MORPHOLOGICAL ALTERATIONS OF HIPPOCAMPAL PYRAMIDAL NEURONS HETEROTOPICALLY TRANSPLANTED INTO THE SOMATOSENSORY CORTEX OF ADULT-RATS - A QUANTITATIVE GOLGI-STUDY

A characteristic feature of hippocampal organization is the laminated termination of extrinsic and intrinsic afferents. At present, it is no t known to what extent these layer-specific fiber projections modulate the development and final shape of the dendritic arbor of hippocampal target neurons. In the present study, pieces of late embryonic (E18) rat hippocampus were transplanted heterotopically into a cavity in the somatosensory cortex of 6-8 week-old recipient rats. Here, the transp lanted neurons differentiated and survived up to several months in the absence of their specific extrinsic afferents. Moreover, tracing of t ransplant connections with the carbocyanine dye DiI revealed only a li mited projection between the transplant and the host neocortex. Golgi- impregnated transplants were used to analyze the postsynaptic structur es (dendrites and spines) of hippocampal pyramidal cells quantitativel y. Compared with controls, the transplanted pyramidal neurons showed a significant reduction of apical primary dendrites and basal dendritic branches, i.e. of peripheral dendritic portions that originate farthe r from the soma. In contrast, the number of basal primary dendrites or iginating directly from the perikaryon was enhanced. Spine density on the main apical dendritic shaft was significantly lower in all periphe ral dendritic segments in transplanted neurons. We conclude from our r esults that the absence of layer-specific extrinsic afferents that nor mally terminate on peripheral parts of the dendritic arbor of hippocam pal pyramidal neurons caused a reduction of these peripheral dendrites and spines. In contrast, the increase of dendrites and spines near th e cell body might be induced by intrinsic fibers that normally termina te on these proximal dendritic portions and are known to sprout under transplant conditions.