Cytology · MU Sofia

57. Blood formation. Periods and sites of blood formation. Regulation of hemopoiesis.

Covers how blood cells form, starting with the stages of erythropoiesis from proerythroblast to mature erythrocyte, with its regulation by erythropoietin and hypoxia. It then sets out the prenatal phases (mesoblastic, hepatolienal, medullary) and the postnatal shift to bone marrow, followed by the stem cell, progenitor and blast hierarchy and the wider regulators such as thrombopoietin, colony-stimulating factors and negative feedback. Four pictures accompany the text.

Inside: 7 sections · 4 labelled pictures · 13 flashcards · 7 min

Topic 57 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 7 sections, about 1,091 words (7 min of reading), with 13 flashcards.

In this note

  1. Blood Formation
  2. Hematopoiesis
  3. Periods and Sites of Hematopoiesis
  4. Postnatal Hematopoiesis
  5. Shifting Sites of Hematopoiesis
  6. Cells Involved in Hematopoiesis
  7. Regulation of Hematopoiesis

Blood Formation

Erythropoiesis is the process by which red blood cells (erythrocytes) are formed.

Location and Timing of Erythropoiesis

  • Early Stages: In the early embryo, blood cells originate in the yolk sac mesoderm.
  • Second Trimester: Hemopoiesis (blood cell formation) primarily occurs in the developing liver during the second trimester, with the spleen playing a minor role.
  • Third Trimester Onward: As skeletal elements ossify and bone marrow develops within their medullary cavities, bone marrow becomes the primary site of hemopoiesis during the third trimester and throughout adulthood.

Stem Cells and Differentiation

  • Pluripotent Stem Cells: All blood cells, including erythrocytes, arise from a single type of pluripotent stem cell located in the bone marrow. These stem cells have the capacity to self-renew and give rise to all blood cell lineages.
  • Lineage Commitment: The pluripotent stem cells produce two main lineages of progenitor cells:
  • Lymphoid cells: These cells differentiate into lymphocytes.
  • Myeloid cells: These cells give rise to granulocytes, monocytes, erythrocytes, and megakaryocytes (which produce platelets).

Erythropoiesis: A Multi-Step Process

  1. 1. Proerythroblast: This is the first recognisable progenitor cell in the erythroid lineage. It is a large cell with loose chromatin, prominent nucleoli, and basophilic cytoplasm due to the abundance of ribosomes synthesising haemoglobin.
  2. 2. Basophilic Erythroblast: The basophilic erythroblast has even more intensely basophilic cytoplasm and a condensed nucleus.
  3. 3. Polychromatophilic Erythroblast: As the cell volume reduces and ribosomes decrease, areas of the cytoplasm begin to be filled with haemoglobin, leading to a mix of basophilic and acidophilic staining.
  4. 4. Orthochromatophilic Erythroblast (Normoblast): In this stage, the nucleus becomes highly condensed (pyknotic) and the cytoplasm is uniformly acidophilic due to the high concentration of haemoglobin.
  5. 5. Reticulocyte: The orthochromatophilic erythroblast ejects its nucleus, which is then phagocytosed by macrophages. The remaining cell, called a reticulocyte, still contains a few ribosomes that can be visualised with special staining techniques. Reticulocytes are released into the bloodstream.
  6. 6. Mature Erythrocyte: Reticulocytes mature into erythrocytes within a few days. The mature erythrocyte is a non-nucleated, biconcave disc packed with haemoglobin for oxygen transport.

Regulation of Erythropoiesis

  • Erythropoietin (EPO): Erythropoiesis is primarily regulated by erythropoietin, a glycoprotein hormone produced by cells in the kidneys.
  • Stimulus for EPO Production: EPO production is stimulated by low blood oxygen levels (hypoxia).
  • Action of EPO: EPO acts on erythroid progenitor and precursor cells, stimulating their proliferation and differentiation and promoting haemoglobin synthesis.

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