Making mountains out of molehills: how stem cells become platelets

You have 100,000 km of plumbing (blood vessels) in your body. What then keeps them from leaking? This essential work is carried out by tiny cells called platelets, which are able to form a plug to stop bleeding after blood vessel damage.

Platelets have a volume 10 times smaller than a red blood cell, curiously they are produced from giant cells called megakaryocytes (MKs). These MKs are generated from haematopoietic (blood) stem cells (HSCs) in the bone marrow and in the lungs. In fact, HSCs give rise to all the different cellular components of blood such as white and red blood cells. For the HSCs to produce MKs and platelets the activity of a specific protein called thrombopoietin or TPO is needed. TPO sends a signal through a particular receptor on the cell surface and this triggers changes that push the cells to become MKs and platelets rather than, say, red or white blood cells.     

Heamatopoietic stem cells purified from cord blood or bone
Fig. 1. Heamatopoietic stem cells purified from cord blood or bone marrow samples are stimulated by thrombopoietin (TPO) in culture to differentiate into mature megakaryocytes (MKs). Mks then release functional platelets into the culture medium. Each stem cell can generate up to 420 platelets.

There are diseases, known as thrombocytopenias, characterized by abnormally low platelet numbers in the circulation. The reasons that lead to these diseases are varied, and include immunological problems, genetic conditions, certain medications, infections, cancer and blood transfusions. Low platelet counts can cause internal bleeding (eg, in the intestines or the brain) and in severe cases result in death. Treatment for thrombocytopenia depends on the cause, the status of the patient and other specific needs. One of these treatments is platelet transfusion, using platelets isolated from blood donors. This treatment is effective in the short term, however many patients become refractory to platelet transfusion and this is a serious clinical issue. This happens, for instance, in patients that need ongoing transfusions due to thrombocytopenia induced by chemotherapy. Many of these patients develop antibodies that destroy the transfused platelets and the patients remain in danger of serious bleeding. Finding compatible or matching platelets is not always feasible.

There are diseases, known as thrombocytopenias, characterized by abnormally low platelet numbers in the circulation. The reasons that lead to these diseases are varied, and include immunological problems, genetic conditions, certain medications, infections, cancer and blood transfusions. Low platelet counts can cause internal bleeding (eg, in the intestines or the brain) and in severe cases result in death. Treatment for thrombocytopenia depends on the cause, the status of the patient and other specific needs. One of these treatments is platelet transfusion, using platelets isolated from blood donors. This treatment is effective in the short term, however many patients become refractory to platelet transfusion and this is a serious clinical issue. This happens, for instance, in patients that need ongoing transfusions due to thrombocytopenia induced by chemotherapy. Many of these patients develop antibodies that destroy the transfused platelets and the patients remain in danger of serious bleeding. Finding compatible or matching platelets is not always feasible. This shortage of matching platelets could be overcome by producing platelets in the laboratory as required. Our method published in the Journal of Visualized Experiments shows that functional platelets can be produced in the laboratory from HSCs. HSCs can be purified from cord blood or from bone marrow samples and expanded and differentiated in the presence of TPO under serum free conditions, thus avoiding the use of animal products in platelet production for potential human use. This method produces MKs in 10-12 days which release platelets in the next two to three days (Fig. 1). The platelets produced are similar in size to those in human blood and show biochemical features characteristic of functional human platelets. The number of platelets generated is up to 420 platelets per input HSC. This yield would not be sufficient to generate enough platelets for transfusion, but it is a proof of principle and also a useful tool for the study of platelet production using primary human cells. There are other methods to generate large numbers of platelets, for example using induced pluripotent stem cells (Moreau et al., 2016) or expanding HSCs with additional growth factors (Matsunaga et al, 2006). Some of these techniques could result in production of sufficient functional platelets for therapeutic use. The demand for platelet transfusions is growing due to the number of patients requiring stem cell transplants or chemotherapy, therefore the need to explore ways to produce matching platelets on demand will continue. Production of platelets from HSCs will advance our understanding of platelet formation and improve the efficiency of culture methodologies to meet patients’ needs.

Jose Perdomo
Haematology Research Unit, St George and Sutherland Clinical School, University of New South Wales, Sydney, Australia

Publication

Megakaryocyte Differentiation and Platelet Formation from Human Cord Blood-derived CD34+ Cells.
Perdomo J, Yan F, Leung HHL, Chong BH
J Vis Exp. 2017 Dec 27
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