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Journal of Neuroscience, Vol 9, 2230-2246, Copyright © 1989 by Society for Neuroscience
Selective elimination of axons extended by developing cortical neurons is dependent on regional locale: experiments utilizing fetal cortical transplants
DD O'Leary and BB Stanfield
Department of Neurology, Washington University School of Medicine, St. Louis, Missouri 63110.
In adult rats, cortical neurons that extend an exon through the pyramidal
tract (a major subcortical efferent projection of the neocortex) are
limited to layer V of about the rostral two-thirds of the neocortex. In
neonates, however, pyramidal tract neurons are distributed throughout the
neocortex, but all of those found in certain areas, such as the posterior
occipital region (including primary visual cortex) selectively lose their
pyramidal tract axon (Stanfield et al., 1982) yet maintain axon collaterals
to other subcortical targets (O'Leary and Stanfield, 1985). To determine if
the regional location of a developing pyramidal tract neuron critically
influences the maintenance or elimination of the axon collaterals it
initially extends, pieces of cortex from embryonic day 17 (E17) rat fetuses
(exposed to 3H-thymidine on E15) were transplanted heterotopically into the
cortex of newborn (PO) rats; rostral cortex was placed into the posterior
occipital region (R----O), or posterior occipital cortex into a rostral
cortical locale (O----R). The retrograde tracers Fast blue (FB) and
Diamidino yellow (DY) were used to assay for the presence of specific
populations of cortical projection neurons within the autoradiographically
identified transplants. In terms of the extension and maintenance of
pyramidal tract axons, the transplanted neurons behave like the host
neurons of the recipient cortical region rather than like those of their
site of origin. At P40, following FB injections into the pyramidal
decussation on P34, pyramidal tract neurons are labeled within the O----R
transplants, but none can be labeled within R----O transplants, although in
the same R----O cases transplanted neurons are labeled by an injection of
DY in the superior colliculus. However, at P13 pyramidal tract neurons can
be identified within the R----O transplants, as well as in the host
occipital cortex, following injections made on P9, a period when the
distribution of pyramidal tract neurons in normal rats is widespread
(Stanfield and O'Leary, 1985b). In a second series of host rats, on P34 FB
was injected in the pyramidal decussation of the O----R cases, or in the
superior colliculus of the R----O cases, and in both groups DY was injected
into the region of contralateral cortex homotopic for the new location of
the transplant. On P40, in both the O----R and R----O transplants, many
neurons singly labeled with FB or DY are found, but no double dye-labeled
cells are seen.(ABSTRACT TRUNCATED AT 400 WORDS)
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