Lecture 28: Bone Marrow and Hematopoiesis

Transcribed by ______________ Date of the Lecture

Transcribed by Joseph Schwimmer

Basic Tissues Lectures #27 – BONE MARROW HEMATOPOIESIS by Dr. Despina Sitara 2/20/14

Slide 1 – CoverDr. Despina Sitara – Ok, a few more slides for today, and then we can be done for this week. Now we are going to talk about bone marrow hematopoiesis, how the blood cells are formed in the bone marrow and the various stages of their maturation.

Slide 2 – Hematopoiesis OverviewDr. Despina Sitara – So the number of circulating cells, the numbers of blood cells in the blood is maintained within narrow limits. However, the blood cells have finite and short lifespans, so they must be continuously renewed and replaced to maintain appropriate circulating levels. This process, the process of blood cell renewal and therefore blood cell formation, is called hematopoiesis. All blood cells are derived from the hematopoietic stem cells, the HSCs. Normal individuals produce about 1011 – 1012 new blood cells per day. There are many factors that can influence hematopoiesis. These can be infection, hemorrhage, tumors that can induce increases or decreases in one or more blood cell lines, and also several growth factors can affect proliferation and differentiation of the hematopoietic cells.

Slide 3 – Ontogeny of HematopoiesisDr. Despina Sitara – Hematopoiesis occurs in various sites, anatomical sites during the development from the embryo to the adult. Prenatally means before birth. So prenatally, before birth, the major site of hematopoiesis – FORGET THAT – before birth, prenatally, the FIRST site of hematopoiesis is the yolk sac. This is the site between 2-10 weeks of gestation. At 10 weeks post conception, mesenchymal cells aggregate and form what they’re called blood islands. These blood islands are the precursors for all erythroid cells.

Slide 4 – Ontogeny of HematopoiesisDr. Despina Sitara – At about 6 weeks of gestation, granulocytes and megakaryocytes are formed in the yolk sac, and after weeks of gestation, lymphocytes are formed in the lymph sacs of the yolk sac. From 6 week gestation to birth, the primary hematopoietic organ is the fetal liver. The yolk sac is the first, but the primary is the fetal liver. At 10 weeks gestation, the fetal spleen can also be used as a secondary hematopoietic organ. As the bones start to form bone cavity that fill switch bone marrow and eventually the bone marrow is the organ that will replace the fetal liver after birth for the production of hematopoietic cells. So the first site is the yolk sac, the primary is the fetal liver. The secondary is the fetal spleen. And the fetal bone marrow starts taking over right around birth.

Slide 5 - Ontogeny of Hematopoiesis

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Dr. Despina Sitara – After birth, postnatally, the primary hematopoietic organ is the bone marrow. However, there are secondary hematopoietic organs, these are the lymphatic organs, and in children hematopoiesis occurs in the bones marrow of the long bones; these are the femur and the tibia. However in adults hematopoiesis occurs in the marrow of the pelvis, the cranium, vertebrae, and sternum. The lymphatic such as liver, spleen and thymus only become active when it’s needed postnatal hematopoiesis if there is a defect in the bone marrow and the bone marrow cannot produce efficient number of bone marrow cells – of blood cells, then the secondary lymphatic organs kick in and produce hematopoietic cells. This condition is called extramedullary hematopoiesis.

Slide 6 - Hematopoietic Stem Cells (HSCs)Dr. Despina Sitara – The hematopoietic stem cells reside in the bone marrow, they’re very rare, only 0.1% and have the ability to give rise to all different mature blood cell types. That’s why they’re called pluripotential. They’re capable of self-renewal. However, some of them it means that they remain HSC although they can give rise to all other blood cell types, some of them remain HSCs simply because otherwise the hematopoietic stem cell pull will be depleted if they all matured into different types of cells, so some of them have to remain as HSCs. The HSCs give rise to two types of multi-potential cells, these are the common myeloid progenitors and the common lymphoid progenitors. These are morphologically identical to HSCs but they have limited capacity for self-renewal. The common myeloid progenitors give rise to myeloid cells, these are the granulocytes, the erythroid cells, the monocytes, and the megakaryocytes which are the immature cells of platelets. That’s why the common myeloid progenitors are also called CFU-GEMM, and this process is the process of myelopoiesis. The common lymphoid progenitors give rise to lymphoid cells, the B cells, the T cells, and the natural killer cells, and this process is called lymphopoiesis.

Slide 7 - Hematopoietic Stem Cells (HSCs)Dr. Despina Sitara – This is a cartoon that shows the pluripotential hematopoietic stem cells, all its own by there, then gives rise to either the common myeloid progenitors, CFU-GEMMs, or common lymphoid progenitors CFU-Ls. These gives rise to erythrocytes and all the other myeloid cells, which can be the basophils, the neutrophils, the eosinophils and the monocytes, and the megakaryocytes, and then the lymphoid progenitors give rise only to the 3 different types of lymphocytes: the natural killer cells, the T lymphocytes and the B lymphocytes.

