Hematopoiesis
Hematopoiesis, also known as Hemopoiesis is formation and development of blood cells. Meaning of the term hematopoiesis is “Hemo = blood” and “poiesis = to form”. Hematopoiesis occurs in bone marrow and where is this bone marrow located?? To understand this, let us take a cross section of a bone (figure a).
Figure a. Bone Marrow
In figure a we can see the center cavity of bone has the red spongy tissue. This red spongy tissue in the center of the bone is called bone marrow. OK so we are clear with some terminology and we will see few more as we go on J
The starting point of Hematopoiesis is Hematopoietic Stem Cell (HSC) which is a multipotent stem cell. What is multipotent now?? The meaning of multipotent is “multi = many” and “potent = being able” which means the cell has ability to differentiate or give rise to several different types of cells. We will see how HSC which is a pluripotent stem cell gives rise to all different types of blood cells (figure b).
Figure b. Hematopoiesis
The first differentiation of HSC results in two lineages – Myeloid stem cell and Lymphoid stem cell. Now let me tell you couple of points which would make this chart (figure b) very easy to understand and remember.
First, the lymphoid stem cell lineage gives rise to all the lymphocytes and the myeloid stem cell will differentiate in rest of the blood cells (other than lymphocytes). Lymphoid stem cell = all lymphocytes, easy to understand and remember J Second, once the main two lineages are differentiated (myeloid and lymphoid), it will now differentiate into progenitor cells (green in color in fig b), followed by differentiation in precursor cells (blue in color in fig b) which are named by writing suffix “blast”, i.e. proerythroblast, megakaryoblast, monoblast etc. Now this precursor cells will undergo further development and becomes formed element (red in color in fig b).
So briefly it is,
HSC – myeloid and lymphoid lineages – progenitor cells – precursor cells – formed elements
Now that we are clear with these points, keeping them in mind let us see the differentiation process.
So first we will see what happens to myeloid stem cell. It will differentiate into Colony Forming Unit – Erythrocyte (CFU-E), Colony Forming Unit – Megakaryocyte (CFU-Meg) and Colony Forming Unit – Granulocyte and Macrophage (CFU-GM). If you look at these progenitor cell, you can understand what it is going get developed in. There are two more cell developing from myeloid cell but they don’t have progenitor cell phase, it will have direct precursor cell stage.
Alright so now once the progenitor cells are formed, the next stage is the formation of precursor cells. As I mentioned before they will have suffix “blast”. So from CFU-E, proerythroblast will form. From CFU-Meg, megakaryoblast will form. From CFU-GM two precursor cell will form as the term suggest, monoblast and myeloblast. And as I said there will be two more precursor cells directly differentiating from myeloid stem cell and they are eosionophilic myelolast and basophilic myeloblast.
Now proerythroblst is going to get developed in reticulocyte, at this point the nucleus of RBC will be ejected and eventually it will developed in erythrocyte (RBC). Megakaryoblast will differentiate in a huge megakaryocyte cell which gets splinter into thousands of small fragments and that is what we call thrombocyte (platelets). Monoblast differentiates into monocyte and circulating monocytes are called macrophages. The remaining three precursor cells are going to give rise to all granulocytes and if you notice that all the precursor cells of granulocyte are named myeloblast, i.e. myeloblast will developed into neutrophil, eosionophilic myeloblast will developed into eosinophil and basophilic myeloblast will developed into basophil.
Now let us see what happens to lymphoid stem cell. It will differentiate into two progenitor cells, i.e. prothymocyte and pre B-cell and they will develop into precursor cells T lymphoblast and B lymphoblast respectively. Here also there is one direct precursor cell formation without the progenitor cell stage and that is NK lymphoblast (again see suffix “blast” in all precursor cells). T lymphoblast will develop into T lymphocyte (T cell), B lymphoblast will develop into B lymphoblast (B cell) and NK lymphoblast will give rise to Natural killer cell (NK cell).
So that is it!! It is very easy to understand once we know all different stages and how they are named. Last but not least this is how you can remember it 🙂
First, you start with HSC which give rise to two lineages myeloid and lymphoid and here the term itself has the answer that lymphoid stem cell give rise to all lymphoid cells which include T cells, B cells and NK cells. All other remaining blood cells are derived from myeloid stem cell.
Second, progenitor cells are formed and in case of eosinophil, basophil and NK cell there is no progenitor cell.
Third, the progenitor cells differentiates into precursor cells (suffix “blast”) which then differentiates in to formed elements and they are nothing but our blood cells.
For better understanding watch this video on Hematopoiesis.