Histology of murine BM engrafted with CML cells. Engraftment with BP cells: (A) The marrow is infiltrated by sheets of huCD45⁺ cells, with some residual huCD45-negative maturing hematopoietic elements; (B) The infiltrating cells are large, nucleolated blasts with strong surface immunoreactivity for huCD45. The smaller maturing erythoid elements are negative. Murine BM engrafted with CP cells: (C) the marrow contains frequent dispersed huCD45⁺ cells, comprising about 10% of the cellularity. The remaining huCD45-negative marrow elements appear normal. (D) The huCD45⁺ cells are medium sized mononuclear cells with round to oval nuclei and abundant pale cytoplasm. They are dissimilar from the blasts seen in (B).