Section of bone marrow obtained from a transgenic MIIb-tk mouse 4 days after discontinuing GCV treatment. (a) The bone marrow has become reorganized and compact. Numerous maturing cells (*) are present. However, mature MK were still rarely observed. (b) A well-preserved endothelial cell (EC) is delimiting a vascular sinus (arrowheads). (c through e) Typical examples of cells in mitosis and in division (arrows) illustrate the intense cell proliferation. Bars = 5 μm.