ERYTHROPOIESIS

• The process of erythrocyte (red blood cell) development, which, in adults, primarily occurs in the spongy bone marrow.
• Takes place in red marrow of spongy bone, aka, trabecular or cancellous bone.

Stages:

• Common myeloid progenitor cell
  Arises within the red bone marrow*
• Colony-forming unit – erythroid cells (CFU-E)
  • EPO (hormone released from the kidneys) induces entry into the erythroid series.
• Proerythroblast
  Has a large nucleus, so that only a basophilic rim of agranular cytoplasm can be seen.
  Golgi apparatus appears as a white “ghost” in the histological sample; though not shown here, the proerythroblast has an abundance of RNA and ribosome content.
  The nuclear chromatin is fine and granular, with 1 – 2 nucleoli.
• Basophilic erythroblast
  Smaller (~8-16 µm)
  Cytoplasm appears even darker, more basophilic, because the polyribosomes are active in hemoglobin (Hb) synthesis.
  The nuclear chromatin begins to form clumps, which, against the lighter-staining surrounding matter, can create a so-called “checkerboard” pattern.
• Polychromatophilic erythroblast
  Bluish-gray hue – the presence of hemoglobin increases and the cytoplasm becomes more acidophilic (“polychromatophilic” = cell contains both basophilic and acidophilic matter).
  Nucleus decreases in size, and the chromatin condenses; “checkerboard” pattern becomes more visible.
• Orthocromatic erythroblast
  More acidophilic (hence, it is also referred to as an acidophilic erythrobast), due to the high concentration of hemoglobin
  Pyknotic nucleus: as the chromatin degenerates, the nucleus shrivels to form a dense basophilic mass
  • In transition to the next stage, the nucleus is extruded from the cell.
• Reticulocyte
  Light pink/blue staining
  Has no nucleus
  Basophilic remnants of the reticulum and other organelles can still be seen.
  • Eventually, these remnants are also lost, and the mature erythrocyte stains red; however, upon close inspection, we can see that its biconcave center stains lighter – this is referred to as the central pallor.

Additional Information:

• As the erythropoid series progresses, the cells become smaller: the proerythroblast is approximately 15-20 micrometers in diameter; the erythrocyte is only 7-8.5 micrometers.
• Be aware that a great deal of intertextual variation in nomenclature exists; in this tutorial, we’ve used the nomenclature that reflects changes in cytoplasmic staining as it goes from basophilic (dark purple/blue) to acidophilic (bright pinkish).
• Be aware that many texts use “erythroblast” and “normoblast” interchangeably.