LAUSR

Erythroblastic islands, niches for erythroblast maturation and red blood cell development, exhibit altered structure and cell composition after multicompartmental injury, hemorrhage, and stress in rats. BACKGROUND Trauma is associated with widespread inflammation, neuroendocrine activation, and an inadequate bone marrow response to anemia. During late-stage erythropoiesis, erythroid progenitors/erythroblasts form clusters on the surface of specialized bone marrow macrophages where they are supported through terminal differentiation and enucleation. We hypothesized that these erythroblastic islands (EBIs) are adversely impacted by severe trauma. METHODS Male Sprague-Dawley rats (n = 8/group) were subjected to either multiple injuries (PT) (lung contusion, hemorrhagic shock, cecectomy, and bifemoral pseudofractures), PT plus 2 hours of daily chronic restraint stress (PT/CS), or naive controls. Bone marrow was harvested on days 2 and 7. Nuclear-stained, enriched bone marrow EBIs were fixed and stained for CD71, VCAM-1, and CD163, and confocal images were obtained at 20 times magnification. Numbers of erythroid cells/EBI and ratio of reticulocytes/EBI were counted by a blinded observer. Differences were compared using analysis of variance, with significance defined as p < 0.05. RESULTS PT and PT/CS had significantly reduced numbers of erythroid cells per EBI on day 2 when compared with naive (PT: 5.9 ± 1.0 cells [p < 0.05], PT/CS: 6.8 ± 0.8 cells [p < 0.05] vs. naive: 8.5 ± 0.8 cells). On day 7, the number of erythroid cells/EBI increased following PT (8.3 ± 0.4 cells) but remained reduced following PT/CS (5.9 ± 0.5 cells [p < 0.05]). This correlated with an increased proportion of reticulocytes/EBI on day 7 following PT, which was not present following PT/CS (PT: 54% [p < 0.05] vs. PT/CS: 28%). CONCLUSION Late-stage erythropoiesis was altered following multicompartmental PT early after injury, and these alterations persisted with the addition of daily chronic stress. Alterations in EBI structure and function after severe trauma and critical illness may serve as a promising new area of study to improve mechanistic understanding of persistent anemia after trauma.