Identification of proerythroblasts in tissue sections of bone marrow

In paraffin-embedded sections of bone marrow fixed in B5-formalin, proerythroblasts from patients with marked erythroid hyperplasia displayed a characteristic nuclear chromatin pattern. This pattern consisted of an "arm" of chromatin extending from the center of the nucleus to the inner aspect of the nuclear envelope. In some instances the arm showed an "elbow bend." Depending on the plane of section, approximately 30% of proerythroblasts demonstrated these chromatin patterns. Ultrastructurally a similar "arm" of chromatin was seen in proerythroblasts. Embedded within the chromatin was a nucleolus. Although the significance of this pattern is as yet unknown, it provides a morphologic clue to identification of proerythroblasts in tissue sections.