Haemopoiesis Notes

Haemopoiesis (Blood Cell Development)

  • Identify histological features of immature/developing blood cells.

  • Differentiate between bone marrow smear/aspiration and a section.

  • Investigate the appearance of cells in bone marrow smears.

  • Understand maturational processes and requirements for erythropoiesis (red blood cell development).

  • Understand maturational processes in granulopoiesis (granulocyte development) and consequences of disturbances (e.g., leukaemia).

  • Understand the process involved in thrombopoiesis (formation of platelets).

Terminology of Blood Cell Production

  • Haemopoiesis / Haematopoiesis: Process of blood cell production (all cell types), including formation & maturation of mature blood cells from stem cells.

  • Granulopoiesis: Process via which new granular leukocytes are produced.

  • Erythropoiesis: Process via which new erythrocytes (RBCs) are produced.

  • Thrombopoiesis: Process via which new thrombocytes/platelets are produced.

Histological Features of Immature Blood Cells

  • Large cells with a large euchromatic nucleus and many nucleoli.

  • Undifferentiated, lacking specific distinguishing nuclear or cytoplasmic characteristics.

  • Enables sustained cell division.

  • Stem & progenitor cells are morphologically indistinguishable; progenitor cells are more mitotically active.

  • Precursor cells (CFUs) respond to environmental GFs, leading to targeted growth, development & maturation.

Sites of Haemopoiesis – Shift with Age

  • Prenatal: Yolk sac, liver, spleen, then bone marrow.

  • Child & Adult: Red bone marrow (myeloid tissue).

Location of Red Bone Marrow in Adult

  • Iliac crest (pelvis) is a common site for bone marrow biopsy.

  • Aspiration needle insertion site.

Bone Marrow Histological Section (in situ)

  • Thin-walled blood vessels called sinusoids contain many RBCs.

  • Cords/islands/clusters of hemopoietic cells; also contains megakaryocytes.

Red Bone Marrow Smear (from Aspiration)

  • Characteristic appearance involves the presence of many clusters of immature blood cells.

  • No structural/architectural details of bone marrow are seen in a smear.

  • Identification of cell types & trends in differentiation processes is the focus.

Origin of Blood Cells - Overview

  • Multipotential stem cells: Progenitor cells with the gene activation potential to differentiate into discrete cell types.

  • Pluripotential stem cell: A stem cell that has the potential to differentiate into any of the three germ layers.

  • Differentiation pathways:

    • Pluripotent stem cell → Myeloid stem cell or Lymphoid stem cell

    • Myeloid stem cell → CFU-E, CFU-Meg, CFU-GM; Lymphoid stem cell → Pre-B cells, Prothymocyte

    • CFU-E → Proerythroblast → Erythrocytes (RBCs)

    • CFU-Meg → Megakaryoblast → Megakaryocytes → Platelets

    • CFU-GM → Monoblast or Myeloblast

    • Monoblast → Monocytes → Macrophages

    • Myeloblast → Granular leukocytes (Neutrophils, Eosinophils, Basophils)

    • Pre-B cells → B lymphoblasts → B lymphocytes → Plasma Cells

    • Prothymocyte → T lymphoblasts → T lymphocytes

Erythropoiesis

  • Proerythroblast undergoes 3-5 mitoses.

  • Nucleus loses its nucleoli.

  • Cell gains many ribosomes; referred to as Basophilic erythroblast.

  • Accumulation of haemoglobin.

  • Grey-green cytoplasm; referred to as Polychromatophilic erythroblast.

  • Normoblast: Nucleus has shrunk, ↑Haemoglobin, ↓ ribosomes.

  • Extrusion of nucleus to become Reticulocyte.

  • Mature RBC: Release into circulation from bone marrow involving the loss of organelles (a few ribosomes remain).

Trends in Erythrocyte Differentiation

  • Cell size decreases.

  • Cell shape changes from spherical to biconcave disc.

  • Cytoplasmic staining changes from basophilia (blueish/purplish) due to initial RNA production and cell organelles (including ribosomes), to acidophilia (reddish) as a consequence of subsequent haemoglobin production.

Nuclear Changes in Erythrocyte Differentiation

  • Shrinkage & condensation of the round nucleus, followed by its extrusion from the cell.

  • Coiling of chromatin.

Nucleic Acid Distribution during Differentiation

  • Reduction & eventual disappearance of all DNA & RNA from cell.

  • Cell terminally differentiated, with no repair capacity.

Combined Trends in Erythrocyte Differentiation

  • Change in appearance between precursor cells & circulating erythrocytes.

  • Initially, DNA, transcription (RNA), & ribosomes dominate.

  • In mature blood cells, protein dominates in the form of haemoglobin.

Requirements for, & Control of, Erythropoiesis

  • Growth factors: e.g., colony-stimulating factor (CSF).

  • Hormones:

    • Erythropoietin (EPO) from the kidney promotes:

      • Proerythroblast production (↑ numbers & mitosis).

      • Early release of reticulocytes.

  • Iron:

    • Essential for haemoglobin synthesis.

    • Iron deficiency → small RBCs.

  • Folic Acid & Vitamin B12:

    • For DNA synthesis.

    • Folic acid or vitamin B12 deficiency → fewer, large RBCs.

Iron (Fe2+) for Erythropoiesis

  • Haemoglobin (Hb) consists of:

    • 2 ⍺ & 2 β globin chains (in adults) that each associate with a heme/haem unit.

