Imaging Flow Cytometric Analysis of Primary Bone Marrow Erythroblastic Islands

The erythroblastic island (EBI) is a multicellular functional erythropoietic unit comprising a central macrophage nurturing a rosette of maturing erythroblasts. Since the discovery of EBIs more than half a century ago, EBIs are still studied by traditional microscopy methods after enrichment by sedimentation. These isolation methods are not quantitative and do not enable precise quantification of EBI numbers or frequency in the bone marrow or spleen tissues. Conventional flow cytometric methods have enabled quantification of cell aggregates co-expressing macrophage and erythroblast markers; however, it is unknown whether these aggregates contain EBIs as these aggregates cannot be visually assessed for EBI content. Combining the strengths of both microscopy and flow cytometry methods, in this chapter we describe an imaging flow cytometry method to analyze and quantitatively measure EBIs from the mouse bone marrow. This method is adaptable to other tissues such as the spleen or to other species provided that fluorescent antibodies specific to macrophages and erythroblasts are available.

Cellular dynamics of mammalian red blood cell production in the erythroblastic island niche

Red blood cells, or erythrocytes, make up approximately a quarter of all cells in the human body with over 2 billion new erythrocytes made each day in a healthy adult human. This massive cellular production system is coupled with a set of cell biological processes unique to mammals, in particular, the elimination of all organelles, and the expulsion and destruction of the condensed erythroid nucleus. Erythrocytes from birds, reptiles, amphibians and fish possess nuclei, mitochondria and other organelles: erythrocytes from mammals lack all of these intracellular components. This review will focus on the dynamic changes that take place in developing erythroid cells that are interacting with specialized macrophages in multicellular clusters termed erythroblastic islands. Proerythroblasts enter the erythroblastic niche as large cells with active nuclei, mitochondria producing heme and energy, and attach to the central macrophage via a range of adhesion molecules. Proerythroblasts then mature into erythroblasts and, following enucleation, in reticulocytes. When reticulocytes exit the erythroblastic island, they are smaller cells, without nuclei and with few mitochondria, possess some polyribosomes and have a profoundly different surface molecule phenotype. Here, we will review, step-by-step, the biophysical mechanisms that regulate the remarkable process of erythropoiesis with a particular focus on the events taking place in the erythroblastic island niche. This is presented from the biological perspective to offer insight into the elements of red blood cell development in the erythroblastic island niche which could be further explored with biophysical modelling systems.

Ineffective Erythropoiesis: Anemia and Iron Overload

Stress erythropoiesis (SE) is characterized by an imbalance in erythroid proliferation and differentiation under increased demands of erythrocyte generation and tissue oxygenation. β-thalassemia represents a chronic state of SE, called ineffective erythropoiesis (IE), exhibiting an expansion of erythroid-progenitor pool and deposition of alpha chains on erythrocyte membranes, causing cell death and anemia. Concurrently, there is a decrease in hepcidin expression and a subsequent state of iron overload. There are substantial investigative efforts to target increased iron absorption under IE. There are also avenues for targeting cell contact and signaling within erythroblastic islands under SE, for therapeutic benefits.

The erythroblastic island as an emerging paradigm in the anemia of inflammation

Terminal erythroid differentiation occurs in the bone marrow, within specialized niches termed erythroblastic islands. These functional units consist of a macrophage surrounded by differentiating erythroblasts and have been described more than five decades ago, but their function in the pathophysiology of erythropoiesis has remained unclear until recently. Here we propose that the central macrophage in the erythroblastic island contributes to the pathophysiology of anemia of inflammation. After introducing erythropoiesis and the interactions between the erythroblasts and the central macrophage within the erythroblastic islands, we will discuss the immunophenotypic characterization of this specific subpopulation of macrophages. We will then integrate these concepts into the currently known pathophysiological drivers of anemia of inflammation and address the role of the central macrophage in this disorder. Finally, as a means of furthering our understanding of the various concepts, we will discuss the differences between murine and rat models with regard to developmental and stress erythropoiesis in an attempt to define a model system representative of human pathophysiology.

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

The erythroblastic island provides an important nutritional and survival support niche for efficient erythropoietic differentiation. Island integrity is reliant on adhesive interactions between erythroid and macrophage cells. We show that erythroblastic islands can be formed from single progenitor cells present in differentiating embryoid bodies, and that these correspond to erythro-myeloid progenitors (EMPs) that first appear in the yolk sac of the early developing embryo. Erythroid Krüppel-like factor (EKLF; KLF1), a crucial zinc finger transcription factor, is expressed in the EMPs, and plays an extrinsic role in erythroid maturation by being expressed in the supportive macrophage of the erythroblastic island and regulating relevant genes important for island integrity within these cells. Together with its well-established intrinsic contributions to erythropoiesis, EKLF thus plays a coordinating role between two different cell types whose interaction provides the optimal environment to generate a mature red blood cell.