Slide 8 - Hematopoietic Cell DifferentiationDr. Despina Sitara – Hematopoietic precursors arise from progenitor cells, either form the common myeloid progenitors, or the common lymphoid progenitors. The precursors are incapable of cell renewal, these are a stage after the progenitor stage. They are the first cells to become lineage committed, to be the first cells of a particular cell line. They undergo cell division and differentiation and these are the ones that give rise finally to the mature cells. As they mature they become smaller, they lose their nucleus, and their chromatin becomes denser. They have the suffix

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blast. The mature hematopoietic cells do not undergo cell division, and they are apparent/evident in peripheral blood.

Slide 9 - Differentiation of Pluripotential Stem Cells during HematopoiesisDr. Despina Sitara – that’s another cartoon here that shows that you’ve got the stem cells, and then the progenitor cells and then the precursor cells which will be the lymphoblasts. Remember you have the suffix blast, erythroblasts, megakaryoblasts, etc., and then these give rise to mature cells. This will make sense in a second, when we talk about individual lines.

Slide 10 - Differentiation of Pluripotential Stem Cells during HematopoiesisDr. Despina Sitara – Several growth factors which are also called colony stimulating factors affect blood cell proliferation and differentiation. These factors can stimulate proliferation of immature cells, this can be either progenitors or precursors. They can also support the differentiation of the maturing cells, and they can enhance the functions of mature cells. The growth factors, these colony stimulating factors can also stimulate cell division and differentiation of progenitor cells of granulocytic and monocytic series.

Slide 11 - Hematopoietic Growth FactorsDr. Despina Sitara – Some of these factors are stem cells factors, the granulocyte-macrophage colony-stimulating factors GM-CSF, Interleukin 3 and Interleukin 7. These growth factors play a role in proliferation of stem cells. There are other factors which play a role in mobilization and differentiation of the steam cells into progenitor cells. These are the GCSF granulocyte colony-stimulating factors G-CSF, and the M-CSF, the Monocyte colony-stimulating factors. An important factor for erythrocytes, for the red blood cells, is erythropoietin, which activates erythroid cells. And another factor important for the platelets is thrombopoietin, which stimulates platelet production.

Slide 12 - ErythropoiesisDr. Despina Sitara – So in erythropoiesis, this is the process of red cell formation, this takes about 7 days for an erythroid cell to mature. The maturation process involves the synthesis of hemoglobin, and the formation of enucleated – without nucleus – biconcave small erythrocytes, small red blood cells. The red blood cell production is under the control of erythropoietin – Epo. This is a kidney secreted protein, in response to hypoxia, the body senses when the oxygen levels are low, the kidneys produce erythropoietin, erythropoietin travels to the bone marrow through the blood stream, and tells the bone marrow cells, the erythroid cells in the bone marrow to multiply, to make more. It also stimulates the production of globin, which is the protein component of hemoglobin. Certain factors such as iron and B12 and Folic Acid are slap essential for red cell production. Now from the hematopoietic stem cell, the progenitor for erythroid cells are this common myeloid progenitor, the CFU-GEMM, which gives rise to two other progenitors, the BFU-E this is the best

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forming unit erythroid, and the CFU-E which is the colony forming unit erythroid, or erythrocyte.

Slide 13 – Stages in Erythroid (Red Cell) MaturationDr. Despina Sitara – Now these are the stages of erythroid cell maturation. It is important that you know which stage comes after which. So, the first stage, the first precursor after CFU-E is the proerythroblast. This up here is a cartoon, and this is a real picture of a real proerythroblast. Proerythroblasts are large cells, they have very little cytoplasm, which is pale-grayish blue, but what is most important is that their nucleus is large, is sort of round, and its dark red, it has visible nucleoli. The second stage is the basophilic erythroblast. This is a cartoon and this is a basophilic erythroblast in a blood smear. It’s a smaller cell than a proerythroblast, the cytoplasm is pale blue, and the nucleus is still round, but smaller and coarser than the proerythroblast. The next stage is the polychromatic erythroblast. This is much smaller, as you can see, than the proerythroblast, this is a polychromatic erythroblast here. The cytoplasm is gray, but the nucleus is small, round, and coarse.

Slide 14 - Stages in Erythroid (Red Cell) MaturationDr. Despina Sitara – The next stage up towards maturation is the orthochromatic, I would say, some people call it orthochromatophilic orthochromatic erythroblast. This is also called normoblast. From this point onwards, the cells are not capable of division; any division has already happened. As you can see, the cytoplasm it’s not blue, it’s pinkish with a little hint of blue. The nucleus is small, its compact, and its densely stained, and it’s pushed towards the edge of the cell as you can see. That’s because it’s ready to be extruded. The next stage is the reticulocytes. We introduced reticulocytes on Tuesday. They’re still not capable of division, they have cytoplasm and some residual RNA. They have no nucleus at this point. Mature red blood cells, now they are in the blood, they cannot divide, they have no nucleus, and they have the typical biconcave round shape. So the stages are important to know what comes after what.

