER) and
mitochondria. We propose that by releasing material into the host neuron, through
a process triggered by synaptic activity and analogous to axonal pruning, synaptic
spinules could function as a mechanism for synapse tagging, synaptic remodeling and
neural plasticity. Future directions of experimental work to investigate this theory are proposed.