Pronormoblasts

Pronormoblasts, also called proerythroblasts, are the earliest morphologically identifiable precursors in the bone marrow during erythropoiesis. They arise from erythroid progenitors in response to erythropoietic stimuli and give rise to successive stages that culminate in mature red blood cells.

Morphology and staining characteristics: Pronormoblasts are large cells, typically 12–20 micrometers in diameter, with a large, round nucleus that exhibits finely dispersed chromatin and one or more prominent nucleoli. The cytoplasm is intensely basophilic due to abundant RNA and ribosomes. As they mature, the nucleus condenses and is eventually extruded during the late erythroblast stages.

Developmental sequence: The pronormoblast represents the initial erythroid precursor stage, followed by basophilic erythroblasts, then polychromatophilic erythroblasts, and later orthochromatic (normoblasts). The nucleus is expelled during the transition from orthrochromatic erythroblast to reticulocyte, after which hemoglobin synthesis continues in circulating erythrocytes.

Markers and localization: In early erythroid precursors, CD71 (transferrin receptor) is prominently expressed and supports iron uptake for heme synthesis. Glycophorin A (CD235a) and other erythroid-related markers increase as differentiation proceeds. Pronormoblasts reside in the bone marrow and are not typically present in circulating blood under normal conditions.

Clinical relevance: Abnormalities in pronormoblasts can appear in certain conditions, notably parvovirus B19 infection, which can cause giant pronormoblasts with intranuclear inclusions and transient red cell aplasia. Understanding pronormoblasts helps illuminate normal erythropoiesis and its disruptions in hematologic disorders.