Sections of human malignant marrow tissue engrafted in SCID mice. (a) Sample R49, a pre-pre-B-ALL, 60 days posttransplantation; blasts diffusely infiltrate the tissue. Arrow, bone trabecula (HE staining, original magnification × 100). (b) Many blast cells (in the same graft) are mitotic (arrows) (Giemsa staining, original magnification × 100). (c) In this same case, but in none of the others studied, blast cells also disseminated in the mouse tissues immediately adjacent to the graft, notably around nerves (arrow) (HE staining, original magnification × 100). (d) Same sample as in a, b, and c, showing myelofibrosis (Gordon-Sweet dye, original magnification × 33). (e) R41, another pre-pre-B-ALL, was one of 6 samples in which blastic infiltration was only partial and contained adipocytes; here, at 11 weeks posttransplantation (HE staining, original magnification × 33). (f ) R46, an AML-2 in which evidence of myeloid differentiation is observed, 11 weeks after engraftment (arrows) (HE staining, original magnification × 100).