Selective elimination of axons extended by developing cortical neurons is dependent on regional locale: experiments utilizing fetal cortical transplants.

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)