Defective EBI formation in Se-D mice is partially responsible for the ineffective stress erythropoiesis. (A) Representative picture of spleen EBI structure. EBIs are Ter119⁺F4/80⁺ aggregates. (B) Flow cytometry analysis of EBIs on the indicated days after 50% PHZ treatment; n = 2-4 per diet. Two independent experiments. (C) Flow cytometry analysis of RPMs in EBIs on the indicated days after 50% PHZ treatment; n = 2-4. (D-E) Flow cytometry analysis of (D) Vcam-1 and (E) CD169 expression on EBI macrophages on the indicated days after 50% PHZ treatment; n = 2-4 per diet. (F) Representative FlowSight analysis of EBIs at day 2 and day 3 after 50% PHZ treatment. Four independent experiments. Bars are representative of mean ± SEM. (G) Schematic representation of ineffective stress erythropoiesis during Se and selenoprotein deficiency. Red dashed arrows indicate the process is delayed or impaired. Red solid arrows indicate downregulation or decreased level of the molecules. In the normal condition, in response to stress, SEPs and erythroid precursors rapidly expand and differentiate. SelenoW expression is upregulated in the erythroblasts by GATA-1, regulating cell cycle and hemoglobin synthesis. Increased number of RPMs mature from monocytes, expanding the erythropoietic niche to support erythroblast maturation. Adhesion molecules are upregulated in the EBIs, as well as heme biosynthesis-related genes and pathways to bolster the process. However, in the absence of Se or selenoproteins, SEP expansion and differentiation is delayed. Erythroblast maturation in the niche is less efficient because of the deficits in both erythroblasts and macrophages. Heme homeostasis is perturbed, resulting in improper downstream signaling. **P < .001; ****P < .0001.