Distribution and Western blot analysis of Lamin B in erythroid progenitors. (A) Fluorescence micrograph of early and late mouse erythroblasts from freshly harvested bone marrow labeled with antilamin. The pattern of rim staining was similar in early and late erythroblasts. Bar is 2.5 μm. (B) Western blot analysis of FVA-cell lysates obtained from cells cultured from 0 hour to 43 hours. Gel lanes were loaded with equivalent cell numbers and blot probed with anti-lamin B. Immunoreactive bands of lamin B were observed at 69 kDa; no apoptotic products were detected. A staurosporine-treated sample of FVA cells, serving as positive control for apoptosis, showed the classic apoptotic 49-kDa lamin B cleavage product.