Cell hierarchy and colony-forming units
Hematopoiesis is based on a strict hierarchy. The hematopoietic stem cell gives rise to two global lineages via multipotent progenitor cells. In international nomenclature, CFU (Colony Forming Units) and BFU (Burst Forming Units) are used to designate them.
Multipotent progenitors:
- CFU-GEMM — the common progenitor of myelopoiesis. This cell subsequently gives rise to granulocytes, erythrocytes, monocytes, and megakaryocytes.
- CFU-Ly — the basic progenitor of lymphocytopoiesis.
- CFU-GM — a more specialized progenitor giving rise to granulocytes and monocytes.
- CFU-G — progenitor for neutrophils and basophils.
Next come unipotent progenitors, destined to generate only a single cell type:
- BFU-E and CFU-E — develop exclusively into erythrocytes.
- CFU-Meg — responsible for the formation of megakaryocytes.
- CFU-Eo — differentiate into eosinophils.
- CFU-M — become mature monocytes.
Main differentiation lineages
Three main myeloid hematopoietic lineages branch off from the multipotent CFU-GEMM cell:
- Erythroid lineage: development follows the chain CFU-GEMM $\rightarrow$ BFU-E $\rightarrow$ CFU-E, ultimately resulting in the formation of a mature erythrocyte.
- Megakaryocytic lineage: the pathway from CFU-GEMM to CFU-Meg leads to the appearance of a giant cell, the megakaryocyte. Subsequently, blood platelets pinch off from it, forming platelets.
- Granulocyte-monocyte lineage: implemented via the intermediate stage of CFU-GM. The line then diverges: neutrophils and basophils are formed via CFU-G, eosinophils mature via CFU-Eo, and monocytes are formed via CFU-M.
The lymphoid lineage develops from a separate progenitor, CFU-Ly. Final maturation of B lymphocytes occurs directly in the bone marrow. In turn, cells destined to become T lymphocytes migrate and undergo differentiation in the thymus.
Functional self-regulation system
The body must maintain an optimal level of cellular elements in the blood, adequate to the current metabolic rate. A unique feature of this functional system is that it has only an internal self-regulation link (ensuring homeostasis without involving external behavior).
A special place in this process is occupied by local mechanisms in the bone marrow. The process of blood cell destruction is a crucial factor. The destruction of old formed elements occurs not in the circulating blood, but in specialized organs: the spleen, lymph nodes, and bone marrow. At the same time, the breakdown products of destroyed cells act as stimulants of hematopoiesis, activating the corresponding lineages (erythropoiesis, leukopoiesis, or thrombopoiesis).
Neurohumoral regulation of hematopoiesis
Hematopoiesis is controlled by an extensive network of mechanisms:
- Nervous factors: activity of the sympathetic and parasympathetic nervous system.
- Hormones: the endocrine system actively modulates cell maturation. Pituitary hormones (ACTH, somatotropin), thyroid hormones, and thymic hormones (signaling molecules that specifically regulate leukocyte counts) exert an influence.
- Thrombopoietin: synthesized in the liver, purposefully stimulates the formation of megakaryocytes.
- Interleukin-3 (IL-3): a potent cytokine secreted by T lymphocytes and bone marrow stromal cells. Its targets are stem cells and myeloid progenitors. IL-3 supports the proliferation of these early cells and stimulates the formation of most myeloid cells.
Briefly: answers to topical questions
Where is erythropoietin synthesized and what function does it perform?
Erythropoietin is synthesized predominantly in the kidneys (about 90% in adults) as well as in the liver. The main function of this glycoprotein is to regulate the proliferation and differentiation cycle of erythroid progenitor cells.
Effects of erythropoietin:
- Stimulation of erythropoiesis — activates hematopoiesis in the red bone marrow.
- Proliferation and differentiation — acts on unipotent erythrocyte progenitors and subsequent cells of the erythroid lineage.
- Prevention of apoptosis — binds to receptors on erythroid cells, blocking their programmed death.
What stages of morphologically recognizable cells does an erythrocyte go through during maturation?
During maturation, an erythrocyte goes through a series of stages accompanied by a decrease in cell size, reduction of organelles, accumulation of hemoglobin, and loss of the nucleus. The sequence of differentiation includes the following steps:
- Proerythroblast — the largest cell with a basophilic nucleus.
- Basophilic erythroblast — characterized by marked cytoplasmic basophilia.
- Polychromatophilic erythroblast — has grayish-pink cytoplasm and is the last dividing cell of the erythroid series.
- Oxyphilic erythroblast (normoblast) — a cell with oxyphilic (pink) cytoplasm and a condensed pyknotic nucleus.
- Reticulocyte — an anucleate postcellular structure.
- Mature erythrocyte.
What is the role of vitamin B12 and folic acid in hematopoiesis?
Vitamin B12 and folic acid act as nutrients and coenzymes necessary for DNA synthesis, normal functioning, and cell division during hematopoiesis.
In case of vitamin B12 or folic acid deficiency, DNA synthesis is impaired, leading to the development of megaloblastic hematopoiesis. In megaloblastic anemias, large progenitor cells called megaloblasts appear in the bone marrow and transform into megalocytes.
The myeloid progenitor cells (CFU-GEMM) can be remembered by decoding their first letters: Granulocyte, Erythrocyte, Monocyte, Megakaryocyte.
Class and exam questions
Where does the destruction of old blood cells take place?
The destruction of blood cells takes place not in the systemic circulation, but in filter organs: the spleen, lymph nodes, and the bone marrow itself.
What role do the breakdown products of formed elements play?
They are not simply eliminated from the body, but act as powerful natural stimulators of the respective hematopoietic lineages.
How does the development of T- and B-lymphocytes differ?
Both lineages originate from a common progenitor, CFU-Ly. However, B lymphocytes mature in the bone marrow, while T lymphocytes undergo final differentiation in the thymus.
What is Interleukin-3 and where does it come from?
This is a humoral regulator produced by T lymphocytes and bone marrow stroma. It stimulates stem cells and myeloid progenitors.
What else to study on this topic
- Mechanisms of the influence of ACTH and somatotropin on hematopoiesis.
- Features of local microenvironmental self-regulation in the red bone marrow.
- Specific thymic hormones and their detailed effect on leukopoiesis.
- The role of the autonomic nervous system (sympathetic and parasympathetic) in hematopoiesis.
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