Haemopoiesis is the process by which mature blood cells develop from precursor cells. It continues continuously throughout embryonic and adult life and as a result new cells formed in the so-called haemopoietic regions constantly replace mature blood cells in the circulation.

In the embryo, haemopoiesis occurs at different stages in the yolk sac, the liver, the spleen, lymph nodes and the bone marrow.
In the adult, erythrocytes, granulocytes, monocytes and platelets are formed in the bone marrow while the lymphocytes are formed mainly in the lymph nodes, spleen, thymus and lymphatic nodules of the gastrointestinal tract. However, the lymphocytes present in these organs originate directly or indirectly from the bone marrow.

Question. Which bones contain haemopoietic marrow?

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Look at this eMicroscope slide, a smear of normal bone marrow, taken using a x40 objective.

Move around the section using the mouse and try to identify some of the different cell types present with the aid of a textbook - you will probably find this very difficult!

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Precursor cells

All precursor cells have some common features:

  1. They are larger in diameter than mature red and white blood cells
  2. The nuclei have non-condensed chromatin
  3. The cytoplasm is rich in free ribosomes

Question. What do these last two points tell you about the activities of precursor cells?

Although you may have difficulty in identifying precursors of WBCs and RBCs you should have no trouble in identifying huge cells (up to 150µm in diameter).

Question. What type of cells are these and what is their function?