Ps. Buckmaster et al., AXON ARBORS AND SYNAPTIC CONNECTIONS OF HIPPOCAMPAL MOSSY CELLS IN THE RAT IN-VIVO, Journal of comparative neurology, 366(2), 1996, pp. 270-292
The axon collateralization patterns and synaptic connections of intrac
ellularly labeled and electrophysiologically identified messy cells we
re studied in rat hippocampus. Light microscopic analysis of 11 biocyt
in-filled cells showed that messy cell axon arbors extended through an
average of 57% of the total septotemporal length of the hippocampus (
summated two-dimensional length, not adjusted for tissue shrinkage). A
xon collaterals were densest in distant lamellae rather than in lamell
ae near the soma. Most of the axon was concentrated in the inner one-t
hird of the molecular layer, with the hilus containing an average of o
nly 26% Of total axon length and the granule cell layer containing an
average of only 7%. Ultrastructural analysis was carried out on three
additional intracellularly stained messy cells, in which axon collater
als and synaptic targets were examined in serial sections of chosen ax
on segments. In the central and subgranular regions of the hilus, mess
y cell axons established a low density of synaptic contacts onto dendr
itic shafts, neuronal somata, and occasional dendritic spines. Most hi
lar synapses were made relatively close to the messy cell somata. At g
reater distances from the labeled messy cell (1-2 mm along the septote
mporal axis), the axon collaterals ramified predominantly within the i
nner molecular layer and made a high density of asymmetric synaptic co
ntacts almost exclusively onto dendritic spines. Quantitative measurem
ents indicated that more than 90% of messy cell synaptic contacts in t
he ipsilateral hippocampus are onto spines of proximal dendrites of pr
esumed granule cells. These results are consistent with a primary mess
y cell role in an excitatory associational network with granule cells
of the dentate gyrus. (C) 1996 Wiley-Liss, Inc.