Electron micrographs illustrating the bone marrow of a normal mouse and a MIIb-tk transgenic mouse both treated with GCV. (a) A mature MK in a nontransgenic mouse surrounded by numerous cells belonging to the granulocytic lineage. An erythroblastic islet can also be seen in the bottom right hand corner. (b) MIIb- tk bone marrow after GCV treatment showing the intense disorganization of the marrow. Note the enhanced presence of small mononuclear cells (arrowheads) having a high nucleus-cytoplasm ratio. MK were absent. Bars = 5 μm.