Development of Formed Elements

For the most part, blood cells and platelets develop from a common precursor cell in bone marrow. The precursor cell, because it gives rise to many different cell types yet still remains to make more, is also called a stem cell. The term "stem cell" is generic, and could refer not only to a cell type that manufactures blood cells but also to cells that generate the mature cells of other tissues.

The particular stem cell that produces our blood cells is the hemocytoblast. "Hemo-" is for blood, "cyto" is for cell, and "-blast" means a cell that can divide to make more cells. Taken together, a hemocytoblast is a cell that can divide to make more and more blood cells.

The hemocytoblast remains in bone marrow. Remember, there are two types of bone marrow: red marrow and yellow marrow. Do you remember enough about them to remember which type produces blood cells? ...

... I hope you remembered it was red bone marrow! One of the types of cells that hemocytoblasts will make is the red blood cell, so, as it makes them, the marrow it is in is forced to look red!

Anyway, the hemocytoblast divides a lot. Each time it divides (mitosis), one of the two daughter cells remains as a hemocytoblast; this ensures the continued existence of hemocytoblasts in the bone marrow. But the other daughter cell will change into a different type of cell. The type of cell it becomes depends on the signals HCBdividing.gif (18945 bytes) (like hormones) that find their way to the bone marrow from the blood. For example,

	erythropoietin acts as a signal to cause the daughter cell of the hemocytoblast to turn into a "proerythroblast," which will keep dividing and eventually form into erythrocytes.
	thrombopoietin acts as a signal to cause the daughter cell of the hemocytoblast to turn into a "megakaryoblast," which will keep dividing and make megakaryocytes which pinch off platelets.

I would like you to know the names of these signals, but you do not need to know any of the intermediary names for the cells (except for a few that will be specified on the platelet and red blood cell pages).

I have tried to illustrate cell division (mitosis) of hemocytoblasts in the image above. In this image you should be able to see that the signal that is present at the time of mitosis influences the fate of a daughter cell. At least that's what I tried to draw! Note that we will not run out of hemocytoblasts because every time they make new types of cells, they also make more hemocytoblasts.

As we continue to study the blood cells, you will see why we might need to make signals (hormones, really) like erythropoietin to promote manufacturing new blood cells.

Meanwhile, if you take a look at Figure 14.3 in your textbook (which is also the same one I showed you on the overhead in lab), you'll see a clear diagram of how the cells develop. What I'd like you to pick out of this diagram is that once the hemocytoblast has divided and its daughter cell's fate has been determined, it is pretty clear what you're going to make. Like, if a proerythroblast is born, only red blood cells will develop-- not white blood cells. And, if a "monoblast" is born, only monocytes will develop, not lymphocytes! Also, note that if a "progranulocyte" is born, any one of the three granulocytes (neutrophils, eosinophils, or basophils) could develop. Again, the specific types that arise depend on more signals.

Some blood cells develop in places other than bone marrow, and some blood cells also leave the blood to take on their jobs elsewhere. Let me explain briefly:

	lymphocytes, although they can develop in bone marrow, can also mature in lymph nodes. One type can even mature to become called "plasma cells." Lymph nodes and plasma cells are involved in our immune systems, so we'll defer discussion of these until we get to the immune system unit.
	white blood cells can leave the blood to enter other body tissues. When monocytes do that, they are referred to as macrophages in those tissues. You'll learn more about this on the white blood cell page.

Please note: only erythrocytes, platelets, neutrophils, basophils, eosinophils, monocytes, and lymphocytes are found in the blood. All the other cells in Figure 14.3 remain in the bone marrow, serving only to manufacture the blood cells. Once the blood cells are born, they enter the blood from the bone marrow.

We have now completed discussion of the development of blood formed elements. It is now time to learn about the cells themselves.