Bone marrow-derived Lyn^-/-erythroblasts falter in their development to Ter119^pos erythroblasts. (A) Defects in Lyn^-/- bone marrow-derived erythroblast development at a Ter119^pos stage. Following 2.5 days of culture in SP34-EX medium, expanded Lyn^+/+ and Lyn^-/- erythroid progenitor cells were shifted to a transferrin, insulin, and Epo-containing differentiation medium. At day 1.5, frequencies of maturing erythroblasts were assayed based on Ter119 marker expression (i). Frequencies of CD71^highTer119^pos copositive cells also were assayed (bottom panels, circled populations). Overall frequencies of Ter119-positive and CD71⁺ erythroblasts from 3 independent experiments (n = 3 mice per experiment) are also graphed (i; mean ± SE). (B) CD71 and Ter119 marker expression among expanded erythroblasts also was assessed prior to differentiation. During this short-term expansion, marker profiles for Lyn^+/+ and Lyn^-/- populations were essentially equivalent. (C) Lyn^-/- erythroblasts exhibit decreased survival at a Ter119^pos stage. Bone marrow-derived erythroblasts from Lyn^-/- and Lyn^+/+ mice were expanded and shifted to differentiation conditions. At 24 hours of culture, frequencies of annexin V-positive cells among the maturing erythroblasts were assayed. Increased death among Lyn^-/- erythroblasts also was confirmed by propidium iodide staining (data not shown). (D) In these cell populations (and at 24 hours of differentiation), levels of Bcl-xL and of activated PY416-p60-Src (p-Src) expression also were analyzed (by Western blotting) together with Lyn and GAPDH.