 Erythropoiesis is the process of origin, development and maturation of red blood cells. The site of Erythropoiesis where all these changes take place changes with age. In early weeks of embryonic life they are synthesized from yolk sac from the mesonchym of yolk sac and this stage of synthesis of RBCs is known as mesoblastic stage when we are referring to the site where Erythropoiesis is taking place. Now in this stage primitive kind of RBCs are synthesized and these are actually nucleated. Then after first trimester of pregnancy they are produced mainly in liver and also somewhat in spleen and lymph nodes and because the major site of synthesis of RBCs is liver this stage is known as hepatic stage. Then during last month of gestation and after birth up to 20 years of age site of Erythropoiesis ships to bone marrow of all the bones of the body and after 20 years what happens marrow of long bones becomes fatty so basically it becomes inactive except upper part of humerus and femur bones. So after 20 years of age RBCs are synthesized only in the marrow of membranous bones and in the upper ends of humerus and femur. So what are these membranous bones? You can simply remember this as bones in central axis of your body that is skull, sternum, ribs, vertebrae and pelvis. Now what is the significance of knowing all this that where this Erythropoiesis is taking place? Well first of all you see in case of hemolysis when a lot of RBC breakdown is taking place then in that case body compensates by producing RBCs even more. So then what happens that there is expansion of the site of Erythropoiesis. So for example in skull there is expansion of the site of Erythropoiesis and in such cases when we take a skull x-rays we see something known as hair on end appearance which are basically vertical striations of calcified spicules and these are perpendicular to the surface of the bone and it looks like hair is standing on end and this is very commonly seen in children and adolescents with hemolytic anemias. Not only that when excessive RBC synthesis is required the site of synthesis may even involve the fetal site that is liver and spleen for RBC synthesis and this is known as extra medullary hematopoiesis. So in case of hemolytic anemia there is extra medullary hematopoiesis in liver and spleen and this leads to enlargement of liver and spleen that is hepatomigaly and spleen omigaly. Then there is another significance of knowing the site of Erythropoiesis. See sometimes in cases of anemia we need to examine the bone marrow to determine the cause. So if we know where exactly the synthesis is taking place we can take the sample from there. So for example in adults it is mainly the membranous bone. So we take the sample from either the sternum or the iliac crest for examining the bone marrow sample. Fine. So that was about the site of Erythropoiesis. Now coming to that how the development and maturation of RBCs is taking place. Well first of all all blood cells develop from pluripotent hematopoietic stem cell and as the name of the cell suggests it is pluripotent hematopoietic that means it has the potency potent means potency to produce multiple types pluris multiple types of hematic cells hematopoietic isn't it? So pluripotent hematopoietic cell can produce multiple types of blood cells. Now this cell can multiply and form many of its own types right so it can form other pluripotent hematopoietic cells but to form a particular type of blood cell it is important that some of the cells which are formed after multiplication they should differentiate towards the lineage of forming that particular blood cell. So when we are talking about red blood cells some of the cells should differentiate to RBC lineage. So two things that these cells hematopoietic stem cells can self renew and they can differentiate to particular lineage. So this differentiation to a particular lineage is based on the presence of certain interleukins and growth factors so there is interleukin 3 is there then interleukin 6 then there is a grandocyte monocyte colony stimulating factor is there which causes its differentiation to the myeloid lineage the other lineage is lymphoid lineage which will produce lymphocytes. So these RBCs are produced from the myeloid lineage so with this differentiation there is production of committed cells which can form cells of the myeloid lineage and the cells of the myeloid lineage include erythrocytes, megakaryocytes then other WBCs apart from lymphocytes. So that means this particular cell now has to differentiate further into different lineages so there will be differentiation towards a line to form the WBCs right so the other line we will discuss so that forms a colony forming unit. So first one which we are talking it is colony forming unit is plain which can differentiate into formation of leukocytes and the other one differentiates into formation of either erythrocytes or megakaryocytes so this is known as colony forming unit erythrocytes slash megakaryocytes or CFUEM in short. So as the term suggests it will further differentiate into CFUE that is colony forming unit erythrocytes and CFUEM that is colony forming unit megakaryocytes and obviously the differentiation to different lineages depends on the presence of different types of growth factors fine. Now this CFU further differentiates into various stages so first stage is pro-ethroblast or pro-norma blast then there is a basophilic erythroblast or also known as early norma blast then there is polychromatophilic erythroblast or intermediate norma blast and then finally there is orthochromatic erythroblast or also known as late norma blast and finally there is reticulocyte formation so this reticulocyte enters into blood and matures into RBCs now each of these stages have certain characteristics so let us see the characteristics of each of these stage. So first of all remember that as different stages progress there is decrease in size of the cell so this one is the largest from 25 micrometers it comes to 7.2 micrometers which is the size of the RBC so cell size decreases size of the nucleus also decreases and ultimately it disappears we will see in which stage it disappears and there is increase in the cytoplasm so nuclear cytoplasmic ratio that is how much nucleus is there and how much cytoplasm there if we see the ratio that nucleus cytoplasmic ratio decreases because nucleus content is decreasing so that is the first thing you should remember common thing the next common thing you should remember that all these stages can multiply and differentiate so they can divide also and they can undergo differentiation also until the stage of late norma blast so this late norma blast doesn't has the capacity of multiplying it can only differentiate into further cells right so after this the number of cells is not going to increase fine now let's see the characteristic of each of these cells first of all proethroblast you see as I said the proethroblast will be a very large cell around 25 micrometers and its nucleus is also very large and it is almost completely occupying the cell so it just fills the cell and there is only a very small rim of basophilic cytoplasm so cytoplasm is also blue in color then the next stage is basophilic erythrocyte or early norma blast so as the name suggests it is basophilic because the cytoplasm again stains the blue and the size of the nucleus will decrease little bit then intermediate norma blast or polychromatophilic erythroblast what we said you see the name says polychromatophilic so it is getting stained with multiple colors polychrome okay so in this stage what happens that the hemoglobin appears in the cytoplasm and that is why it stains with both blue and red stains so that is why the name polychromatophilic so hemoglobin appears in intermediate norma blast so the size of the nucleus is decreased further size of the cell is decreased further and hemoglobin has appeared in this stage then the next is late norma blast and this is basically acidophilic because in this stage the nucleus is expelled out and the hemoglobin almost fills the cell completely so that is late norma blast and as i told earlier this cell does not has the capacity to multiply also then comes the reticulocyte in reticulocyte there is only remnants of RNA which are present as the nucleus has been thrown out and this remnant of RNA forms a reticulum in the cell and that is why the name reticulocyte finally it is that reticulocyte stage which actually enters into the circulation and matures into RBC in the circulation in one to two days and these RBCs become biconcave in shape so those are the various stages of erythropoiesis and the characteristics of various stages now from proetroblast to mature RBCs it takes approximately seven days so if you see if erythropoietin is given which is a hormone which acts on the RBC precursors so in certain diseases it is administered so if erythropoietin is given for treatment you can understand that this CPU which acts on precursors that is this proetroblast it will take minimum one week to act for RBC number 2 increase so that was all about the erythropoiesis for the factors affecting erythropoiesis i have made another video the link for which i have given in the description section below you can check that out too thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you