Cell-sized microspheres in the hippocampus show cleavage planes and passive displacement

Fluorescent microspheres (6 or 10 micron in average diameter) dispersed in fluid were injected into the hippocampus, neocortex or striatum. In the hippocampus the microspheres were located in one of three cleavage planes. Cleavage planes were found above the alveus, in the obliterated hippocampal fissure and on the hilar side of the dentate granule cells. When the injections were made into the infragranular cleavage plane, the adjacent granule cells degenerated, presumably because the cavity separated the axons from their cell bodies. Some microspheres were passively displaced beyond the boundary of the injection site. If the microspheres gained access to the subarachnoid space, some of the displaced microspheres were found at considerable distances from the injection site. There were no cleavage planes in neocortex or striatum but there was passive displacement of microspheres into the host parenchyma. In cell suspension transplants, the passive displacement of cells should be distinguished from migration and the possibility of a widespread distribution of transplanted cells needs to be considered.