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I am currently studying the textbook Hoffbrand's Essential Haematology, eighth edition, by A. Victor Hoffbrand and David P. Steensma. Chapter 1 Haemopoiesis says the following:

SCF, TPO and FLT3 ligand act locally on the pluripotential stem cells and on myeloid/lymphoid progenitors (Fig. 1.6). Interleukin-3 (IL-3) has widespread activity on lymphoid/myeloid and megakaryocytic/erythroid progenitors. Granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF and macrophage colony-stimulating factor (M-CSF) enhance neutrophil and macrophage/monocyte production, IL-5 eosinophil, KIT mast cell, TPO platelet and EPO red cell production. These lineage-specific growth factors also enhance the effects of SCF, FLT3-L and IL-3 on the survival and differentiation of early haemopoietic cells. Interleukin 7 is involved at all stages of lymphocyte production and various other interleukins and Toll-like receptor ligand (not shown) direct B and T lymphocyte and NK cell production (Fig 1.6).

These factors maintain a pool of haemopoietic stem and progenitor cells on which later-acting factors, EPO, G-CSF, M-CSF, IL-5 and TPO, act to increase production of one or other cell lineage in response to the body's need. enter image description here enter image description here enter image description here

This isn't written in a clear way. Consequently, it isn't clear to me what the author is saying here:

These factors maintain a pool of haemopoietic stem and progenitor cells on which later-acting factors, EPO, G-CSF, M-CSF, IL-5 and TPO, act to increase production of one or other cell lineage in response to the body's need.

In particular, it is not clear what "these factors" is referring to, since the author then immediately refers to "EPO, G-CSF, M-CSF, IL-5 and TPO", which were the subject of the last paragraph.

I would greatly appreciate it if people would please take the time to clarify what the author is referring to in the second part when he says "these factors".

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    The author is talking about SCF, TPO, FLT3 and IL-3 which promote early differentiation to colony forming units (CFU) = progenitor cells. The later factors are responsible for differentiating progenitor cells to the specific cell lineages.
    – Narusan
    Jul 26 '20 at 19:58
  • @Narusan thanks for the clarification. Jul 27 '20 at 0:34
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I'll admit, this is far from being clear and I don't have enough points to make this as comment yet but let me try to give you a help with this pseudo-answer.

After birth, hematopoiesis is mainly limited to the red bone marrow, with some lymphatic tissues assisting in the production of lymphocytes.

All blood cell elements are derived from a single population of stem cells called hemocytoblasts, located in the red bone marrow, these are precursor cells capable of dividing and producing daughter cells that can then differentiate into various types of cells blood.

When a hemocytoblast divides, one daughter cell remains hemocytoblast, while the other daughter cell differentiates to form one or more types of intermediate cells: a myeloid stem cell or a lymphoid stem cell.

Red blood cells, platelets and many leukocytes can be developed from myeloid stem cells. Specifically, myeloid stem cells can differentiate into pro-erythroblasts, from which red blood cells develop; myeloblasts, from which basophils, eosinophils and neutrophils develop; monoblasts, from which monocytes develop; and megacarioblasts, from which platelets develop. Lymphocytes develop from lymphoid stem cells.

Now we get to the factors. Chemical signals regulate the development of different types of cellular elements. These chemical signals include CSF's (colony stimulating factors) and hormones transported to the bone marrow by blood or substances released by bone marrow cells.

Erythropoietin (EPO) is an example of an hormone, secreted by kidney endocrine cells, that stimulates myeloid stem cells to develop into red blood cells.

I based my resources on Seeley's Essentials of Anatomy & Physiology, even though the title suggests an off-topic content

Seeley's Essentials of Anatomy & Physiology

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  • Thanks for the answer. "When a hemocytoblast divides, one daughter cell remains hemocytoblast, while the other daughter cell differentiates to form one or more types of intermediate cells: a myeloid stem cell or a lymphoid stem cell." Couldn't it also become a megakaryocytic erythroid progenitor, as shown in the figure? Jul 26 '20 at 17:22
  • Anyway, I will upvote your answer, since it seems to be informative. If no one else is able to answer the question, then I'll just assume that the author screwed up the writing, and I'll accept this answer. Jul 26 '20 at 19:21

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