A 45-year-old woman develops symptoms of shortness of breath on exertion, easy fatigue, and jaundice. On examination she is pale, and there is a palpable spleen, but no lymphadenopathy. Her hemoglobin is 9.0 g/dL, the reticulocyte count 9%, and the direct antibody test (Coombs) is positive. Which of the following bone marrow findings is most likely to be seen in this patient?