Slides 15 - GranulopoiesisDr. Despina Sitara – Granulopoiesis, this is the process of granulocyte formation. The formation of neutrophils, eosinophils and basophils. This happens under the influence of IL-3, IL-5, G-CSF, granulocyte colony-stimulating factors GM-CSF, the granulocyte monocyte colony-stimulating factors. The granulocytes also originate from the common myeloid progenitor, CFU-GEMM, which can then differentiate into neutrophils, and it’s called CFU-GM, eosinophils CFU-Eo, and basophils CFU-Ba. Now for neutrophils, eosinophils and basophils, the first two stages of granulocytic series, the myeloblasts and the promyelocyte possess no specific granules. The granules appear later in the third stage, the myelocyte stage, and they are distinguished between neutrophils, eosinophils and basophils. And this is what I mean. So the myeloblast and the promyelocytes is the same for all 3 white blood cell types, the neutrophils, eosinophils and basophils, and then from this stage onwards, after the second stage, they start looking differently, depending on their type of granules.

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Slide 16 - Stages in Granulocyte Maturation Dr. Despina Sitara – So I am not going to go through all 3 of them, I’ll take as an example the neutrophils, and what you need to know for the eosinophils and basophils are that the eosinophils its exactly the same maturation process but they have large orange granules, and the basophils have dark purple granules cover all of them, like I described earlier in the lecture. So, I’m gonna take as an example the neutrophil, and the stages as you can see are similar, only the color of granules changes. So the first stage is myeloblast, that’s a cartoon, that’s a real myeloblasts. It’s a medium sized cell, the cytoplasm is light blue, there are no granules at this point, but the nucleus is large and round, it almost covers most of the cell, most of the myeloblast, and it’s sort of reddish, dark with visible nucleoli.

Slide 17 - Stages in Granulocyte MaturationDr. Despina Sitara – The second stage is the promyelocyte, much larger than the first stage, it has a bluish – the cytoplasm has a bluish color, and has many azurophilic granules as you can see. The nucleus is large, but the nucleus to cytoplasm ratio is not as large as it’s in the first – the myeloblast stage, and its reddish, round, and coarser.

Slide 18 - Stages in Granulocyte MaturationDr. Despina Sitara – The myelocyte, the neutrophilic myelocyte, as you can see here, it has a pale blue cytoplasm, it has some granules at this point knowing about neutrophils would be azurophilic and also specific granules. The nucleus is round, off the center, coarse and reddish. The next stage sown is a neutrophilic myelocyte. At this point there is no cell division. The cytoplasm is similar to the previous stage, it’s a little paler, but a typical characteristic of a metamyelocyte is the nucleus – it is sort of a kidney shaped, it has a little bit of indentation , it is off the center, and it is dense.

Slide 19 - Stages in Granulocyte MaturationDr. Despina Sitara – Just before maturation, the stage is called neutrophilic bond, or stab cell. Again, this cell doesn’t undergo cell division, the cytoplasm is bluer than a mature neutrophil, but the important thing is the shape of this nucleus, it’s elongated, and it has the shape of horseshoe. That’s a given, when you see a cell like that it’s definitely a stab, or band, cell. Now again for this one, for Granulopoiesis, the stages in granulocyte maturation please know what is the sequence of these cells. It’s a myeloblast, a promyelocyte, a myelocyte, a metamyelocyte, a band cell, and a mature neutrophil.

Slide 20 - Stages in Granulocyte MaturationDr. Despina Sitara – So that’s the last stage, its segmented neutrophil, it’s in the blood already at this point, no cell division, and the nucleus has 2-5 blobs, and in females, the Barr Body may appear, like here.

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And actually I would like to stop here for today, we’ll continue with the megakaryocytes and the B and T cells next week, and we have also the platelets and the blood clotting and that would be the end of the blood lecture series. I believe we have a conference starting on Monday afternoon, we will be showing you slide of different cell types and we will be asking you question s as to identify what type of cell is in the picture, and what kind of function. I realize it’s very close to the end of these lectures. I don’t know, will there be a quiz at the end, Dr. Wishe? I think we did last year. So, but just if you know, please go through some of these things. Don’t focus too much on prognosis of the diseases and management. The things that have been underlined in the slides are important, I realize that there are a lot of diseases, but unfortunately they are in the syllabus for your board exams next year, but try to focus, it’s important to know the types of cells and what’s their function, and also the things that they are diagnostic. For example if there is chromosomal translocation, or like I mentioned the Bence Jones proteins, or whatever there is underlined in the slides, these are things to know for the upcoming exams. Ok? Have a good weekend.

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Lecture 28: Bone Marrow and Hematopoiesis