- They develop at various sites during embryonic periods.
- Initially yolk sac produces them, then later placenta.
- During second and third trimester the site shifts to bone marrow and spleen.
- Yolk sac- produces embryonic hemoglobin and tissue resident macrophages.
- intraembryonic sites- RBC, platelets, cells of adaptive immunity.
- differentiation of multipotent stem cells -> primitive erythroid precursors.
- IL-3, GM-CSF, SCF (stem cell factor/ kit ligand or steel factor) are important for amplification of progenitor cells.
- EPO (erythropoietin) is a growth factor essential for amplification and terminal differentiation of erythroid progenitors and precursors.
- BFU-E and CFU-E are two erythroid progenitors.
- EPO is produced in the kidney (inner cortex and outer medulla) and fetal liver in response to hypoxia/ anemia.
BFU-E: burst forming unit (seen in in vitro cultures)
- EPO + SCF/IL-3/ GM-CSF = BFU-E CFU-E.
- This process takes approximately 2 weeks in vitro.
CFU-E: colony forming unit.
- Under the influence of EPO small colonies of erythroblasts in 7 days’ time.
- Proerythroblasts -> basophilic normoblast -> polychromatic normoblast -> orthochromatic normoblast -> reticulocyte -> mature erythrocyte.
- This process involves reduction of cell size, nuclear condensation and extrusion and hemoglobin accumulation.
- Each proerythroblast 8 reticulocytes (5 days’ time)
- Erythrocyte mature RBC (1 day time in circulation)
- Life span of circulating mature RBC is 4 months (120 days)
In acute anemia-
- The length of this transit time may decrease to as little as one or two days because of skipped divisions.
- The red cells are macrocytic; they may also bear surface i antigen and other fetal characteristics, such as increased (HbF).
- As a result, stress erythropoiesis is associated with circulating-
- Pappenheimer bodies (iron granules),
- basophilic stippling (ribosomes),
- Heinz bodies (haemoglobin inclusions), and
- Howell-Jolly bodies (nuclear remnants)