Fates of visual cortical neurons in the ferret after isochronic and heterochronic transplantation

Abstract

In the mammalian cerebral cortex, neurons in a given layer are generated at about the same time in development. These cells also tend to share similar sets of morphological and physiological properties and have projection patterns characteristic of that layer. This correspondence between the birthday and eventual fate of a cortical neuron suggests the possibility that the commitment of a cell to a particular laminar position and set of connections may occur very early on in cortical development. The experiments described here constitute an attempt to manipulate the fates of newly generated cortical neurons upon transplantation. The first set of experiments addressed the normal development of neurons in the primary visual cortex (area 17) of the ferret. Injections of 3H-thymidine into newborn ferrets showed that neurons generated after birth are destined to sit in layer 2/3 of the cortex, whereas neurons born on embryonic day (E) 32 populate primarily layers 5 and 6. Many layer 2/3 neurons in adult ferrets could be retrogradely labeled with HRP from visual cortical areas 18 and 19, while about half of the neurons in layer 6 were found to project to the lateral geniculate nucleus (LGN). In the second set of experiments, presumptive layer 2/3 cells were labeled in vivo by injecting ferrets with 3H-thymidine on P1 and P2. Before the cells had a change to migrate, they were removed from the donor brain, incubated in a fluorescent dye (DAPI or fast blue), and dissociated into a single-cell suspension. The labeled cells were then transplanted into the proliferative zone of a littermate host ferret ('isochronic' transplants). Over the next few weeks, many of these dye-labeled cells underwent changes in their position and morphology that were consistent witha radially directed migration and subsequent differentiation into cortical neurons. The final positions of isochronically transplanted neurons in the host brain were mapped out by using the 3H-thymidine marker after long survival periods. About 97% of radioactively labeled cells had migrated out into the visual cortex, where they attained a compact laminar distribution: 99% were found in layer 2/3, their normal destination. The labeled cells had normal mostly pyramidal neuronal morphologies and appeared to be well integrated with host neurons when viewed in Nissi-stained sections. Ten isochronically transplanted neurons were successfully labeled after HRP injection into 2 normal target regions, areas 18 and 19. Thus, newly generated cortical neurons are capable of survival, migration, and differentiation in a host brain, and transplantation per se does not alter their fate. In a final set of experiments, the commitment of presumptive deep-layer neurons to their normal fate was tested by challenging them to alter that fate. In these 'heterochronic' transplants, cells from the occipital proliferative zone of E31 or 32 donor ferrets were labeled with 3H-thymidine and then transplanted into the proliferative zone of a newborn host ferret. In contrast to the isochronic transplants, about 80% of 3H-thymidine-labeled cells in heterochronic transplants failed to migrate and were instead found at the site where they were injected. The remainder migrated out to the visual cortex and developed neuronal morphologies. Of the labeled cells that reached the cortex, 43% were found in layer 2/3, and the remaining 57% occupied the deeper cortical layers, primarily layers 5 and 6, their normal destination. Five transplanted neurons sitting in layer 6 were retrogradely labeled with LGN, a normal target of layer 6 cells. These results indicate that at least a subpopulation of embryonically generated neurons appears to be committed to a deep-layer fate prior to migration.