The Journal of Neuroscience, August 1, 2002, 22(15):6682-6695
Reorganization of Exuberant Axonal Arbors Contributes to the
Development of Laminar Specificity in Ferret Visual Cortex
Victor
Borrell and
Edward M.
Callaway
Systems Neurobiology Laboratories, The Salk Institute for
Biological Studies, 10010 North Torrey Pines Road, La Jolla, California
92037
Layer-specific cortical axons are believed to develop precisely
from the outset without making exuberant branches in incorrect cortical
layers. We tested this hypothesis by following the development of
axonal arbors of layer 2/3 pyramidal neurons in ferret visual cortex,
which in the adult have dense axonal arbors in layers 2/3 and 5 and not
in layer 4. We began our studies at earlier ages and sampled at closer
intervals than in previous studies. Our studies reveal that between
postnatal day (P) 14 and P18 the initial growth of axonal arbors lacks
specificity for layers 2/3 and 5 and involves the formation of a small
number of branches incorrectly in layer 4. After P18 the number of
incorrect axonal branches in layer 4 decreased, whereas there was
extensive axonal growth specifically in layers 2/3 and 5. To further
study the mechanisms involved in directing the layer-specific growth
and elimination of axonal branches, we studied the development of layer
2/3 neurons in slices kept in culture for 5 d. In these studies
the initial imprecise growth at P14-18 followed by precise growth
after P18 was mimicked in vitro, but the elimination of incorrect axons from layer 4 did not occur. These studies reveal that
most axonal arbors grow precisely in the correct layers, but the
initial growth involves the formation of a small number of incorrect
axonal branches that are later eliminated. Therefore the mechanisms
that regulate axonal growth are not as precise initially as later, and
mechanisms must exist to allow selective elimination of axon
collaterals from incorrect layers.
Key words:
local circuits; axonal branching; layer 2/3; pyramidal
cells; visual cortex; ferret
Copyright © 2002 Society for Neuroscience 0270-6474/02/22156682-14$05.00/0
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