    • 2 ⍺ & 2 ɣ globin chains = HbF (in fetus).

  • Each heme/haem unit has a Fe^{2+} in its core.

  • Fe^{2+} must be obtained from the diet (can’t be synthesised by humans).

  • Hb carries oxygen (also binds CO_2 & NO).

  • Each heme group binds one oxygen molecule [Fe^{2+} essential for haem formation].

Vitamin B12 & Folate for Erythropoiesis

  • Vitamin B12 required for DNA synthesis.

  • Helps in the conversion of dUMP to dTMP.

  • Vitamin B12 also required for recycling of tetrahydrofolate (THF).

  • Folate = term for many folic acids e.g., tetrahydrofolate (THF) and methyl THF.

  • Folate required for DNA & RNA synthesis through methylation [deficit delays development & division of cells].

Lifespan & Recycling of Erythrocytes

  • RBCs live for 120 days.

  • 2 million RBCs/sec enter circulation from red bone marrow.

  • 2 million RBCs/sec are removed from circulation in the spleen (& liver).

  • Resource intensive.

  • Macrophages in the spleen break down haemoglobin.

  • Heme is broken down to bilirubin, which is secreted from the liver in bile.

  • Iron is recycled back to bone marrow (or stored in the spleen, liver).

  • Amino acids from globin are reutilised.

Granulopoiesis

  • Neutrophils, eosinophils & basophils all have the same precursor cell = myeloblast.

  • Differentiation process: myeloblast → promyelocyte → myelocyte → metamyelocyte → band form → neutrophil.

  • Synthesise different granules.

  • Differentiation of eosinophils.

  • Differentiation of basophils.

Trends in Granulopoiesis

  • Slight decrease in cell size.

  • Heterochromatin in the nucleus increases, and the shape changes from large & round to segmented.

  • Nucleoli become less visible.

  • Non-specific granules appear in the cytoplasm at the promyelocyte stage, followed by specific granules.

  • Medullary storage ~ 7 days

Morphological Changes - Summary

Feature

Immature

Mature

Cytoplasm

Basophilic color

Loses basophilic color

Chromatin

Less condensed

More condensed

Specific Granules

Fewer

Increase in number

Acute Myeloid Leukaemia (AML)

  • Increased proliferation of blast (immature) cells.

  • Myeloblasts & promyelocytes are common in peripheral blood.

  • Note the presence of nucleoli (arrows) in these immature cells.

Chronic Myeloid Leukaemia (CML)

  • Increased proliferation of more mature cells.

  • Myelocytes, metamyelocytes & band cells are common in peripheral blood.

Thrombopoiesis

  • Thrombopoiesis = platelet production, occurs in bone marrow.

  • Platelets share a common progenitor cell with erythrocytes & leukocytes.

  • Developmental pathway is influenced by granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 13 & thrombopoietin (from the liver & kidneys).

  • Leads to development into megakaryocytes.

  • Large complex cells with multi-lobed nuclei and long cytoplasmic processes.

  • Fragmentation into platelets at the ends of the cytoplasmic processes = platelet shedding.

Histological features are directly related to the structure and function of blood cell components. For instance:

  1. Immature Blood Cells:

    • Large cells with a large euchromatic nucleus and many nucleoli: This structure supports the high level of transcriptional activity required for rapid cell division and differentiation.

    • Undifferentiated, lacking specific distinguishing nuclear or cytoplasmic characteristics: This reflects their role as progenitor cells capable of differentiating into various cell types; they haven't yet committed to a specific function.

  2. Red Bone Marrow:

    • Thin-walled blood vessels called sinusoids contain many RBCs: These structures facilitate efficient nutrient and waste exchange, crucial for erythropoiesis.

    • Cords/islands/clusters of hemopoietic cells; also contains megakaryocytes: This arrangement supports the localized production and maturation of blood cells.

  3. Erythrocytes (RBCs):

    • Change in cytoplasmic staining from basophilia (blueish/purplish) to acidophilia (reddish): Reflects the shift from RNA production and organelles to hemoglobin as the dominant protein, supporting oxygen transport.

    • Shrinkage & condensation of the round nucleus, followed by its extrusion from the cell: This increases space for hemoglobin, optimizing their oxygen-carrying capacity.

  4. Granulocytes (Neutrophils, Eosinophils, Basophils):

    • Heterochromatin in the nucleus increases, and the shape changes from large & round to segmented: This accompanies their maturation and readiness to perform specific immune functions.

    • Non-specific granules appear in the cytoplasm at the promyelocyte stage, followed by specific granules: These granules contain enzymes and other substances that mediate their functions in the immune response.

  5. Megakaryocytes:
    Large complex cells with multi-lobed nuclei and long cytoplasmic processes: Their large size and multi-lobed nuclei support high levels of protein synthesis required for platelet production. The long cytoplasmic processes extend into the bone marrow sinusoids, facilitating platelet shedding directly into the bloodstream.
    Fragmentation into platelets at the ends of the cytoplasmic processes: This is the mechanism by which megakaryocytes produce platelets, ensuring a continuous supply for blood clotting.

In each case, the histological features—the structural details visible under a microscope—directly enable the cells to perform their specific functions within the blood and immune systems. The changes observed during cell maturation reflect the acquisition of specialized roles and the optimization of cellular machinery to fulfill those roles efficiently and effectively.