Effects of antimitotic treatment on haematopoietic tissues in mice

The antimitotic drug hydroxyurea was injected into mice twice daily for one, two, three or four consecutive days, and its effects assessed in bone marrow (BM) and spleen (SP). The number of nucleated cells decreased with the number of injections, reaching 25% of normal control values in SP and less than 10% in BM after four cycles. In contrast, the number of red blood cells (RBC) also dropped in SP, but increased in BM. Histological and morphometric studies showed that the increased number of RBC in BM could result from two major modifications in the marrow tissue: an expansion of sinusoids and a possible extravasation of cells from the marrow vasculature.