erytropoese

Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. In healthy adults, it occurs primarily in the red bone marrow of the axial skeleton and the ends of long bones; during fetal development the liver and spleen contribute significantly. The process begins with hematopoietic stem cells and proceeds through erythroid-committed progenitors to mature erythrocytes.

The erythroid lineage passes through a defined sequence of stages: proerythroblast, basophilic erythroblast, polychromatic erythroblast, and orthochromatic erythroblast. During maturation, hemoglobin is synthesized, iron is incorporated via transferrin, and the cell progressively loses its nucleus and organelles in late stages (enucleation). The cell then becomes a reticulocyte, which enters the bloodstream and matures into a fully functional erythrocyte within a day or two. The overall maturation from progenitor to circulating red cell takes several days.

Regulation of erythropoiesis is tightly controlled, predominantly by erythropoietin (EPO). EPO is produced mainly by kidney interstitial cells in response to hypoxia. Binding of EPO to its receptor on erythroid progenitors promotes their survival, proliferation, and differentiation. Hypoxic signaling involves hypoxia-inducible factors (HIFs). Adequate iron, folate, and vitamin B12 are essential for hemoglobin synthesis; various growth factors and transcription factors, such as GATA-1, also influence erythroid development.

Clinical relevance includes the maintenance of circulating red cell mass, with normal lifespan about 120 days and production rates of roughly millions per second. Defects in erythropoiesis can cause anemia (insufficient production) or polycythemia (excess production), and some conditions exhibit extramedullary hematopoiesis when marrow function is impaired.