Expression of myeloid markers by marrow MKs. (A) CD45 is highly expressed by MKs. The expression level correlates with cell maturation as determined by ploidy. (B) CD36 (thrombospondin receptor) is robustly expressed by MKs and is highly correlated with MK size, ploidy, and the expression of CD41a. (C) The expression of IgG-FcγRII (CDw32, the only IgG-Fcγ receptor expressed on platelets-MKs) correlates directly with cell maturation. Receptor expression is assessed by the binding of IV.3 MoAb. (D) Mature MKs express relatively low levels of HLA-DR compared with other marrow cells (represented by gray dots).